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git://git.kernel.org/pub/scm/linux/kernel/git/mfasheh/btrfs-error-handling into for-linus
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In addition to properly handling allocation failure from btrfs_alloc_path, I
also fixed up the kzalloc error handling code immediately below it.
Signed-off-by: Mark Fasheh <mfasheh@suse.com>
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I also removed the BUG_ON from error return of find_next_chunk in
init_first_rw_device(). It turns out that the only caller of
init_first_rw_device() also BUGS on any nonzero return so no actual behavior
change has occurred here.
do_chunk_alloc() also needed an update since it calls btrfs_alloc_chunk()
which can now return -ENOMEM. Instead of setting space_info->full on any
error from btrfs_alloc_chunk() I catch and return every error value _except_
-ENOSPC. Thanks goes to Tsutomu Itoh for pointing that issue out.
Signed-off-by: Mark Fasheh <mfasheh@suse.com>
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Dealing with this seems trivial - the only caller of btrfs_balance() is
btrfs_ioctl() which passes the error code directly back to userspace. There
also isn't much state to unwind (if I'm wrong about this point, we can
always safely move the allocation to the top of btrfs_balance() anyway).
Signed-off-by: Mark Fasheh <mfasheh@suse.com>
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btrfs_iget() also needed an update so that errors from btrfs_locked_inode()
are caught and bubbled back up.
Signed-off-by: Mark Fasheh <mfasheh@suse.com>
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I moved the path allocation up a few lines to the top of the function so
that we couldn't get into the state where we've dropped delayed items and
the extent cache but fail due to -ENOMEM.
Signed-off-by: Mark Fasheh <mfasheh@suse.com>
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The two ->process_func call sites in tree-log.c which were ignoring a return
code have also been updated to gracefully exit as well.
Signed-off-by: Mark Fasheh <mfasheh@suse.com>
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This patch fixes many callers of btrfs_alloc_path() which BUG_ON allocation
failure. All the sites that are fixed in this patch were checked by me to
be fairly trivial to fix because of at least one of two criteria:
- Callers of the function catch errors from it already so bubbling the
error up will be handled.
- Callers of the function might BUG_ON any nonzero return code in which
case there is no behavior changed (but we still got to remove a BUG_ON)
The following functions were updated:
btrfs_lookup_extent, alloc_reserved_tree_block, btrfs_remove_block_group,
btrfs_lookup_csums_range, btrfs_csum_file_blocks, btrfs_mark_extent_written,
btrfs_inode_by_name, btrfs_new_inode, btrfs_symlink,
insert_reserved_file_extent, and run_delalloc_nocow
Signed-off-by: Mark Fasheh <mfasheh@suse.com>
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The btrfs transaction code will return any errors that come from
reserve_metadata_bytes. We need to make sure we don't return funny
things like 1 or EAGAIN.
Signed-off-by: Chris Mason <chris.mason@oracle.com>
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Now that we are using regular file crcs for the free space cache,
we can deadlock if we try to read the free_space_inode while we are
updating the crc tree.
This commit fixes things by using the commit_root to read the crcs. This is
safe because we the free space cache file would already be loaded if
that block group had been changed in the current transaction.
Signed-off-by: Chris Mason <chris.mason@oracle.com>
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For metadata buffers that don't straddle pages (all of them), btrfs
can safely use the page uptodate bits and extent_buffer uptodate bit
instead of needing to use the extent_state tree.
This greatly reduces contention on the state tree lock.
Signed-off-by: Chris Mason <chris.mason@oracle.com>
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Before the reader/writer locks, btrfs_next_leaf needed to keep
the path blocking to avoid making lockdep upset.
Now that btrfs_next_leaf only takes read locks, this isn't required.
Signed-off-by: Chris Mason <chris.mason@oracle.com>
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This patch was originally from Tejun Heo. lockdep complains about the btrfs
locking because we sometimes take btree locks from two different trees at the
same time. The current classes are based only on level in the btree, which
isn't enough information for lockdep to figure out if the lock is safe.
This patch makes a class for each type of tree, and lumps all the FS trees that
actually have files and directories into the same class.
Signed-off-by: Chris Mason <chris.mason@oracle.com>
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The btrfs metadata btree is the source of significant
lock contention, especially in the root node. This
commit changes our locking to use a reader/writer
lock.
The lock is built on top of rw spinlocks, and it
extends the lock tracking to remember if we have a
read lock or a write lock when we go to blocking. Atomics
count the number of blocking readers or writers at any
given time.
It removes all of the adaptive spinning from the old code
and uses only the spinning/blocking hints inside of btrfs
to decide when it should continue spinning.
In read heavy workloads this is dramatically faster. In write
heavy workloads we're still faster because of less contention
on the root node lock.
We suffer slightly in dbench because we schedule more often
during write locks, but all other benchmarks so far are improved.
Signed-off-by: Chris Mason <chris.mason@oracle.com>
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Hit this nice little deadlock. What happens is this
__btrfs_end_transaction with throttle set, --use_count so it equals 0
btrfs_commit_transaction
<somebody else actually manages to start the commit>
btrfs_end_transaction --use_count so now its -1 <== BAD
we just return and wait on the transaction
This is bad because we just return after our use_count is -1 and don't let go
of our num_writer count on the transaction, so the guy committing the
transaction just sits there forever. Fix this by inc'ing our use_count if we're
going to call commit_transaction so that if we call btrfs_end_transaction it's
valid. Thanks,
Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
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The extent_buffers have a very complex interface where
we use HIGHMEM for metadata and try to cache a kmap mapping
to access the memory.
The next commit adds reader/writer locks, and concurrent use
of this kmap cache would make it even more complex.
This commit drops the ability to use HIGHMEM with extent buffers,
and rips out all of the related code.
Signed-off-by: Chris Mason <chris.mason@oracle.com>
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When we balanced the chunks across the devices, BUG_ON() in
__finish_chunk_alloc() was triggered.
------------[ cut here ]------------
kernel BUG at fs/btrfs/volumes.c:2568!
[SNIP]
Call Trace:
[<ffffffffa049525e>] btrfs_alloc_chunk+0x8e/0xa0 [btrfs]
[<ffffffffa04546b0>] do_chunk_alloc+0x330/0x3a0 [btrfs]
[<ffffffffa045c654>] btrfs_reserve_extent+0xb4/0x1f0 [btrfs]
[<ffffffffa045c86b>] btrfs_alloc_free_block+0xdb/0x350 [btrfs]
[<ffffffffa048a8d8>] ? read_extent_buffer+0xd8/0x1d0 [btrfs]
[<ffffffffa04476fd>] __btrfs_cow_block+0x14d/0x5e0 [btrfs]
[<ffffffffa044660d>] ? read_block_for_search+0x14d/0x4d0 [btrfs]
[<ffffffffa0447c9b>] btrfs_cow_block+0x10b/0x240 [btrfs]
[<ffffffffa044dd5e>] btrfs_search_slot+0x49e/0x7a0 [btrfs]
[<ffffffffa044f07d>] btrfs_insert_empty_items+0x8d/0xf0 [btrfs]
[<ffffffffa045e973>] insert_with_overflow+0x43/0x110 [btrfs]
[<ffffffffa045eb0d>] btrfs_insert_dir_item+0xcd/0x1f0 [btrfs]
[<ffffffffa0489bd0>] ? map_extent_buffer+0xb0/0xc0 [btrfs]
[<ffffffff812276ad>] ? rb_insert_color+0x9d/0x160
[<ffffffffa046cc40>] ? inode_tree_add+0xf0/0x150 [btrfs]
[<ffffffffa0474801>] btrfs_add_link+0xc1/0x1c0 [btrfs]
[<ffffffff811dacac>] ? security_inode_init_security+0x1c/0x30
[<ffffffffa04a28aa>] ? btrfs_init_acl+0x4a/0x180 [btrfs]
[<ffffffffa047492f>] btrfs_add_nondir+0x2f/0x70 [btrfs]
[<ffffffffa046af16>] ? btrfs_init_inode_security+0x46/0x60 [btrfs]
[<ffffffffa0474ac0>] btrfs_create+0x150/0x1d0 [btrfs]
[<ffffffff81159c63>] ? generic_permission+0x23/0xb0
[<ffffffff8115b415>] vfs_create+0xa5/0xc0
[<ffffffff8115ce6e>] do_last+0x5fe/0x880
[<ffffffff8115dc0d>] path_openat+0xcd/0x3d0
[<ffffffff8115e029>] do_filp_open+0x49/0xa0
[<ffffffff8116a965>] ? alloc_fd+0x95/0x160
[<ffffffff8114f0c7>] do_sys_open+0x107/0x1e0
[<ffffffff810bcc3f>] ? audit_syscall_entry+0x1bf/0x1f0
[<ffffffff8114f1e0>] sys_open+0x20/0x30
[<ffffffff81484ec2>] system_call_fastpath+0x16/0x1b
[SNIP]
RIP [<ffffffffa049444a>] __finish_chunk_alloc+0x20a/0x220 [btrfs]
The reason is:
Task1 Space balance task
do_chunk_alloc()
__finish_chunk_alloc()
update device info
in the chunk tree
alloc system metadata block
relocate system metadata block group
set system metadata block group
readonly, This block group is the
only one that can allocate space. So
there is no free space that can be
allocated now.
find no space and don't try
to alloc new chunk, and then
return ENOSPC
BUG_ON() in __finish_chunk_alloc()
was triggered.
Fix this bug by allocating a new system metadata chunk before relocating the
old one if we find there is no free space which can be allocated after setting
the old block group to be read-only.
Reported-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Tested-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
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Everybody else does this, we need to do it too. If we're syncing, we need to
tag the pages we're going to write for writeback so we don't end up writing the
same stuff over and over again if somebody is constantly redirtying our file.
This will keep us from having latencies with heavy sync workloads. Thanks,
Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
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So I had this brilliant idea to use atomic counters for outstanding and reserved
extents, but this turned out to be a bad idea. Consider this where we have 1
outstanding extent and 1 reserved extent
Reserver Releaser
atomic_dec(outstanding) now 0
atomic_read(outstanding)+1 get 1
atomic_read(reserved) get 1
don't actually reserve anything because
they are the same
atomic_cmpxchg(reserved, 1, 0)
atomic_inc(outstanding)
atomic_add(0, reserved)
free reserved space for 1 extent
Then the reserver now has no actual space reserved for it, and when it goes to
finish the ordered IO it won't have enough space to do it's allocation and you
get those lovely warnings.
Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
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Kill the check to see if we have 512mb of reserved space in delalloc and
shrink_delalloc if we do. This causes unexpected latencies and we have other
logic to see if we need to throttle. Thanks,
Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
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grab_cache_page will use mapping_gfp_mask(), which for all inodes is set to
GFP_HIGHUSER_MOVABLE. So instead use find_or_create_page in all cases where we
need GFP_NOFS so we don't deadlock. Thanks,
Signed-off-by: Josef Bacik <josef@redhat.com>
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A user reported a deadlock when copying a bunch of files. This is because they
were low on memory and kthreadd got hung up trying to migrate pages for an
allocation when starting the caching kthread. The page was locked by the person
starting the caching kthread. To fix this we just need to use the async thread
stuff so that the threads are already created and we don't have to worry about
deadlocks. Thanks,
Reported-by: Roman Mamedov <rm@romanrm.ru>
Signed-off-by: Josef Bacik <josef@redhat.com>
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First, we can sometimes free the state we're merging, which means anybody who
calls merge_state() may have the state it passed in free'ed. This is
problematic because we could end up caching the state, which makes caching
useless as the state will no longer be part of the tree. So instead of free'ing
the state we passed into merge_state(), set it's end to the other->end and free
the other state. This way we are sure to cache the correct state. Also because
we can merge states together, instead of only using the cache'd state if it's
start == the start we are looking for, go ahead and use it if the start we are
looking for is within the range of the cached state. Thanks,
Signed-off-by: Josef Bacik <josef@redhat.com>
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We used to store the checksums of the space cache directly in the space cache,
however that doesn't work out too well if we have more space than we can fit the
checksums into the first page. So instead use the normal checksumming
infrastructure. There were problems with doing this originally but those
problems don't exist now so this works out fine. Thanks,
Signed-off-by: Josef Bacik <josef@redhat.com>
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We keep having problems with early enospc, and that's because our method of
making space is inherently racy. The problem is we can have one guy trying to
make space for himself, and in the meantime people come in and steal his
reservation. In order to stop this we make a waitqueue and put anybody who
comes into reserve_metadata_bytes on that waitqueue if somebody is trying to
make more space. Thanks,
Signed-off-by: Josef Bacik <josef@redhat.com>
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We have to do weird things when handling enospc in the transaction joining code.
Because we've already joined the transaction we cannot commit the transaction
within the reservation code since it will deadlock, so we have to return EAGAIN
and then make sure we don't retry too many times. Instead of doing this, just
do the reservation the normal way before we join the transaction, that way we
can do whatever we want to try and reclaim space, and then if it fails we know
for sure we are out of space and we can return ENOSPC. Thanks,
Signed-off-by: Josef Bacik <josef@redhat.com>
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I've been watching how many btrfs_search_slot()'s we do and I noticed that when
we create a file with selinux enabled we were doing 2 each time we initialize
the security context. That's because we lookup the xattr first so we can delete
it if we're setting a new value to an existing xattr. But in the create case we
don't have any xattrs, so it is completely useless to have the extra lookup. So
re-arrange things so that we only lookup first if we specifically have
XATTR_REPLACE. That way in the basic case we only do 1 search, and in the more
complicated case we do the normal 2 lookups. Thanks,
Signed-off-by: Josef Bacik <josef@redhat.com>
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git://git.kernel.org/pub/scm/linux/kernel/git/jwessel/linux-2.6-kgdb
* 'for_linus' of git://git.kernel.org/pub/scm/linux/kernel/git/jwessel/linux-2.6-kgdb:
sparc,kgdbts: fix compile regression with kgdb test suite
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Commit 63ab25ebbc (kgdbts: unify/generalize gdb breakpoint adjustment)
introduced a compile regression on sparc.
kgdbts.c: In function 'check_and_rewind_pc':
kgdbts.c:307: error: implicit declaration of function 'instruction_pointer_set'
Simply add the correct macro definition for instruction pointer on the
Sparc architecture.
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: David S. Miller <davem@davemloft.net>
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* git://git.kernel.org/pub/scm/linux/kernel/git/sfrench/cifs-2.6:
CIFS: Fix wrong length in cifs_iovec_read
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Signed-off-by: Pavel Shilovsky <piastryyy@gmail.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>
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git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip
* 'x86-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip:
x86: Make Dell Latitude E6420 use reboot=pci
x86: Make Dell Latitude E5420 use reboot=pci
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Yet another variant of the Dell Latitude series which requires
reboot=pci.
From the E5420 bug report by Daniel J Blueman:
> The E6420 is affected also (same platform, different casing and
> features), which provides an external confirmation of the issue; I can
> submit a patch for that later or include it if you prefer:
> http://linux.koolsolutions.com/2009/08/04/howto-fix-linux-hangfreeze-during-reboots-and-restarts/
Reported-by: Daniel J Blueman <daniel.blueman@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: <stable@kernel.org>
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Rebooting on the Dell E5420 often hangs with the keyboard or ACPI
methods, but is reliable via the PCI method.
[ hpa: this was deferred because we believed for a long time that the
recent reshuffling of the boot priorities in commit
660e34cebf0a11d54f2d5dd8838607452355f321 fixed this platform.
Unfortunately that turned out to be incorrect. ]
Signed-off-by: Daniel J Blueman <daniel.blueman@gmail.com>
Link: http://lkml.kernel.org/r/1305248699-2347-1-git-send-email-daniel.blueman@gmail.com
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: <stable@kernel.org>
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git://git.kernel.org/pub/scm/linux/kernel/git/keithp/linux-2.6
* 'drm-intel-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/keithp/linux-2.6:
drm/i915: Fix unfenced alignment on pre-G33 hardware
drm/i915: Add quirk to disable SSC on Lenovo U160 LVDS
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Align unfenced buffers on older hardware to the power-of-two object
size. The docs suggest that it should be possible to align only to a
power-of-two tile height, but using the already computed fence size is
easier and always correct. We also have to make sure that we unbind
misaligned buffers upon tiling changes.
In order to prevent a repetition of this bug, we change the interface
to the alignment computation routines to force the caller to provide
the requested alignment and size of the GTT binding rather than assume
the current values on the object.
Reported-and-tested-by: Sitosfe Wheeler <sitsofe@yahoo.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=36326
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Keith Packard <keithp@keithp.com>
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We've tried several times to make this machine 'just work', but every
patch that does causes many other machines to fail. This adds a quirk
which special cases this hardware and forces ssc to be
disabled. There's no way to override this from the command line; that
would be a significantly more invasive change.
This patch fixes #36656 on fdo bugzilla:
https://bugs.freedesktop.org/show_bug.cgi?id=36656
Signed-off-by: Keith Packard <keithp@keithp.com>
References: https://bugs.freedesktop.org/show_bug.cgi?id=36656
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
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It seems to hurt performance in real life. Yes, the inode will be used
later, but the conditional doesn't seem to predict all that well
(negative dentries are not uncommon) and it looks like the cost of
prefetching is simply higher than depending on the cache doing the right
thing.
As usual.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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The compiler, at least for ix86 and m68k, validly warns that the
comparison:
next <= (loff_t)-1
is always true (and it's always true also for x86-64 and probably all
other arches - as long as pgoff_t isn't wider than loff_t). The
intention appears to be to avoid wrapping of "next", so rather than
eliminating the pointless comparison, fix the loop to indeed get exited
when "next" would otherwise wrap.
On m68k the following warning is observed:
fs/fscache/page.c: In function '__fscache_uncache_all_inode_pages':
fs/fscache/page.c:979: warning: comparison is always false due to limited range of data type
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reported-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Cc: Suresh Jayaraman <sjayaraman@suse.de>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip
* 'core-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip:
signal: align __lock_task_sighand() irq disabling and RCU
softirq,rcu: Inform RCU of irq_exit() activity
sched: Add irq_{enter,exit}() to scheduler_ipi()
rcu: protect __rcu_read_unlock() against scheduler-using irq handlers
rcu: Streamline code produced by __rcu_read_unlock()
rcu: Fix RCU_BOOST race handling current->rcu_read_unlock_special
rcu: decrease rcu_report_exp_rnp coupling with scheduler
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git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-2.6-rcu into core/urgent
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The __lock_task_sighand() function calls rcu_read_lock() with interrupts
and preemption enabled, but later calls rcu_read_unlock() with interrupts
disabled. It is therefore possible that this RCU read-side critical
section will be preempted and later RCU priority boosted, which means that
rcu_read_unlock() will call rt_mutex_unlock() in order to deboost itself, but
with interrupts disabled. This results in lockdep splats, so this commit
nests the RCU read-side critical section within the interrupt-disabled
region of code. This prevents the RCU read-side critical section from
being preempted, and thus prevents the attempt to deboost with interrupts
disabled.
It is quite possible that a better long-term fix is to make rt_mutex_unlock()
disable irqs when acquiring the rt_mutex structure's ->wait_lock.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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The rcu_read_unlock_special() function relies on in_irq() to exclude
scheduler activity from interrupt level. This fails because exit_irq()
can invoke the scheduler after clearing the preempt_count() bits that
in_irq() uses to determine that it is at interrupt level. This situation
can result in failures as follows:
$task IRQ SoftIRQ
rcu_read_lock()
/* do stuff */
<preempt> |= UNLOCK_BLOCKED
rcu_read_unlock()
--t->rcu_read_lock_nesting
irq_enter();
/* do stuff, don't use RCU */
irq_exit();
sub_preempt_count(IRQ_EXIT_OFFSET);
invoke_softirq()
ttwu();
spin_lock_irq(&pi->lock)
rcu_read_lock();
/* do stuff */
rcu_read_unlock();
rcu_read_unlock_special()
rcu_report_exp_rnp()
ttwu()
spin_lock_irq(&pi->lock) /* deadlock */
rcu_read_unlock_special(t);
Ed can simply trigger this 'easy' because invoke_softirq() immediately
does a ttwu() of ksoftirqd/# instead of doing the in-place softirq stuff
first, but even without that the above happens.
Cure this by also excluding softirqs from the
rcu_read_unlock_special() handler and ensuring the force_irqthreads
ksoftirqd/# wakeup is done from full softirq context.
[ Alternatively, delaying the ->rcu_read_lock_nesting decrement
until after the special handling would make the thing more robust
in the face of interrupts as well. And there is a separate patch
for that. ]
Cc: Thomas Gleixner <tglx@linutronix.de>
Reported-and-tested-by: Ed Tomlinson <edt@aei.ca>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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Ensure scheduler_ipi() calls irq_{enter,exit} when it does some actual
work. Traditionally we never did any actual work from the resched IPI
and all magic happened in the return from interrupt path.
Now that we do do some work, we need to ensure irq_{enter,exit} are
called so that we don't confuse things.
This affects things like timekeeping, NO_HZ and RCU, basically
everything with a hook in irq_enter/exit.
Explicit examples of things going wrong are:
sched_clock_cpu() -- has a callback when leaving NO_HZ state to take
a new reading from GTOD and TSC. Without this
callback, time is stuck in the past.
RCU -- needs in_irq() to work in order to avoid some nasty deadlocks
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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The addition of RCU read-side critical sections within runqueue and
priority-inheritance lock critical sections introduced some deadlock
cycles, for example, involving interrupts from __rcu_read_unlock()
where the interrupt handlers call wake_up(). This situation can cause
the instance of __rcu_read_unlock() invoked from interrupt to do some
of the processing that would otherwise have been carried out by the
task-level instance of __rcu_read_unlock(). When the interrupt-level
instance of __rcu_read_unlock() is called with a scheduler lock held
from interrupt-entry/exit situations where in_irq() returns false,
deadlock can result.
This commit resolves these deadlocks by using negative values of
the per-task ->rcu_read_lock_nesting counter to indicate that an
instance of __rcu_read_unlock() is in flight, which in turn prevents
instances from interrupt handlers from doing any special processing.
This patch is inspired by Steven Rostedt's earlier patch that similarly
made __rcu_read_unlock() guard against interrupt-mediated recursion
(see https://lkml.org/lkml/2011/7/15/326), but this commit refines
Steven's approach to avoid the need for preemption disabling on the
__rcu_read_unlock() fastpath and to also avoid the need for manipulating
a separate per-CPU variable.
This patch avoids need for preempt_disable() by instead using negative
values of the per-task ->rcu_read_lock_nesting counter. Note that nested
rcu_read_lock()/rcu_read_unlock() pairs are still permitted, but they will
never see ->rcu_read_lock_nesting go to zero, and will therefore never
invoke rcu_read_unlock_special(), thus preventing them from seeing the
RCU_READ_UNLOCK_BLOCKED bit should it be set in ->rcu_read_unlock_special.
This patch also adds a check for ->rcu_read_unlock_special being negative
in rcu_check_callbacks(), thus preventing the RCU_READ_UNLOCK_NEED_QS
bit from being set should a scheduling-clock interrupt occur while
__rcu_read_unlock() is exiting from an outermost RCU read-side critical
section.
Of course, __rcu_read_unlock() can be preempted during the time that
->rcu_read_lock_nesting is negative. This could result in the setting
of the RCU_READ_UNLOCK_BLOCKED bit after __rcu_read_unlock() checks it,
and would also result it this task being queued on the corresponding
rcu_node structure's blkd_tasks list. Therefore, some later RCU read-side
critical section would enter rcu_read_unlock_special() to clean up --
which could result in deadlock if that critical section happened to be in
the scheduler where the runqueue or priority-inheritance locks were held.
This situation is dealt with by making rcu_preempt_note_context_switch()
check for negative ->rcu_read_lock_nesting, thus refraining from
queuing the task (and from setting RCU_READ_UNLOCK_BLOCKED) if we are
already exiting from the outermost RCU read-side critical section (in
other words, we really are no longer actually in that RCU read-side
critical section). In addition, rcu_preempt_note_context_switch()
invokes rcu_read_unlock_special() to carry out the cleanup in this case,
which clears out the ->rcu_read_unlock_special bits and dequeues the task
(if necessary), in turn avoiding needless delay of the current RCU grace
period and needless RCU priority boosting.
It is still illegal to call rcu_read_unlock() while holding a scheduler
lock if the prior RCU read-side critical section has ever had either
preemption or irqs enabled. However, the common use case is legal,
namely where then entire RCU read-side critical section executes with
irqs disabled, for example, when the scheduler lock is held across the
entire lifetime of the RCU read-side critical section.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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Given some common flag combinations, particularly -Os, gcc will inline
rcu_read_unlock_special() despite its being in an unlikely() clause.
Use noinline to prohibit this misoptimization.
In addition, move the second barrier() in __rcu_read_unlock() so that
it is not on the common-case code path. This will allow the compiler to
generate better code for the common-case path through __rcu_read_unlock().
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
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The RCU_BOOST commits for TREE_PREEMPT_RCU introduced an other-task
write to a new RCU_READ_UNLOCK_BOOSTED bit in the task_struct structure's
->rcu_read_unlock_special field, but, as noted by Steven Rostedt, without
correctly synchronizing all accesses to ->rcu_read_unlock_special.
This could result in bits in ->rcu_read_unlock_special being spuriously
set and cleared due to conflicting accesses, which in turn could result
in deadlocks between the rcu_node structure's ->lock and the scheduler's
rq and pi locks. These deadlocks would result from RCU incorrectly
believing that the just-ended RCU read-side critical section had been
preempted and/or boosted. If that RCU read-side critical section was
executed with either rq or pi locks held, RCU's ensuing (incorrect)
calls to the scheduler would cause the scheduler to attempt to once
again acquire the rq and pi locks, resulting in deadlock. More complex
deadlock cycles are also possible, involving multiple rq and pi locks
as well as locks from multiple rcu_node structures.
This commit fixes synchronization by creating ->rcu_boosted field in
task_struct that is accessed and modified only when holding the ->lock
in the rcu_node structure on which the task is queued (on that rcu_node
structure's ->blkd_tasks list). This results in tasks accessing only
their own current->rcu_read_unlock_special fields, making unsynchronized
access once again legal, and keeping the rcu_read_unlock() fastpath free
of atomic instructions and memory barriers.
The reason that the rcu_read_unlock() fastpath does not need to access
the new current->rcu_boosted field is that this new field cannot
be non-zero unless the RCU_READ_UNLOCK_BLOCKED bit is set in the
current->rcu_read_unlock_special field. Therefore, rcu_read_unlock()
need only test current->rcu_read_unlock_special: if that is zero, then
current->rcu_boosted must also be zero.
This bug does not affect TINY_PREEMPT_RCU because this implementation
of RCU accesses current->rcu_read_unlock_special with irqs disabled,
thus preventing races on the !SMP systems that TINY_PREEMPT_RCU runs on.
Maybe-reported-by: Dave Jones <davej@redhat.com>
Maybe-reported-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
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PREEMPT_RCU read-side critical sections blocking an expedited grace
period invoke rcu_report_exp_rnp(). When the last such critical section
has completed, rcu_report_exp_rnp() invokes the scheduler to wake up the
task that invoked synchronize_rcu_expedited() -- needlessly holding the
root rcu_node structure's lock while doing so, thus needlessly providing
a way for RCU and the scheduler to deadlock.
This commit therefore releases the root rcu_node structure's lock before
calling wake_up().
Reported-by: Ed Tomlinson <edt@aei.ca>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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