fs/locks: Replace lg_global with a percpu-rwsem

Replace the global part of the lglock with a percpu-rwsem.

Since fcl_lock is a spinlock and itself nests under i_lock, which too
is a spinlock we cannot acquire sleeping locks at
locks_{insert,remove}_global_locks().

We can however wrap all fcl_lock acquisitions with percpu_down_read
such that all invocations of locks_{insert,remove}_global_locks() have
that read lock held.

This allows us to replace the lg_global part of the lglock with the
write side of the rwsem.

In the absense of writers, percpu_{down,up}_read() are free of atomic
instructions. This further avoids the very long preempt-disable
regions caused by lglock on larger machines.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: dave@stgolabs.net
Cc: der.herr@hofr.at
Cc: paulmck@linux.vnet.ibm.com
Cc: riel@redhat.com
Cc: tj@kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 21 insertions, 0 deletions

diff --git a/fs/locks.c b/fs/locks.c
index ee1b15f6fc13..8f609ec03364 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -164,6 +164,7 @@ int lease_break_time = 45;
  */
 DEFINE_STATIC_LGLOCK(file_lock_lglock);
 static DEFINE_PER_CPU(struct hlist_head, file_lock_list);
+DEFINE_STATIC_PERCPU_RWSEM(file_rwsem);
 
 /*
  * The blocked_hash is used to find POSIX lock loops for deadlock detection.
@@ -587,6 +588,8 @@ static int posix_same_owner(struct file_lock *fl1, struct file_lock *fl2)
 /* Must be called with the flc_lock held! */
 static void locks_insert_global_locks(struct file_lock *fl)
 {
+        percpu_rwsem_assert_held(&file_rwsem);
+
         lg_local_lock(&file_lock_lglock);
         fl->fl_link_cpu = smp_processor_id();
         hlist_add_head(&fl->fl_link, this_cpu_ptr(&file_lock_list));
@@ -596,6 +599,8 @@ static void locks_insert_global_locks(struct file_lock *fl)
 /* Must be called with the flc_lock held! */
 static void locks_delete_global_locks(struct file_lock *fl)
 {
+        percpu_rwsem_assert_held(&file_rwsem);
+
         /*
          * Avoid taking lock if already unhashed. This is safe since this check
          * is done while holding the flc_lock, and new insertions into the list
@@ -915,6 +920,7 @@ static int flock_lock_inode(struct inode *inode, struct file_lock *request)
                         return -ENOMEM;
         }
 
+        percpu_down_read(&file_rwsem);
         spin_lock(&ctx->flc_lock);
         if (request->fl_flags & FL_ACCESS)
                 goto find_conflict;
@@ -955,6 +961,7 @@ find_conflict:
 
 out:
         spin_unlock(&ctx->flc_lock);
+        percpu_up_read(&file_rwsem);
         if (new_fl)
                 locks_free_lock(new_fl);
         locks_dispose_list(&dispose);
@@ -991,6 +998,7 @@ static int posix_lock_inode(struct inode *inode, struct file_lock *request,
                 new_fl2 = locks_alloc_lock();
         }
 
+        percpu_down_read(&file_rwsem);
         spin_lock(&ctx->flc_lock);
         /*
          * New lock request. Walk all POSIX locks and look for conflicts. If
@@ -1162,6 +1170,7 @@ static int posix_lock_inode(struct inode *inode, struct file_lock *request,
         }
  out:
         spin_unlock(&ctx->flc_lock);
+        percpu_up_read(&file_rwsem);
         /*
          * Free any unused locks.
          */
@@ -1436,6 +1445,7 @@ int __break_lease(struct inode *inode, unsigned int mode, unsigned int type)
                 return error;
         }
 
+        percpu_down_read(&file_rwsem);
         spin_lock(&ctx->flc_lock);
 
         time_out_leases(inode, &dispose);
@@ -1487,9 +1497,13 @@ restart:
         locks_insert_block(fl, new_fl);
         trace_break_lease_block(inode, new_fl);
         spin_unlock(&ctx->flc_lock);
+        percpu_up_read(&file_rwsem);
+
         locks_dispose_list(&dispose);
         error = wait_event_interruptible_timeout(new_fl->fl_wait,
                                                 !new_fl->fl_next, break_time);
+
+        percpu_down_read(&file_rwsem);
         spin_lock(&ctx->flc_lock);
         trace_break_lease_unblock(inode, new_fl);
         locks_delete_block(new_fl);
@@ -1506,6 +1520,7 @@ restart:
         }
 out:
         spin_unlock(&ctx->flc_lock);
+        percpu_up_read(&file_rwsem);
         locks_dispose_list(&dispose);
         locks_free_lock(new_fl);
         return error;
@@ -1660,6 +1675,7 @@ generic_add_lease(struct file *filp, long arg, struct file_lock **flp, void **pr
                 return -EINVAL;
         }
 
+        percpu_down_read(&file_rwsem);
         spin_lock(&ctx->flc_lock);
         time_out_leases(inode, &dispose);
         error = check_conflicting_open(dentry, arg, lease->fl_flags);
@@ -1730,6 +1746,7 @@ out_setup:
                 lease->fl_lmops->lm_setup(lease, priv);
 out:
         spin_unlock(&ctx->flc_lock);
+        percpu_up_read(&file_rwsem);
         locks_dispose_list(&dispose);
         if (is_deleg)
                 inode_unlock(inode);
@@ -1752,6 +1769,7 @@ static int generic_delete_lease(struct file *filp, void *owner)
                 return error;
         }
 
+        percpu_down_read(&file_rwsem);
         spin_lock(&ctx->flc_lock);
         list_for_each_entry(fl, &ctx->flc_lease, fl_list) {
                 if (fl->fl_file == filp &&
@@ -1764,6 +1782,7 @@ static int generic_delete_lease(struct file *filp, void *owner)
         if (victim)
                 error = fl->fl_lmops->lm_change(victim, F_UNLCK, &dispose);
         spin_unlock(&ctx->flc_lock);
+        percpu_up_read(&file_rwsem);
         locks_dispose_list(&dispose);
         return error;
 }
@@ -2703,6 +2722,7 @@ static void *locks_start(struct seq_file *f, loff_t *pos)
         struct locks_iterator *iter = f->private;
 
         iter->li_pos = *pos + 1;
+        percpu_down_write(&file_rwsem);
         lg_global_lock(&file_lock_lglock);
         spin_lock(&blocked_lock_lock);
         return seq_hlist_start_percpu(&file_lock_list, &iter->li_cpu, *pos);
@@ -2721,6 +2741,7 @@ static void locks_stop(struct seq_file *f, void *v)
 {
         spin_unlock(&blocked_lock_lock);
         lg_global_unlock(&file_lock_lglock);
+        percpu_up_write(&file_rwsem);
 }
 
 static const struct seq_operations locks_seq_operations = {