[PATCH] srcu-3: RCU variant permitting read-side blocking

Updated patch adding a variant of RCU that permits sleeping in read-side
critical sections.  SRCU is as follows:

o	Each use of SRCU creates its own srcu_struct, and each
	srcu_struct has its own set of grace periods.  This is
	critical, as it prevents one subsystem with a blocking
	reader from holding up SRCU grace periods for other
	subsystems.

o	The SRCU primitives (srcu_read_lock(), srcu_read_unlock(),
	and synchronize_srcu()) all take a pointer to a srcu_struct.

o	The SRCU primitives must be called from process context.

o	srcu_read_lock() returns an int that must be passed to
	the matching srcu_read_unlock().  Realtime RCU avoids the
	need for this by storing the state in the task struct,
	but SRCU needs to allow a given code path to pass through
	multiple SRCU domains -- storing state in the task struct
	would therefore require either arbitrary space in the
	task struct or arbitrary limits on SRCU nesting.  So I
	kicked the state-storage problem up to the caller.

	Of course, it is not permitted to call synchronize_srcu()
	while in an SRCU read-side critical section.

o	There is no call_srcu().  It would not be hard to implement
	one, but it seems like too easy a way to OOM the system.
	(Hey, we have enough trouble with call_rcu(), which does
	-not- permit readers to sleep!!!)  So, if you want it,
	please tell me why...

[josht@us.ibm.com: sparse notation]
Signed-off-by: Paul E. McKenney <paulmck@us.ibm.com>
Signed-off-by: Josh Triplett <josh@freedesktop.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 3 insertions, 0 deletions

diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
index 820fee236967..e0d6d99b8f9b 100644
--- a/Documentation/RCU/whatisRCU.txt
+++ b/Documentation/RCU/whatisRCU.txt
@@ -778,6 +778,8 @@ Markers for RCU read-side critical sections:
         rcu_read_unlock
         rcu_read_lock_bh
         rcu_read_unlock_bh
+        srcu_read_lock
+        srcu_read_unlock
 
 RCU pointer/list traversal:
 
@@ -804,6 +806,7 @@ RCU grace period:
         synchronize_net
         synchronize_sched
         synchronize_rcu
+        synchronize_srcu
         call_rcu
         call_rcu_bh