rwsem: do not block readers at head of queue if other readers are active
authorMichel Lespinasse <walken@google.com>
Tue, 7 May 2013 13:46:00 +0000 (06:46 -0700)
committerLinus Torvalds <torvalds@linux-foundation.org>
Tue, 7 May 2013 14:20:17 +0000 (07:20 -0700)
commit25c39325968bbcebe6cd2a1991228c9dfb48d655
tree627e32434338adcf4f7583219e9b4014b738dcc3
parentfe6e674c6187d4f452a679ced7e95262bd517936
rwsem: do not block readers at head of queue if other readers are active

This change fixes a race condition where a reader might determine it
needs to block, but by the time it acquires the wait_lock the rwsem has
active readers and no queued waiters.

In this situation the reader can run in parallel with the existing
active readers; it does not need to block until the active readers
complete.

Thanks to Peter Hurley for noticing this possible race.

Signed-off-by: Michel Lespinasse <walken@google.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
lib/rwsem.c