rwsem: wake queued readers when writer blocks on active read lock

This change addresses the following situation:

- Thread A acquires the rwsem for read
- Thread B tries to acquire the rwsem for write, notices there is already
  an active owner for the rwsem.
- Thread C tries to acquire the rwsem for read, notices that thread B already
  tried to acquire it.
- Thread C grabs the spinlock and queues itself on the wait queue.
- Thread B grabs the spinlock and queues itself behind C. At this point A is
  the only remaining active owner on the rwsem.

In this situation thread B could notice that it was the last active writer
on the rwsem, and decide to wake C to let it proceed in parallel with A
since they both only want the rwsem for read.

Signed-off-by: Michel Lespinasse <[email protected]>
Acked-by: David Howells <[email protected]>
Cc: Mike Waychison <[email protected]>
Cc: Suleiman Souhlal <[email protected]>
Cc: Ying Han <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

0 files changed, 0 insertions, 0 deletions