percpu_rw_semaphore: reimplement to not block the readers unnecessarily

Currently the writer does msleep() plus synchronize_sched() 3 times to
acquire/release the semaphore, and during this time the readers are
blocked completely.  Even if the "write" section was not actually started
or if it was already finished.

With this patch down_write/up_write does synchronize_sched() twice and
down_read/up_read are still possible during this time, just they use the
slow path.

percpu_down_write() first forces the readers to use rw_semaphore and
increment the "slow" counter to take the lock for reading, then it
takes that rw_semaphore for writing and blocks the readers.

Also.  With this patch the code relies on the documented behaviour of
synchronize_sched(), it doesn't try to pair synchronize_sched() with
barrier.

Signed-off-by: Oleg Nesterov <[email protected]>
Reviewed-by: Paul E. McKenney <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Mikulas Patocka <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Srikar Dronamraju <[email protected]>
Cc: Ananth N Mavinakayanahalli <[email protected]>
Cc: Anton Arapov <[email protected]>
Cc: Jens Axboe <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

0 files changed, 0 insertions, 0 deletions