On Fri, 2013-03-15 at 03:54 -0700, Michel Lespinasse wrote:
> 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 just in parallel with the existing active
^^^
start ?
> 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 <wal...@google.com>
>
> ---
> lib/rwsem.c | 10 ++++++++--
> 1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/lib/rwsem.c b/lib/rwsem.c
> index 09bf03e7808c..4e4c8893dc00 100644
> --- a/lib/rwsem.c
> +++ b/lib/rwsem.c
> @@ -162,8 +162,14 @@ struct rw_semaphore __sched
> *rwsem_down_read_failed(struct rw_semaphore *sem)
> /* we're now waiting on the lock, but no longer actively locking */
> count = rwsem_atomic_update(adjustment, sem);
^^^^^^^^^
>
> - /* If there are no active locks, wake the front queued process(es). */
> - if (!(count & RWSEM_ACTIVE_MASK))
> + /* If there are no active locks, wake the front queued process(es).
> + *
> + * If there are no writers and we are first in the queue,
> + * wake our own waiter to join the existing active readers !
> + */
> + if (count == RWSEM_WAITING_BIAS ||
> + (count > RWSEM_WAITING_BIAS &&
> + adjustment != -RWSEM_ACTIVE_READ_BIAS))
> sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);
Thanks for fixing this.
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