Messages in this thread | | | Subject | Re: [PATCH v2 12/13] rwsem: do not block readers at head of queue if other readers are active | From | Peter Hurley <> | Date | Thu, 28 Mar 2013 13:25:09 -0400 |
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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 <walken@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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