Re: [PATCH v2] locking/rwsem: Avoid double checking before try acquiring write lock
From: Tim Chen
Date: Tue Sep 16 2014 - 16:51:51 EST
On Tue, 2014-09-16 at 16:08 -0400, Peter Hurley wrote:
> Hi Jason,
>
> On 09/16/2014 03:01 PM, Jason Low wrote:
> > Commit 9b0fc9c09f1b checks for if there are known active lockers in
> > order to avoid write trylocking using expensive cmpxchg() when it
> > likely wouldn't get the lock.
> >
> > However, a subsequent patch was added such that we directly check for
> > sem->count == RWSEM_WAITING_BIAS right before trying that cmpxchg().
> > Thus, commit 9b0fc9c09f1b now just adds extra overhead. This patch
> > deletes it.
>
> It would be better to just not reload sem->count, and check the parameter
> count == RWSEM_WAITING_BIAS instead. The count parameter is a very recent
> load of sem->count (one of which is the latest exclusive read from an
> atomic operation), so likely to be just as accurate as a reload of
> sem->count without causing more cache line contention.
>
Agree with Peter.
I think the extra check in the original code was to try to
avoid reloading sem->count.
So checking directly here (count == RWSEM_WAITING_BIAS) will
accomplish that end. You'll need to modify your comment slightly
to say
Try acquiring the write lock. Check count first ...
Thanks.
Tim
> Regards,
> Peter Hurley
>
> > Also, add a comment on why we do an "extra check" of sem->count before
> > the cmpxchg().
> >
> > Signed-off-by: Jason Low <jason.low2@xxxxxx>
> > ---
> > kernel/locking/rwsem-xadd.c | 24 +++++++++++++-----------
> > 1 files changed, 13 insertions(+), 11 deletions(-)
> >
> > diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
> > index d6203fa..63d3ef2 100644
> > --- a/kernel/locking/rwsem-xadd.c
> > +++ b/kernel/locking/rwsem-xadd.c
> > @@ -247,18 +247,20 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
> > return sem;
> > }
> >
> > -static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
> > +static inline bool rwsem_try_write_lock(struct rw_semaphore *sem)
> > {
> > - if (!(count & RWSEM_ACTIVE_MASK)) {
> > - /* try acquiring the write lock */
> > - if (sem->count == RWSEM_WAITING_BIAS &&
> > - cmpxchg(&sem->count, RWSEM_WAITING_BIAS,
> > - RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_WAITING_BIAS) {
> > - if (!list_is_singular(&sem->wait_list))
> > - rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
> > - return true;
> > - }
> > + /*
> > + * Try acquiring the write lock. Check sem->count first
> > + * in order to reduce unnecessary expensive cmpxchg() operations.
> > + */
> > + if (sem->count == RWSEM_WAITING_BIAS &&
> > + cmpxchg(&sem->count, RWSEM_WAITING_BIAS,
> > + RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_WAITING_BIAS) {
> > + if (!list_is_singular(&sem->wait_list))
> > + rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
> > + return true;
> > }
> > +
> > return false;
> > }
> >
> > @@ -446,7 +448,7 @@ struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
> > /* wait until we successfully acquire the lock */
> > set_current_state(TASK_UNINTERRUPTIBLE);
> > while (true) {
> > - if (rwsem_try_write_lock(count, sem))
> > + if (rwsem_try_write_lock(sem))
> > break;
> > raw_spin_unlock_irq(&sem->wait_lock);
> >
> >
>
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