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    SubjectRe: [PATCH v4 11/16] locking/rwsem: Enable readers spinning on writer
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    On 04/17/2019 09:58 AM, Peter Zijlstra wrote:
    > On Sat, Apr 13, 2019 at 01:22:54PM -0400, Waiman Long wrote:
    >> +/*
    >> + * Try to acquire read lock before the reader is put on wait queue.
    >> + * Lock acquisition isn't allowed if the rwsem is locked or a writer handoff
    >> + * is ongoing.
    >> + */
    >> +static inline bool rwsem_try_read_lock_unqueued(struct rw_semaphore *sem)
    >> +{
    >> + long count = atomic_long_read(&sem->count);
    >> +
    >> + if (RWSEM_COUNT_WLOCKED_OR_HANDOFF(count))
    >> + return false;
    >> +
    >> + count = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS, &sem->count);
    >> + if (!RWSEM_COUNT_WLOCKED_OR_HANDOFF(count)) {
    >> + rwsem_set_reader_owned(sem);
    >> + lockevent_inc(rwsem_opt_rlock);
    >> + return true;
    >> + }
    >> +
    >> + /* Back out the change */
    >> + atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
    >> + return false;
    >> +}
    > Doesn't a cmpxchg 'loop' make more sense here?

    Not really. A cmpxchg loop will have one more correctible failure mode -
    a new reader acquire the lock or a reader owner does an unlock. Failures
    caused by the setting of the handoff bit or writer acquiring the lock
    are the same for both cases. I don't see any advantage in using cmpxchg
    loop.

    Cheers,
    Longman


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