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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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