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SubjectRe: [PATCH 5/5] locking/percpu-rwsem: Remove the embedded rwsem
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On 11/19/19 10:58 AM, Oleg Nesterov wrote:
> On 11/19, Waiman Long wrote:
>> On 11/13/19 5:21 AM, Peter Zijlstra wrote:
>>> +static int percpu_rwsem_wake_function(struct wait_queue_entry *wq_entry,
>>> + unsigned int mode, int wake_flags,
>>> + void *key)
>>> +{
>>> + struct task_struct *p = get_task_struct(wq_entry->private);
>>> + bool reader = wq_entry->flags & WQ_FLAG_CUSTOM;
>>> + struct percpu_rw_semaphore *sem = key;
>>> +
>>> + /* concurrent against percpu_down_write(), can get stolen */
>>> + if (!__percpu_rwsem_trylock(sem, reader))
>>> + return 1;
>>> +
>>> + list_del_init(&wq_entry->entry);
>>> + smp_store_release(&wq_entry->private, NULL);
>>> +
>>> + wake_up_process(p);
>>> + put_task_struct(p);
>>> +
>>> + return !reader; /* wake 'all' readers and 1 writer */
>>> +}
>>> +
>> If I read the function correctly, you are setting the WQ_FLAG_EXCLUSIVE
>> for both readers and writers and __wake_up() is called with an exclusive
>> count of one. So only one reader or writer is woken up each time.
> This depends on what percpu_rwsem_wake_function() returns. If it returns 1,
> __wake_up_common() stops, exactly because all waiters have WQ_FLAG_EXCLUSIVE.
>
>> However, the comment above said we wake 'all' readers and 1 writer. That
>> doesn't match the actual code, IMO.
> Well, "'all' readers" probably means "all readers before writer",
>
>> To match the comments, you should
>> have set WQ_FLAG_EXCLUSIVE flag only on writer. In this case, you
>> probably don't need WQ_FLAG_CUSTOM to differentiate between readers and
>> writers.
> See above...
>
> note also the
>
> if (!__percpu_rwsem_trylock(sem, reader))
> return 1;
>
> at the start of percpu_rwsem_wake_function(). We want to stop wake_up_common()
> as soon as percpu_rwsem_trylock() fails. Because we know that if it fails once
> it can't succeed later. Although iiuc this can only happen if another (new)
> writer races with __wake_up(&sem->waiters).
>
>
> I guess WQ_FLAG_CUSTOM can be avoided, percpu_rwsem_wait() could do
>
> if (read)
> __add_wait_queue_entry_tail(...);
> else {
> wq_entry.flags |= WQ_FLAG_EXCLUSIVE;
> __add_wait_queue(...);
> }
>
> but this is "unfair".

Thanks for the explanation. That clarifies my understanding of the patch.

Cheers,
Longman

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