Re: [PATCH v3 07/13] epoll: call ep_add_event_to_uring() from ep_poll_callback()

Roman Penyaev
2019-05-31T11:22:57Z

On 2019-05-31 11:56, Peter Zijlstra wrote:
> On Thu, May 16, 2019 at 10:58:04AM +0200, Roman Penyaev wrote:
>> Each ep_poll_callback() is called when fd calls wakeup() on epfd.
>> So account new event in user ring.
>>
>> The tricky part here is EPOLLONESHOT. Since we are lockless we
>> have to be deal with ep_poll_callbacks() called in paralle, thus
>> use cmpxchg to clear public event bits and filter out concurrent
>> call from another cpu.
>>
>> Signed-off-by: Roman Penyaev <rpenyaev@suse.de>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Al Viro <viro@zeniv.linux.org.uk>
>> Cc: Linus Torvalds <torvalds@linux-foundation.org>
>> Cc: linux-fsdevel@vger.kernel.org
>> Cc: linux-kernel@vger.kernel.org
>>
>> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
>> index 2f551c005640..55612da9651e 100644
>> --- a/fs/eventpoll.c
>> +++ b/fs/eventpoll.c
>> @@ -1407,6 +1407,29 @@ struct file *get_epoll_tfile_raw_ptr(struct
>> file *file, int tfd,
>> }
>> #endif /* CONFIG_CHECKPOINT_RESTORE */
>>
>> +/**
>> + * Atomically clear public event bits and return %true if the old
>> value has
>> + * public event bits set.
>> + */
>> +static inline bool ep_clear_public_event_bits(struct epitem *epi)
>> +{
>> + __poll_t old, flags;
>> +
>> + /*
>> + * Here we race with ourselves and with ep_modify(), which can
>> + * change the event bits. In order not to override events updated
>> + * by ep_modify() we have to do cmpxchg.
>> + */
>> +
>> + old = epi->event.events;
>> + do {
>> + flags = old;
>> + } while ((old = cmpxchg(&epi->event.events, flags,
>> + flags & EP_PRIVATE_BITS)) != flags);
>> +
>> + return flags & ~EP_PRIVATE_BITS;
>> +}
>
> AFAICT epi->event.events also has normal writes to it, eg. in
> ep_modify(). A number of architectures cannot handle concurrent normal
> writes and cmpxchg() to the same variable.

Yes, we race with the current function and with ep_modify(). Then,
ep_modify()
should do something as the following:

- epi->event.events = event->events
+ xchg(&epi->event.events, event->events);

Is that ok?

Just curious: what are these archs?

Thanks.

--
Roman