linux-kernel - Re: [PATCH v3 07/13] epoll: call ep_add_event_to_uring() from ep_poll

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Date:   Fri, 31 May 2019 13:22:54 +0200
From:   Roman Penyaev <rpenyaev@...e.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     azat@...event.org, akpm@...ux-foundation.org,
        viro@...iv.linux.org.uk, torvalds@...ux-foundation.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 07/13] epoll: call ep_add_event_to_uring() from
 ep_poll_callback()

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@...e.de>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Cc: Al Viro <viro@...iv.linux.org.uk>
>> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
>> Cc: linux-fsdevel@...r.kernel.org
>> Cc: linux-kernel@...r.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