linux-kernel - Re: [RFC PATCH 1/1] epoll: use rwlock in order to reduce ep_poll

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Message-ID: <51e8202b163d7522efb18aa2ca79d952@suse.de>
Date:   Wed, 05 Dec 2018 12:25:39 +0100
From:   Roman Penyaev <rpenyaev@...e.de>
To:     Andrea Parri <andrea.parri@...rulasolutions.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/1] epoll: use rwlock in order to reduce
 ep_poll_callback() contention

On 2018-12-05 00:59, Andrea Parri wrote:
> Hi Roman,
> 
> On Tue, Dec 04, 2018 at 12:50:58PM +0100, Roman Penyaev wrote:
>> On 2018-12-03 18:34, Linus Torvalds wrote:
> 
>> > This also ends up making the memory ordering of "xchg()" very very
>> > important. Yes, we've documented it as being an ordering op, but I'm
>> > not sure we've relied on it this directly before.
>> 
>> Seems exit_mm() does exactly the same, the following chunk:
>> 
>> 		up_read(&mm->mmap_sem);
>> 
>> 		self.task = current;
>> 		self.next = xchg(&core_state->dumper.next, &self);
>> 
>> 
>> At least code pattern looks similar.
> 
> Maybe add a comment on top of (your) xchg() to note/justify these 
> memory
> ordering requirements?  As Paul said: "if there are races, this would
> help force them to happen" (and simplify the review, this/future).

Hi Andrea,

Sure, this path is tricky, so will I cook something up.

--
Roman