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

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Date:   Wed, 5 Dec 2018 00:59:07 +0100
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     Roman Penyaev <rpenyaev@...e.de>
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

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).

  Andrea