linux-kernel - Re: [PATCH v3 06/13] epoll: introduce helpers for adding/removing events to uring

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Message-ID: <20190603090906.GE3436@hirez.programming.kicks-ass.net>
Date:   Mon, 3 Jun 2019 11:09:06 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Roman Penyaev <rpenyaev@...e.de>
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 06/13] epoll: introduce helpers for adding/removing
 events to uring

On Fri, May 31, 2019 at 08:58:19PM +0200, Roman Penyaev wrote:
> On 2019-05-31 18:51, Peter Zijlstra wrote:

> > But like you show, it can be done. It also makes the thing wait-free, as
> > opposed to merely lockless.
> 
> You think it's better?  I did not like this variant from the very
> beginning because of the unnecessary complexity.  But maybe you're
> right.  No busy loops on user side makes it wait-free.  And also
> I can avoid c11 in kernel using cmpxchg along with atomic_t.

Imagine the (v)CPU going for an extended nap right between publishing the
new tail and writing the !0 entry. Then your userspace is stuck burning
cycles without getting anything useful done.