From: Al Viro <viro@ZenIV.linux.org.uk>
To: Eric Wong <normalperson@yhbt.net>
Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [RFC] pipe: prevent compiler reordering in pipe_poll
Date: Sat, 25 Aug 2018 00:05:49 +0100 [thread overview]
Message-ID: <20180824230549.GP6515@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20180824225431.tpaxuck7idgnj3b7@dcvr>
On Fri, Aug 24, 2018 at 10:54:31PM +0000, Eric Wong wrote:
> The pipe_poll function does not use locks, and adding an entry
> to the waitqueue is not guaranteed to happen before pipe->nrbufs
> (or other fields) are read, leading to missed wakeups.
>
> Looking at Ruby CI build logs and backtraces, I've noticed
> occasional instances where processes are stuck in select(2) or
> ppoll(2) with a pipe.
>
> I don't have access to the systems where this is happening to
> test/reproduce the problem, and haven't been able to reproduce
> it locally on less-powerful hardware, either. However, it seems
> like a problem based on similar comments in
> fs/eventfd.c::eventfd_poll made by Paolo.
You are misinterpreting those comments. That load *can't* migrate
to anything earlier than taking queue lock, since that acts as
an acquire barrier. READ_ONCE in eventfd_poll() is not to prevent
compiler reordering - it's to prevent insane compiler doing multiple
loads on possibly changing variable (ctx->count there).
next prev parent reply other threads:[~2018-08-25 2:42 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-24 22:54 [RFC] pipe: prevent compiler reordering in pipe_poll Eric Wong
2018-08-24 23:05 ` Al Viro [this message]
2018-09-10 8:55 ` Paolo Bonzini
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