Re: [RFC] pipe: prevent compiler reordering in pipe_poll

From: Al Viro
Date: Fri Aug 24 2018 - 19:05:53 EST

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).