Re: [PATCH 1/1] [RFC] workqueue: fix ghost PENDING flag while doing MQ IO

From: Tejun Heo <tj@kernel.org>
To: Roman Penyaev <roman.penyaev@profitbricks.com>
Cc: Jens Axboe <axboe@kernel.dk>,
	linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
	David Howells <dhowells@redhat.com>
Subject: Re: [PATCH 1/1] [RFC] workqueue: fix ghost PENDING flag while doing MQ IO
Date: Mon, 25 Apr 2016 13:51:15 -0400	[thread overview]
Message-ID: <20160425175115.GC7822@mtj.duckdns.org> (raw)
In-Reply-To: <CAJrWOzBtqY2ngy3HA-53XX1o-=dUB0u3RPHX38O=S4C-ozdq4g@mail.gmail.com>

Hello, Roman.

On Mon, Apr 25, 2016 at 07:39:52PM +0200, Roman Penyaev wrote:
> Ok, that's clear now. Thanks. I was confused also by a spin lock, which
> is being released just after clear pending:
> 
>    set_work_pool_and_clear_pending(work, pool->id);
>    spin_unlock_irq(&pool->lock);
>    ...
>    worker->current_func(work);
> 
> But seems memory operations of execution can leak-in and appear before
> pended bit is cleared and spin lock is released.
> (according to Documentation/memory-barriers.txt, (6) RELEASE operations)

Yeap, release doesn't prevent following reads from creeping ahead of
it.

> Tejun, do you need an updated patch for that? With a proper smp_mb()?

Yes, can you please also add comment explaining what the mb is
interlocking?

Thanks a lot!

-- 
tejun