linux-kernel - Re: [PATCH] slow_work_execute() needs mb() before test_bit(SLOW_WORK

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Date:	Mon, 13 Apr 2009 22:47:46 +0100
From:	David Howells <dhowells@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	dhowells@...hat.com, Andrew Morton <akpm@...ux-foundation.org>,
	David Miller <davem@...emloft.net>,
	Serge Hallyn <serue@...ibm.com>,
	Steve Dickson <steved@...hat.com>,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Daire Byrne <Daire.Byrne@...mestore.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] slow_work_execute() needs mb() before test_bit(SLOW_WORK_PENDING)

Oleg Nesterov <oleg@...hat.com> wrote:

> clear_bit_unlock() implies release semantics, iow we have a one-way barrier
> before clear_bit().

That is correct.  The stuff before the clear_bit_unlock() must not leak out
past it.  SLOW_WORK_EXECUTING is a lock around the execution of the work item.

> But we need the mb() semantics after clear_bit(), before we test
> SLOW_WORK_PENDING. Otherwise we can miss SLOW_WORK_ENQ_DEFERRED if we race
> slow_work_enqueue().

But if you move the mb() to the other side, and reduce to clear_bit() you
break the above non-reentrancy guarantee.

> However, given that both clear_bit() and set_bit() use the same word,
> perhaps this is not possible.

I would assume clear_bit() and set_bit() on different bits of the same word
must interact as if they're on the same bit, with regard to atomiticity, but I
don't know for certain that it is guaranteed.

> But in that case I don't understand why do we need clear_bit_unlock(), not
> just clear_bit(), and how "mb is not needeed" could be derived from
> documentation.

As mentioned above, SLOW_WORK_EXECUTING is a lock against multiple entrance to
the execution function.

Perhaps I should amend the comments to make this clearer.

David
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