Re: [PATCH] slow_work_execute() needs mb() before test_bit(SLOW_WORK_PENDING)
From: David Howells
Date: Mon Apr 13 2009 - 17:49:11 EST
Oleg Nesterov <oleg@xxxxxxxxxx> 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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/