linux-kernel - Re: [PATCH 4/9] firewire: don't use PREPARE_DELAYED

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Date:	Fri, 21 Feb 2014 18:01:29 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	Tejun Heo <tj@...nel.org>
CC:	laijs@...fujitsu.com, linux-kernel@...r.kernel.org,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	linux1394-devel@...ts.sourceforge.net,
	Chris Boot <bootc@...tc.net>, linux-scsi@...r.kernel.org,
	target-devel@...r.kernel.org
Subject: Re: [PATCH 4/9] firewire: don't use PREPARE_DELAYED_WORK

On 02/21/2014 11:57 AM, Tejun Heo wrote:
> Yo,
>
> On Fri, Feb 21, 2014 at 11:53:46AM -0500, Peter Hurley wrote:
>> Ok, I can do that. But AFAIK it'll have to be an smp_rmb(); there is
>> no mb__after unlock.
>
> We do have smp_mb__after_unlock_lock().
>
>> [ After thinking about it some, I don't think preventing speculative
>>    writes before clearing PENDING if useful or necessary, so that's
>>    why I'm suggesting only the rmb. ]
>
> But smp_mb__after_unlock_lock() would be cheaper on most popular
> archs, I think.

smp_mb__after_unlock_lock() is only for ordering memory operations
between two spin-locked sections on either the same lock or by
the same task/cpu. Like:

    i = 1
    spin_unlock(lock1)
    spin_lock(lock2)
    smp_mb__after_unlock_lock()
    j = 1

This guarantees that the store to j happens after the store to i.
Without it, a cpu can

    spin_lock(lock2)
    j = 1
    i = 1
    spin_unlock(lock1)

Regards,
Peter Hurley
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