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

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Message-ID: <5307E550.4040004@hurleysoftware.com>
Date:	Fri, 21 Feb 2014 18:46:24 -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 06:18 PM, Tejun Heo wrote:
> On Fri, Feb 21, 2014 at 06:01:29PM -0500, Peter Hurley wrote:
>> 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)
> ;
> Hmmm?  I'm pretty sure that's a full barrier.  Local processor is
> always in order (w.r.t. the compiler).

It's a long story but the short version is that
Documentation/memory-barriers.txt recently was overhauled to reflect
what cpus actually do and what the different archs actually
deliver.

Turns out that unlock + lock is not guaranteed by all archs to be
a full barrier. Thus the smb_mb__after_unlock_lock().

This is now all spelled out in memory-barriers.txt under the
sub-heading "IMPLICIT KERNEL MEMORY BARRIERS".

Regards,
Peter Hurley

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