Message-ID: <5308F0E2.3030804@hurleysoftware.com>
Date: Sat, 22 Feb 2014 13:48:02 -0500
From: Peter Hurley <peter@hurleysoftware.com>
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CC: Tejun Heo <tj@kernel.org>, laijs@cn.fujitsu.com,
        linux-kernel@vger.kernel.org,
        Stefan Richter <stefanr@s5r6.in-berlin.de>,
        linux1394-devel@lists.sourceforge.net, Chris Boot <bootc@bootc.net>,
        linux-scsi@vger.kernel.org, target-devel@vger.kernel.org
Subject: Re: [PATCH 4/9] firewire: don't use PREPARE_DELAYED_WORK
References: <1392929071-16555-5-git-send-email-tj@kernel.org>	 <5306AF8E.3080006@hurleysoftware.com>	 <20140221015935.GF6897@htj.dyndns.org>	 <5306B4DF.4000901@hurleysoftware.com>	 <20140221021341.GG6897@htj.dyndns.org>	 <5306E06C.5020805@hurleysoftware.com>	 <20140221100301.GA14653@mtj.dyndns.org>	 <53074BE4.1020307@hurleysoftware.com>	 <20140221130614.GH6897@htj.dyndns.org>	 <5307849A.9050209@hurleysoftware.com>	 <20140221165730.GA10929@htj.dyndns.org>	 <5307DAC9.2020103@hurleysoftware.com> <1393094608.11497.1.camel@dabdike.int.hansenpartnership.com>
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On 02/22/2014 01:43 PM, James Bottomley wrote:
>
> On Fri, 2014-02-21 at 18:01 -0500, Peter Hurley wrote:
>> 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)
>
> No the CPU cannot.  If the CPU were allowed to reorder locking
> sequences, we'd get speculation induced ABBA deadlocks.  The rules are
> quite simple: loads and stores cannot speculate out of critical
> sections.

If you look carefully, you'll notice that the stores have not been
moved from their respective critical sections; simply that the two
critical sections overlap because they use different locks.

Regards,
Peter Hurley

PS - Your reply address is unroutable.

> All architectures have barriers in place to prevent this ...
> I know from personal experience because the barriers on PARISC were
> originally too weak and we did get some speculation out of the critical
> sections, which was very nasty to debug.
>
> Stuff may speculate into critical sections from non-critical but never
> out of them and critical section boundaries may not reorder to cause an
> overlap.

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