linux-kernel - Re: [PATCH] ipc/sem.c: Update/correct memory barriers.

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Date:	Sun, 01 Mar 2015 17:07:44 +0100
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
CC:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>, 1vier1@....de,
	Kirill Tkhai <ktkhai@...allels.com>,
	Ingo Molnar <mingo@...hat.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>, stable@...r.kernel.org
Subject: Re: [PATCH] ipc/sem.c: Update/correct memory barriers.

Hi Oleg,

On 03/01/2015 02:22 PM, Oleg Nesterov wrote:
> On 02/28, Peter Zijlstra wrote:
>> On Sat, Feb 28, 2015 at 09:36:15PM +0100, Manfred Spraul wrote:
>>> +/*
>>> + * Place this after a control barrier (such as e.g. a spin_unlock_wait())
>>> + * to ensure that reads cannot be moved ahead of the control_barrier.
>>> + * Writes do not need a barrier, they are not speculated and thus cannot
>>> + * pass the control barrier.
>>> + */
>>> +#ifndef smp_mb__after_control_barrier
>>> +#define smp_mb__after_control_barrier()	smp_rmb()
>>> +#endif
>> Sorry to go bike shedding again; but should we call this:
>>
>> smp_acquire__after_control_barrier() ?
>>
>> The thing is; its not a full MB because:
>>
>>   - stores might actually creep into it; while the control dependency
>>     guarantees stores will not creep out, nothing is stopping them from
>>     getting in;
>>
>>   - its not transitive, and our MB is defined to be so.
> I agree, so perhaps it should be named smp_acquire_after_unlock_wait ?
> even if it is actually stronger than "acquire"...
>
> To me "control_barrier" looks a bit confusing. I think this helper should
> be only used after spin_unlock_wait() or spin_is_locked/unlocked().
Then lets make two helpers:
smp_acquire__after_spin_unlock_wait() and 
smp_acquire__after_spin_is_unlocked().

I'll send a new proposal.

Oleg: I would leave the update of task_work_run() to you:
The current code is not buggy, doing an docu update immediately and risk 
that the patch might collide with other changes is probably not worth it.


--
     Manfred
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