linux-kernel - Re: [PATCH] make cancel_rearming_delayed

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Date:	Wed, 16 May 2007 22:52:03 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Tejun Heo <htejun@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	David Chinner <dgc@....com>,
	David Howells <dhowells@...hat.com>,
	Gautham Shenoy <ego@...ibm.com>,
	Jarek Poplawski <jarkao2@...pl>, Ingo Molnar <mingo@...e.hu>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] make cancel_rearming_delayed_work() reliable

Hello Tejun,

On 05/16, Tejun Heo wrote:
>
> >> lock is read arrier, unlock is write barrier.
> 
> Let's say there's a shared data structure protected by a spinlock and
> two threads are accessing it.
> 
> 1. thr1 locks spin
> 2. thr1 updates data structure
> 3. thr1 unlocks spin
> 4. thr2 locks spin
> 5. thr2 accesses data structure
> 6. thr2 unlocks spin
> 
> If spin_unlock is not a write barrier and spin_lock is not a read
> barrier, nothing guarantees memory accesses from step#5 will see the
> changes made in step#2.  Memory fetch can occur during updates in step#2
> or even before that.

Ah, but this is something different. Both lock/unlock are full barriers,
but they protect only one direction. A memory op must not leak out of the
critical section, but it may leak in.

	A = B;		// 1
	lock();		// 2
	C = D;		// 3

this can be re-ordered to

	lock();		// 2
	C = D;		// 3
	A = B;		// 1

but 2 and 3 must not be re-ordered.

To be sure, I contacted Paul E. McKenney privately, and his reply is

	> No.  See for example IA64 in file include/asm-ia64/spinlock.h,
	> line 34 for spin_lock() and line 92 for spin_unlock().  The
	> spin_lock() case uses a ,acq completer, which will allow preceding
	> reads to be reordered into the critical section.  The spin_unlock()
	> uses the ,rel completer, which will allow subsequent writes to be
	> reordered into the critical section.  The locking primitives are
	> guaranteed to keep accesses bound within the critical section, but
	> are free to let outside accesses be reordered into the critical
	> section.
	>
	> Download the Itanium Volume 2 manual:
	>
	>         http://developer.intel.com/design/itanium/manuals/245318.htm
	>
	> Table 2.3 on page 2:489 (physical page 509) shows an example of how
	> the rel and acq completers work.


> > Could you also look at
> > 	http://marc.info/?t=116275561700001&r=1
> > 
> > and, in particular,
> > 	http://marc.info/?l=linux-kernel&m=116281136122456
> 
> This is because spin_lock() isn't a write barrier, right?  I totally
> agree with you there.

Yes, but in fact I think wake_up() needs a full mb() semantics (which we
don't have _in theory_), because try_to_wake_up() first checks task->state
and does nothing if it is TASK_RUNNING.

That is why I think that smp_mb__before_spinlock() may be useful not only
for workqueue.c

Oleg.

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