linux-kernel - Re: a volatile related bug in kernel/timer.c ?

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Message-ID: <4FB615A1.1090203@ladisch.de>
Date:	Fri, 18 May 2012 11:25:53 +0200
From:	Clemens Ladisch <clemens@...isch.de>
To:	KwongYuan Wong <wong.kwongyuan@...il.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: a volatile related bug in kernel/timer.c ?

KwongYuan Wong wrote:
>   in the function "del_timer" in kernel/timer.c, there is the following code
>
> 954  if (timer_pending(timer)) {
> 955    base = lock_timer_base(timer, &flags);
> 956    if (timer_pending(timer)) {
>
> suppose timer_pending(timer) check in line 954 is A, and in line 956 is B.
>
> because the timer_pending(timer) check is very simple, so the result
> may be saved in a register, and that register is reused
> by both A and B.  While this should be wrong? the check at B should
> reload the value from memory instead of using previous
> result kept in register,  because lock_timer_base may have side-effect
> which change the result of time_pending?
>
> so I guess a barrier() is needed, so that the code should be the following?
>
> if (timer_pending(timer)) {
>   base = lock_timer_base(timer, &flags);
>   barrier();
>   if (timer_pending(timer)) {

The spin_lock_irqsave() in lock_timer_base() already implies a barrier.
(Well, if it's written correctly.)

> in my chip, the generated assembly is like the following:
> ( the function "lock_timer_base" in inlined also)
>
> 1035         __raw_local_irq_save $7

This is not the arch_spin_lock() code I see in my copy of
arch/mips/include/asm/spinlock.h.


Regards,
Clemens
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