lkml.org 
[lkml]   [2015]   [Apr]   [24]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] context_tracking: remove local_irq_save from __acct_update_integrals
On 04/24/2015 05:11 AM, Heiko Carstens wrote:
> On Thu, Apr 23, 2015 at 09:57:13PM -0400, Rik van Riel wrote:
>> diff --git a/kernel/tsacct.c b/kernel/tsacct.c
>> index 975cb49e32bf..0b967f116a6b 100644
>> --- a/kernel/tsacct.c
>> +++ b/kernel/tsacct.c
>> @@ -126,23 +126,29 @@ static void __acct_update_integrals(struct task_struct *tsk,
>> if (likely(tsk->mm)) {
>> cputime_t time, dtime;
>> struct timeval value;
>> - unsigned long flags;
>> u64 delta;
>>
>> - local_irq_save(flags);
>> time = stime + utime;
>> dtime = time - tsk->acct_timexpd;
>> + /*
>> + * This code is called both from irq context and from
>> + * task context. There is a race where irq context advances
>> + * tsk->acct_timexpd to a value larger than time, creating
>> + * a negative value. In that case, the irq has already
>> + * updated the statistics.
>> + */
>> + if (unlikely((signed long)dtime <= 0))
>> + return;
>
> FWIW, I think you either need a barrier() before the if-statement or use
> READ_ONCE() when reading tsk->acct_timexpd above.
>
> Otherwise the compiler could (in theory at least) generate code which
> would translate to
> if (unlikely(time <= tsk->acct_timexpd))
> in order to achieve the same result, no?
>
> Besides that cputime_t might be 64 bit in size, therefore you don't have
> much of a guarentee that reading tsk->acct_timexpd happens atomically on
> 32 bit architectures, so you _may_ end up with garbage, no?

You are right on both counts. Thank you for pointing out what
should have been obvious...

Let me post a new patch :)

--
All rights reversed.


\
 
 \ /
  Last update: 2015-04-24 17:41    [W:1.096 / U:0.136 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site