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?
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