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SubjectRe: [PATCH v2] context_tracking: remove local_irq_save from __acct_update_integrals
On Fri, Apr 24, 2015 at 11:16:53AM -0400, Rik van Riel wrote:
> V2: introduce signed_cputime_t to deal with 64 bit cputime_t on
> 32 bit architectures, and use READ_ONCE to ensure the value
> is always read atomically (Heiko Karstens)

Erm, that's not what I said ;)
READ_ONCE() only fixes the isssue that with your previous code the
compiler was free to generate code that accesses the memory value
several times.

But..

> - local_irq_save(flags);
> time = stime + utime;
> - dtime = time - tsk->acct_timexpd;
> + dtime = time - READ_ONCE(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_cputime_t)dtime <= 0))
> + return;
> +

...the READ_ONCE() doesn't give you any guarantees about reading
tsk->acct_timexpd in an atomic way.
Well, actually you don't need atomic semantics, but only to make sure that
the read access happens with a single instruction, since you want to protect
against interrupts.
But still: if the size of acct_timexpd is 64 bit READ_ONCE() may still result
in two instructions on 32 bit architectures.
(or isn't there currently no 32 bit architecture with 64 bit cputime_t left?)



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