On 10/06, Peter Zijlstra wrote:
>
> On Mon, Oct 06, 2014 at 02:25:09AM +0200, Oleg Nesterov wrote:
> >
> > Hmm. I specially checked Documentation/memory-barriers.txt,
> >
> > (*) smp_mb__before_atomic();
> > (*) smp_mb__after_atomic();
> >
> > These are for use with atomic (such as add, subtract, increment and
> > decrement) functions that don't return a value, especially when used
> > for
> > reference counting. These functions do not imply memory barriers.
> >
> > These are also used for atomic bitop functions that do not return a
> > value (such as set_bit and clear_bit).
> > ^^^^^^^^^^^^^^^^^^^^^
> >
> > Either you or memory-barriers.txt should be fixed ;)
Heh.
> Its in there, just not explicitly. All those functions listed are
> read-modify-write ops, test_bit() is not, its just a read.
OOPS! I was hypnotized by "_bit" suffix, I guess.
Of course, of course, test_bit() must be a plain LOAD in any case, can't
understand what I was thinking about.
So in this particular case kthread_kill() needs smp_mb__AFTER_atomic(),
and "after" applies to set_bit(KTHREAD_SHOULD_STOP).
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -1594,12 +1594,9 @@ CPU from reordering them.
> (*) smp_mb__before_atomic();
> (*) smp_mb__after_atomic();
>
> - These are for use with atomic (such as add, subtract, increment and
> - decrement) functions that don't return a value, especially when used for
> - reference counting. These functions do not imply memory barriers.
> -
> - These are also used for atomic bitop functions that do not return a
> - value (such as set_bit and clear_bit).
> + These are for use with atomic/bitop (r-m-w) functions that don't return
> + a value (eg. atomic_{add,sub,inc,dec}(), {set,clear}_bit()). These
> + functions do not imply memory barriers.
Thanks!
Oleg.
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