sx locks and memory barriers

[Attilio Rao]

2009/9/29 John Baldwin <jhb at freebsd.org>:
> On Tuesday 29 September 2009 11:39:37 am Attilio Rao wrote:
>> 2009/9/25 Fabio Checconi <fabio at freebsd.org>:
>> > Hi all,
>> >  looking at sys/sx.h I have some troubles understanding this comment:
>> >
>> >  * A note about memory barriers.  Exclusive locks need to use the same
>> >  * memory barriers as mutexes: _acq when acquiring an exclusive lock
>> >  * and _rel when releasing an exclusive lock.  On the other side,
>> >  * shared lock needs to use an _acq barrier when acquiring the lock
>> >  * but, since they don't update any locked data, no memory barrier is
>> >  * needed when releasing a shared lock.
>> >
>> > In particular, I'm not understanding what prevents the following sequence
>> > from happening:
>> >
>> > CPU A                                   CPU B
>> >
>> > sx_slock(&data->lock);
>> >
>> > sx_sunlock(&data->lock);
>> >
>> > /* reordered after the unlock
>> >   by the cpu */
>> > if (data->buffer)
>> >                                        sx_xlock(&data->lock);
>> >                                        free(data->buffer);
>> >                                        data->buffer = NULL;
>> >                                        sx_xunlock(&data->lock);
>> >
>> >        a = *data->buffer;
>> >
>> > IOW, even if readers do not modify the data protected by the lock,
>> > without a release barrier a memory access may leak past the unlock (as
>> > the cpu won't notice any dependency between the unlock and the fetch,
>> > feeling free to reorder them), thus potentially racing with an exclusive
>> > writer accessing the data.
>> >
>> > On architectures where atomic ops serialize memory accesses this would
>> > never happen, otherwise the sequence above seems possible; am I missing
>> > something?
>>
>> I think your concerns are right, possibly we need this patch:
>> http://www.freebsd.org/~attilio/sxrw_unlockb.diff
>
> Actually, since you are only worried about reads, I think this should be
> an "acq" barrier rather than a "rel".  In some cases "acq" is cheaper, so we
> should prefer the cheapest barrier that provides what we need.  You would
> still need to keep some language about the memory barriers since using "acq"
> for shared unlocking is different from exclusive unlocking.

Actually, I don't think that an acq barrier ensures enough protection
against the reordering of 'earlier' operation thus not fixing the
architecture ordering problem reported by Fabio. Also, I don't think
we just have to care about reads (or  I don't understand what you mean
here).
However, I'm not even sure that we have faster read barriers than the
write one. As long as it should be true in theory I don't think that's
what happen in practice.

> The memory clobber is quite heavyweight.  It actually forces gcc to forget any
> cached memory items in registers and reload everything again.  What I really
> want is just a barrier to tell GCC to not reorder things.  If I read a value
> in the program before acquiring a lock it is in theory fine to keep that
> cached across the barrier.  However, there isn't a way to do this sort of
> thing with GCC currently.

Yes, that's the only tool we have right now with GCC. I will try to
look for another way, but it sounds difficult to discover.

Attilio


-- 
Peace can only be achieved by understanding - A. Einstein