Messages in this thread |  | | Date | Wed, 18 Feb 2015 16:59:04 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH 2/2] [PATCH] sched: Add smp_rmb() in task rq locking cycles |
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(Forgot to add Manfred, resending)
Thanks Paul and Peter, this was the interesting reading ;)
This is almost off-topic (but see below), but perhaps memory-barriers.txt could also mention spin_unlock_wait() to explain that _most probably_ it is pointless without the memory barrier(s), and the barrer before-or-after unlock_wait() pairs with release-or-acquire.
At the same time, the code like
spin_unlock_wait(); STORE;
_can_ be correct because this implies the load-store control dependency.
On 02/17, Paul E. McKenney wrote: > > | mb | wmb | rmb | rbd | acq | rel | ctl | > -----+-------+-------+-------+-------+-------+-------+-------+ > mb | Y | | Y | y | Y | | Y + > -----+-------+-------+-------+-------+-------+-------+-------+ > wmb | Y | | Y | y | Y | | Y + > -----+-------+-------+-------+-------+-------+-------+-------+ > rmb | | | | | | | + > -----+-------+-------+-------+-------+-------+-------+-------+ > rbd | | | | | | | + > -----+-------+-------+-------+-------+-------+-------+-------+ > acq | | | | | | | + > -----+-------+-------+-------+-------+-------+-------+-------+ > rel | Y | | Y | y | Y | | Y + > -----+-------+-------+-------+-------+-------+-------+-------+ > ctl | | | | | | | + > -----+-------+-------+-------+-------+-------+-------+-------+
OK, so "acq" can't pair with "acq", and I am not sure I understand.
First of all, it is not clear to me how you can even try to pair them unless you do something like spin_unlock_wait(). I would like to see an example which is not "obviously wrong".
At the same time, if you play with spin_unlock_wait() or spin_is_locked() then acq can pair with acq?
Let's look at sem_lock(). I never looked at this code before, I can be easily wrong. Manfred will correct me. But at first glance we can write the oversimplified pseudo-code:
spinlock_t local, global;
bool my_lock(bool try_local) { if (try_local) { spin_lock(&local); if (!spin_is_locked(&global)) return true; spin_unlock(&local); }
spin_lock(&global); spin_unlock_wait(&local); return false; }
void my_unlock(bool drop_local) { if (drop_local) spin_unlock(&local); else spin_unlock(&global); }
it assumes that the "local" lock is cheaper than "global", the usage is
bool xxx = my_lock(condition); /* CRITICAL SECTION */ my_unlock(xxx);
Now. Unless I missed something, my_lock() does NOT need a barrier BEFORE spin_unlock_wait() (or spin_is_locked()). Either my_lock(true) should see spin_is_locked(global) == T, or my_lock(false)->spin_unlock_wait() should see that "local" is locked and wait.
Doesn't this mean that acq can pair with acq or I am totally confused?
Another question is do we need a barrier AFTER spin_unlock_wait(). I do not know what ipc/sem.c actually needs, but in general (I think) this does need mb(). Otherwise my_lock / my_unlock itself does not have the proper acq/rel semantics. For example, my_lock(false) can miss the changes which were done under my_lock(true).
So I think that (in theory) sem_wait_array() need smp_mb() at the end. But, given that we have the control dependency, perhaps smp_rmb() is enough?
Oleg.
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