Messages in this thread | | | Date | Thu, 1 Feb 2007 00:13:40 +0300 | From | Oleg Nesterov <> | Subject | Re: [PATCH 3/7] barrier: a scalable synchonisation barrier |
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On 01/31, Paul E. McKenney wrote: > > On Sun, Jan 28, 2007 at 04:24:35PM +0100, Ingo Molnar wrote: > > > > * Christoph Hellwig <hch@infradead.org> wrote: > > > > > On Sun, Jan 28, 2007 at 12:51:21PM +0100, Peter Zijlstra wrote: > > > > This barrier thing is constructed so that it will not write in the > > > > sync() condition (the hot path) when there are no active lock > > > > sections; thus avoiding cacheline bouncing. -- I'm just not sure how > > > > this will work out in relation to PI. We might track those in the > > > > barrier scope and boost those by the max prio of the blockers. > > > > > > Is this really needed? We seem to grow new funky locking algorithms > > > exponentially, while people already have a hard time understanding the > > > existing ones. > > > > yes, it's needed. > > Would it be possible to come up with something common between this primitive > and the one that Oleg Nesterov put together for Jens Axboe? > > http://lkml.org/lkml/2006/11/29/330 > > Oleg's approach acquires a lock on the update side, which Peter would > not want in the uncontended case -- but perhaps there is some way to > make Oleg's approach be able to safely test both counters so as to > avoid acquiring the lock if there are no readers. > > Oleg, any chance of this working? I believe it does, but have not > thought it through fully.
I think no. From the quick reading, barrier_sync() and qrcu/srcu are quite different. Consider:
barrier_lock()
barrier_sync();
barrier_unlock(); ... wake up ... barrier_lock();
schedule again
The last "schedule again" would be a BUG for qrcu/srcu, but probably it is ok for barrier_sync(). It looks like barrier_sync() is more a rw semaphore biased to readers.
A couple of minor off-topic notes,
+static inline void barrier_unlock(struct barrier *b) +{ + smp_wmb(); + if (atomic_dec_and_test(&b->count)) + __wake_up(&b->wait, TASK_INTERRUPTIBLE|TASK_UNINTERRUPTIBLE, 0, b);
This is wake_up_all(&b->wait), yes? I don't undestans why key == b, it could be NULL.
+static inline void barrier_sync(struct barrier *b) +{ + might_sleep(); + + if (unlikely(atomic_read(&b->count))) { + DEFINE_WAIT(wait); + prepare_to_wait(&b->wait, &wait, TASK_UNINTERRUPTIBLE); + while (atomic_read(&b->count)) + schedule(); + finish_wait(&b->wait, &wait); + } +}
This should be open-coded wait_event(), but wrong! With the scenario above this can hang forever! because the first wake_up removes the task from the &b->wait.
Oleg.
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