On Sat, 2007-05-12 at 03:00 +0400, Oleg Nesterov wrote:
> On 05/11, Peter Zijlstra wrote:
> >
> > +static inline int __rw_mutex_read_trylock(struct rw_mutex *rw_mutex)
> > +{
> > + preempt_disable();
> > + if (likely(!__rw_mutex_reader_slow(rw_mutex))) {
>
> --- WINDOW ---
>
> > + percpu_counter_mod(&rw_mutex->readers, 1);
> > + preempt_enable();
> > + return 1;
> > + }
> > + preempt_enable();
> > + return 0;
> > +}
Yeah, I found that one when Andrew asked me about that preempt_disable()
thing.
How about:
int __rw_mutex_read_trylock(struct rw_mutex *rw_mutex)
{
percpu_counter_inc(&rw_mutex->readers);
if (unlikely(rw_mutex_reader_slow(rw_mutex))) {
percpu_counter_dec(&rw_mutex->readers);
/*
* possibly wake up a writer waiting for this reference to
* disappear
*/
wake_up(&rw_mutex->wait_queue);
return 0;
}
return 1;
}
> > [...snip...]
> >
> > +void rw_mutex_write_lock_nested(struct rw_mutex *rw_mutex, int subclass)
> > +{
> > [...snip...]
> > +
> > + /*
> > + * block new readers
> > + */
> > + __rw_mutex_status_set(rw_mutex, RW_MUTEX_READER_SLOW);
> > + /*
> > + * wait for all readers to go away
> > + */
> > + wait_event(rw_mutex->wait_queue,
> > + (percpu_counter_sum(&rw_mutex->readers) == 0));
> > +}
>
> This look a bit suspicious, can't mutex_write_lock() set RW_MUTEX_READER_SLOW
> and find percpu_counter_sum() == 0 in that WINDOW above?
Indeed; however with the above having the reverse sequence this has, it
should be closed no?
> > +void rw_mutex_read_unlock(struct rw_mutex *rw_mutex)
> > +{
> > + rwsem_release(&rw_mutex->dep_map, 1, _RET_IP_);
> > +
> > + percpu_counter_mod(&rw_mutex->readers, -1);
> > + if (unlikely(__rw_mutex_reader_slow(rw_mutex)) &&
> > + percpu_counter_sum(&rw_mutex->readers) == 0)
I took out the percpu_counter_sum()
> > + wake_up_all(&rw_mutex->wait_queue);
> > +}
>
> The same. __rw_mutex_status_set()->wmb() in rw_mutex_write_lock below
> is not enough. percpu_counter_mod() doesn't take fbc->lock if < FBC_BATCH,
> so we don't have a proper serialization.
>
> write_lock() sets RW_MUTEX_READER_SLOW, finds percpu_counter_sum() != 0,
> and sleeps. rw_mutex_read_unlock() decrements cpu-local var, does not
> see RW_MUTEX_READER_SLOW and skips wake_up_all().
write lock read lock read unlock
a) state = slow 1) readers++ I) readers--
b) wait(readers == 0) 2) if (state == slow) II) if (state == slow)
That looks pretty safe to me; however are you suggesting the
percpu_counter_inc() needs some sort of barrier in order to be reliably
picked up by the percpu_counter_sum()?
something like this:
percpu_counter_{inc,dec}
smp_wmb()
vs
smp_rmb()
percpu_counter_sum(()
> > +void rw_mutex_write_lock_nested(struct rw_mutex *rw_mutex, int subclass)
> > +{
> > + might_sleep();
> > + rwsem_acquire(&rw_mutex->dep_map, subclass, 0, _RET_IP_);
> > +
> > + mutex_lock_nested(&rw_mutex->write_mutex, subclass);
> > + mutex_lock_nested(&rw_mutex->read_mutex, subclass);
> > +
> > + /*
> > + * block new readers
> > + */
> > + __rw_mutex_status_set(rw_mutex, RW_MUTEX_READER_SLOW);
> > + /*
> > + * wait for all readers to go away
> > + */
> > + wait_event(rw_mutex->wait_queue,
> > + (percpu_counter_sum(&rw_mutex->readers) == 0));
> > +}
> > +
> > +void rw_mutex_write_unlock(struct rw_mutex *rw_mutex)
> > +{
> > + int waiters;
> > +
> > + rwsem_release(&rw_mutex->dep_map, 1, _RET_IP_);
> > +
> > + /*
> > + * let the readers rip
> > + */
> > + __rw_mutex_status_set(rw_mutex, RW_MUTEX_READER_FAST);
> > + waiters = atomic_read(&rw_mutex->read_waiters);
> > + mutex_unlock(&rw_mutex->read_mutex);
> > + /*
> > + * wait for at least 1 reader to get through
> > + */
> > + if (waiters) {
> > + wait_event(rw_mutex->wait_queue,
> > + (atomic_read(&rw_mutex->read_waiters) < waiters));
> > + }
> > + /*
> > + * before we let the writers rip
> > + */
> > + mutex_unlock(&rw_mutex->write_mutex);
> > +}
>
> Looks like we can have only one task on rw_mutex->wait_queue, and it holds
> ->write_mutex. Can't we use just a "task_struct *write_waiter" instead of
> ->wait_queue ? This makes rw_mutex smaller.
Good point; I'll try and figure out how to sleep and wake a single task
without the waitqueue.
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