On Sat, 2007-05-12 at 20:04 +0400, Oleg Nesterov wrote:
> On 05/12, Peter Zijlstra wrote:
> >
> > +static inline void rw_mutex_readers_dec(struct rw_mutex *rw_mutex)
> > +{
> > + percpu_counter_dec(&rw_mutex->readers);
> > + smp_wmb();
> > +}
> >
> > +void rw_mutex_read_unlock(struct rw_mutex *rw_mutex)
> > +{
> > + rw_mutex_readers_dec(rw_mutex);
> > + /*
> > + * on the slow path;
> > + * nudge the writer waiting for the last reader to go away
> > + */
> > + if (unlikely(rw_mutex_reader_slow(rw_mutex)))
> > + rw_mutex_writer_wake(rw_mutex);
> > +}
> >
> > +void rw_mutex_write_lock_nested(struct rw_mutex *rw_mutex, int subclass)
> > +{
> > + mutex_lock_nested(&rw_mutex->write_mutex, subclass);
> > +
> > + /*
> > + * block new readers
> > + */
> > + mutex_lock_nested(&rw_mutex->read_mutex, subclass);
> > + rw_mutex_status_set(rw_mutex, RW_MUTEX_READER_SLOW);
> > + /*
> > + * and wait for all current readers to go away
> > + */
> > + rw_mutex_writer_wait(rw_mutex, (rw_mutex_readers(rw_mutex) == 0));
> > +}
>
> I think this is still not right, but when it comes to barriers we
> need a true expert (Paul cc-ed).
>
> this code roughly does (the only reader does unlock)
>
> READER WRITER
>
> readers = 0; state = 1;
> wmb(); wmb();
> CHECK(state != 0) CHECK(readers == 0)
>
> We need to ensure that we can't miss both CHECKs. Either reader
> should see RW_MUTEX_READER_SLOW, o writer sees "readers == 0"
> and does not sleep.
>
> In that case both barriers should be converted to smp_mb(). There
> was a _long_ discussion about STORE-MB-LOAD behaviour, and experts
> seem to believe everething is ok.
Ah, but note that both those CHECK()s have a rmb(), so that ends up
being:
READER WRITER
readers = 0; state = 1;
wmb(); wmb();
rmb(); rmb();
if (state != 0) if (readers == 0)
and a wmb+rmb is a full mb, right?
> Another question. Isn't it possible to kill rw_mutex->status ?
>
> I have a vague feeling you can change the code so that
>
> rw_mutex_reader_slow() <=> "->waiter != NULL"
>
> , but I am not sure.
If not now, it might be possible to make it so. Thanks for the
suggestion.
Peter
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