Paul, thanks for looking!
On 10/15, Paul E. McKenney wrote:
>
> > +void brw_start_read(struct brw_mutex *brw)
> > +{
> > + for (;;) {
> > + bool done = false;
> > +
> > + preempt_disable();
> > + if (likely(!atomic_read(&brw->write_ctr))) {
> > + __this_cpu_inc(*brw->read_ctr);
> > + done = true;
> > + }
>
> brw_start_read() is not recursive -- attempting to call it recursively
> can result in deadlock if a writer has shown up in the meantime.
Yes, yes, it is not recursive. Like rw_semaphore.
> Which is often OK, but not sure what you intended.
I forgot to document this in the changelog.
> > +void brw_end_read(struct brw_mutex *brw)
> > +{
>
> I believe that you need smp_mb() here.
I don't understand why...
> The wake_up_all()'s memory barriers
> do not suffice because some other reader might have awakened the writer
> between this_cpu_dec() and wake_up_all().
But __wake_up(q) takes q->lock? And the same lock is taken by
prepare_to_wait(), so how can the writer miss the result of _dec?
> > + this_cpu_dec(*brw->read_ctr);
> > +
> > + if (unlikely(atomic_read(&brw->write_ctr)))
> > + wake_up_all(&brw->write_waitq);
> > +}
>
> Of course, it would be good to avoid smp_mb on the fast path. Here is
> one way to avoid it:
>
> void brw_end_read(struct brw_mutex *brw)
> {
> if (unlikely(atomic_read(&brw->write_ctr))) {
> smp_mb();
> this_cpu_dec(*brw->read_ctr);
> wake_up_all(&brw->write_waitq);
Hmm... still can't understand.
It seems that this mb() is needed to ensure that brw_end_read() can't
miss write_ctr != 0.
But we do not care unless the writer already does wait_event(). And
before it does wait_event() it calls synchronize_sched() after it sets
write_ctr != 0. Doesn't this mean that after that any preempt-disabled
section must see write_ctr != 0 ?
This code actually checks write_ctr after preempt_disable + enable,
but I think this doesn't matter?
Paul, most probably I misunderstood you. Could you spell please?
> > +void brw_start_write(struct brw_mutex *brw)
> > +{
> > + atomic_inc(&brw->write_ctr);
> > + synchronize_sched();
> > + /*
> > + * Thereafter brw_*_read() must see write_ctr != 0,
> > + * and we should see the result of __this_cpu_inc().
> > + */
> > + wait_event(brw->write_waitq, brw_read_ctr(brw) == 0);
>
> This looks like it allows multiple writers to proceed concurrently.
> They both increment, do a synchronize_sched(), do the wait_event(),
> and then are both awakened by the last reader.
Yes. From the changelog:
Unlike rw_semaphore it allows multiple writers too,
just "read" and "write" are mutually exclusive.
> Was that the intent? (The implementation of brw_end_write() makes
> it look like it is in fact the intent.)
Please look at 2/2.
Multiple uprobe_register() or uprobe_unregister() can run at the
same time to install/remove the system-wide breakpoint, and
brw_start_write() is used to block dup_mmap() to avoid the race.
But they do not block each other.
Thanks!
Oleg.
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