Messages in this thread | | | Date | Tue, 16 Oct 2012 11:58:52 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 1/2] brw_mutex: big read-write mutex |
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On Tue, Oct 16, 2012 at 05:56:23PM +0200, Oleg Nesterov wrote: > 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.
Hey, I had to ask. ;-)
> > > +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?
Suppose that the writer arrives and sees that the value of the counter is zero, and thus never sleeps, and so is also not awakened? Unless I am missing something, there are no memory barriers in that case.
Which means that you also need an smp_mb() after the wait_event() in the writer, now that I think on it.
> > > + 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?
Let me try outlining the sequence of events that I am worried about...
1. Task A invokes brw_start_read(). There is no writer, so it takes the fastpath.
2. Task B invokes brw_start_write(), atomically increments &brw->write_ctr, and executes synchronize_sched().
3. Task A invokes brw_end_read() and does this_cpu_dec().
4. Task B invokes wait_event(), which invokes brw_read_ctr() and sees the result as zero. Therefore, Task B does not sleep, does not acquire locks, and does not execute any memory barriers. As a result, ordering is not guaranteed between Task A's read-side critical section and Task B's upcoming write-side critical section.
So I believe that you need smp_mb() in both brw_end_read() and brw_start_write().
Sigh... It is quite possible that you also need an smp_mb() in brw_start_read(), but let's start with just the scenario above.
So, does the above scenario show a problem, or am I confused?
> > > +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.
OK, color me blind! ;-)
> > 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.
Ah, makes sense, thank you!
Thanx, Paul
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