From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>,
Linus Torvalds <torvalds@linux-foundation.org>,
Peter Zijlstra <peterz@infradead.org>,
Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
Ananth N Mavinakayanahalli <ananth@in.ibm.com>,
Anton Arapov <anton@redhat.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] brw_mutex: big read-write mutex
Date: Tue, 16 Oct 2012 11:58:52 -0700 [thread overview]
Message-ID: <20121016185852.GH2385@linux.vnet.ibm.com> (raw)
In-Reply-To: <20121016155623.GA4028@redhat.com>
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
next prev parent reply other threads:[~2012-10-16 18:59 UTC|newest]
Thread overview: 103+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-10-15 19:09 [RFC PATCH 0/2] uprobes: register/unregister can race with fork Oleg Nesterov
2012-10-15 19:10 ` [PATCH 1/2] brw_mutex: big read-write mutex Oleg Nesterov
2012-10-15 23:28 ` Paul E. McKenney
2012-10-16 15:56 ` Oleg Nesterov
2012-10-16 18:58 ` Paul E. McKenney [this message]
2012-10-17 16:37 ` Oleg Nesterov
2012-10-17 22:28 ` Paul E. McKenney
2012-10-16 19:56 ` Linus Torvalds
2012-10-17 16:59 ` Oleg Nesterov
2012-10-17 22:44 ` Paul E. McKenney
2012-10-18 16:24 ` Oleg Nesterov
2012-10-18 16:38 ` Paul E. McKenney
2012-10-18 17:57 ` Oleg Nesterov
2012-10-18 19:28 ` Mikulas Patocka
2012-10-19 12:38 ` Peter Zijlstra
2012-10-19 15:32 ` Mikulas Patocka
2012-10-19 17:40 ` Peter Zijlstra
2012-10-19 17:57 ` Oleg Nesterov
2012-10-19 22:54 ` Mikulas Patocka
2012-10-24 3:08 ` Dave Chinner
2012-10-25 14:09 ` Mikulas Patocka
2012-10-25 23:40 ` Dave Chinner
2012-10-26 12:06 ` Oleg Nesterov
2012-10-26 13:22 ` Mikulas Patocka
2012-10-26 14:12 ` Oleg Nesterov
2012-10-26 15:23 ` mark_files_ro && sb_end_write Oleg Nesterov
2012-10-26 16:09 ` [PATCH 1/2] brw_mutex: big read-write mutex Mikulas Patocka
2012-10-19 17:49 ` Oleg Nesterov
2012-10-22 23:09 ` Mikulas Patocka
2012-10-23 15:12 ` Oleg Nesterov
2012-10-19 19:28 ` Paul E. McKenney
2012-10-22 23:36 ` [PATCH 0/2] fix and improvements for percpu-rw-semaphores (was: brw_mutex: big read-write mutex) Mikulas Patocka
2012-10-22 23:37 ` [PATCH 1/2] percpu-rw-semaphores: use light/heavy barriers Mikulas Patocka
2012-10-22 23:39 ` [PATCH 2/2] percpu-rw-semaphores: use rcu_read_lock_sched Mikulas Patocka
2012-10-24 16:16 ` Paul E. McKenney
2012-10-24 17:18 ` Oleg Nesterov
2012-10-24 18:20 ` Paul E. McKenney
2012-10-24 18:43 ` Oleg Nesterov
2012-10-24 19:43 ` Paul E. McKenney
2012-10-25 14:54 ` Mikulas Patocka
2012-10-25 15:07 ` Paul E. McKenney
2012-10-25 16:15 ` Mikulas Patocka
2012-10-23 16:59 ` [PATCH 1/2] percpu-rw-semaphores: use light/heavy barriers Oleg Nesterov
2012-10-23 18:05 ` Paul E. McKenney
2012-10-23 18:27 ` Oleg Nesterov
2012-10-23 18:41 ` Oleg Nesterov
2012-10-23 20:29 ` Paul E. McKenney
2012-10-23 20:32 ` Paul E. McKenney
2012-10-23 21:39 ` Mikulas Patocka
2012-10-24 16:23 ` Paul E. McKenney
2012-10-24 20:22 ` Mikulas Patocka
2012-10-24 20:36 ` Paul E. McKenney
2012-10-24 20:44 ` Mikulas Patocka
2012-10-24 23:57 ` Paul E. McKenney
2012-10-25 12:39 ` Paul E. McKenney
2012-10-25 13:48 ` Mikulas Patocka
2012-10-23 19:23 ` Oleg Nesterov
2012-10-23 20:45 ` Peter Zijlstra
2012-10-23 20:57 ` Peter Zijlstra
2012-10-24 15:11 ` Oleg Nesterov
2012-10-23 21:26 ` Mikulas Patocka
2012-10-23 20:32 ` Peter Zijlstra
2012-10-30 18:48 ` [PATCH 0/2] fix and improvements for percpu-rw-semaphores (was: brw_mutex: big read-write mutex) Oleg Nesterov
2012-10-31 19:41 ` [PATCH 0/1] percpu_rw_semaphore: reimplement to not block the readers unnecessarily Oleg Nesterov
2012-10-31 19:41 ` [PATCH 1/1] " Oleg Nesterov
2012-11-01 15:10 ` Linus Torvalds
2012-11-01 15:34 ` Oleg Nesterov
2012-11-02 18:06 ` [PATCH v2 0/1] " Oleg Nesterov
2012-11-02 18:06 ` [PATCH v2 1/1] " Oleg Nesterov
2012-11-07 17:04 ` [PATCH v3 " Mikulas Patocka
2012-11-07 17:47 ` Oleg Nesterov
2012-11-07 19:17 ` Mikulas Patocka
2012-11-08 13:42 ` Oleg Nesterov
2012-11-08 1:23 ` Paul E. McKenney
2012-11-08 1:16 ` [PATCH v2 " Paul E. McKenney
2012-11-08 13:33 ` Oleg Nesterov
2012-11-08 16:27 ` Paul E. McKenney
2012-11-08 13:48 ` [PATCH RESEND v2 0/1] " Oleg Nesterov
2012-11-08 13:48 ` [PATCH RESEND v2 1/1] " Oleg Nesterov
2012-11-08 20:07 ` Andrew Morton
2012-11-08 21:08 ` Paul E. McKenney
2012-11-08 23:41 ` Mikulas Patocka
2012-11-09 0:41 ` Paul E. McKenney
2012-11-09 3:23 ` Paul E. McKenney
2012-11-09 16:35 ` Oleg Nesterov
2012-11-09 16:59 ` Paul E. McKenney
2012-11-09 12:47 ` Mikulas Patocka
2012-11-09 15:46 ` Oleg Nesterov
2012-11-09 17:01 ` Paul E. McKenney
2012-11-09 18:10 ` Oleg Nesterov
2012-11-09 18:19 ` Oleg Nesterov
2012-11-10 0:55 ` Paul E. McKenney
2012-11-11 15:45 ` Oleg Nesterov
2012-11-12 18:38 ` Paul E. McKenney
2012-11-11 18:27 ` [PATCH -mm] percpu_rw_semaphore-reimplement-to-not-block-the-readers-unnecessari ly.fix Oleg Nesterov
2012-11-12 18:31 ` Paul E. McKenney
2012-11-16 23:22 ` Andrew Morton
2012-11-18 19:32 ` Oleg Nesterov
2012-11-01 15:43 ` [PATCH 1/1] percpu_rw_semaphore: reimplement to not block the readers unnecessarily Paul E. McKenney
2012-11-01 18:33 ` Oleg Nesterov
2012-11-02 16:18 ` Oleg Nesterov
2012-10-15 19:10 ` [PATCH 2/2] uprobes: Use brw_mutex to fix register/unregister vs dup_mmap() race Oleg Nesterov
2012-10-18 7:03 ` Srikar Dronamraju
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