From: Oleg Nesterov <oleg@redhat.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.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: Wed, 17 Oct 2012 18:37:02 +0200 [thread overview]
Message-ID: <20121017163702.GA9872@redhat.com> (raw)
In-Reply-To: <20121016185852.GH2385@linux.vnet.ibm.com>
On 10/16, Paul E. McKenney wrote:
>
> On Tue, Oct 16, 2012 at 05:56:23PM +0200, Oleg Nesterov wrote:
> > >
> > > 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,
after synchronize_sched(). So there are no readers (but perhaps there
are brw_end_read's in flight which already decremented read_ctr)
> and thus never sleeps, and so is also not awakened?
and why do we need wakeup in this case?
> > > 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().
OK. And to simplify this discussion, suppose that A invoked
brw_start_read() on CPU_0 and thus incremented read_ctr[0], and
then it migrates to CPU_1 and brw_end_read() uses read_ctr[1].
My understanding was, brw_start_write() must see read_ctr[0] == 1
after synchronize_sched().
> 4. Task B invokes wait_event(), which invokes brw_read_ctr()
> and sees the result as zero.
So my understanding is completely wrong? I thought that after
synchronize_sched() we should see the result of any operation
which were done inside the preempt-disable section.
No?
Hmm. Suppose that we have long A = B = STOP = 0, and
void func(void)
{
preempt_disable();
if (!STOP) {
A = 1;
B = 1;
}
preempt_enable();
}
Now, you are saying that this code
STOP = 1;
synchronize_sched();
BUG_ON(A != B);
is not correct? (yes, yes, this example is not very good).
The comment above synchronize_sched() says:
return ... after all currently executing
rcu-sched read-side critical sections have completed.
But if this code is wrong, then what "completed" actually means?
I thought that it also means "all memory operations have completed",
but this is not true?
Oleg.
next prev parent reply other threads:[~2012-10-17 16:36 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
2012-10-17 16:37 ` Oleg Nesterov [this message]
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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