From: Oleg Nesterov <oleg@redhat.com>
To: Mikulas Patocka <mpatocka@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
Ingo Molnar <mingo@elte.hu>,
Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
Ananth N Mavinakayanahalli <ananth@in.ibm.com>,
Anton Arapov <anton@redhat.com>,
linux-kernel@vger.kernel.org,
Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH 1/2] brw_mutex: big read-write mutex
Date: Tue, 23 Oct 2012 17:12:24 +0200 [thread overview]
Message-ID: <20121023151224.GA14916@redhat.com> (raw)
In-Reply-To: <Pine.LNX.4.64.1210191948130.12753@file.rdu.redhat.com>
Hi Mikulas,
On 10/22, Mikulas Patocka wrote:
>
> On Fri, 19 Oct 2012, Oleg Nesterov wrote:
>
> > On 10/19, Mikulas Patocka wrote:
> > >
> > > synchronize_rcu() is way slower than msleep(1) -
> >
> > This depends, I guess. but this doesn't mmatter,
> >
> > > so I don't see a reason
> > > why should it be complicated to avoid msleep(1).
> >
> > I don't think this really needs complications. Please look at this
> > patch for example. Or initial (single writer) version below. It is
> > not finished and lacks the barriers too, but I do not think it is
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
please note the comment above ;)
> > more complex.
>
> Hi
>
> My implementation has a smaller structure (it doesn't have
> wait_queue_head_t).
Oh, I don't think sizeof() really matters in this case.
> Your implementation is prone to starvation - if the writer has a high
> priority and if it is doing back-to-back write unlocks/locks, it may
> happen that the readers have no chance to run.
Yes, it is write-biased, this was intent. writers should be rare.
> The use of mutex instead of a wait queue in my implementation is unusual,
> but I don't see anything wrong with it
Neither me.
Mikulas, apart from _rcu/_sched change, my only point was msleep() can
(and imho should) be avoided.
> > static inline long brw_read_ctr(struct brw_sem *brw)
> > {
> > long sum = 0;
> > int cpu;
> >
> > for_each_possible_cpu(cpu)
> > sum += per_cpu(*brw->read_ctr, cpu);
>
> Integer overflow on signed types is undefined - you should use unsigned
> long - you can use -fwrapv option to gcc to make signed overflow defined,
> but Linux doesn't use it.
I don't think -fwrapv can make any difference in this case, but I agree
that "unsigned long" makes more sense.
> > void brw_up_write(struct brw_sem *brw)
> > {
> > brw->writer = NULL;
> > synchronize_sched();
>
> That synchronize_sched should be put before brw->writer = NULL.
Yes, I know. I mentioned this at the start, this lacks the necessary
barrier between this writer and the next reader.
> I had this bug in my implementation too.
Yes, exactly. And this is why I cc'ed you initially ;)
Oleg.
next prev parent reply other threads:[~2012-10-23 15:12 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
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 [this message]
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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