From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Mikulas Patocka <mpatocka@redhat.com>,
Peter Zijlstra <peterz@infradead.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
Subject: Re: [PATCH RESEND v2 1/1] percpu_rw_semaphore: reimplement to not block the readers unnecessarily
Date: Fri, 9 Nov 2012 09:01:07 -0800 [thread overview]
Message-ID: <20121109170107.GB2419@linux.vnet.ibm.com> (raw)
In-Reply-To: <20121109154656.GA26134@redhat.com>
On Fri, Nov 09, 2012 at 04:46:56PM +0100, Oleg Nesterov wrote:
> On 11/08, Andrew Morton wrote:
> >
> > On Thu, 8 Nov 2012 14:48:49 +0100
> > Oleg Nesterov <oleg@redhat.com> wrote:
> >
> > >
> > > include/linux/percpu-rwsem.h | 83 +++++------------------------
> > > lib/Makefile | 2 +-
> > > lib/percpu-rwsem.c | 123 ++++++++++++++++++++++++++++++++++++++++++
> >
> > The patch also uninlines everything.
> >
> > And it didn't export the resulting symbols to modules, so it isn't an
> > equivalent. We can export thing later if needed I guess.
>
> Yes, currently it is only used by block_dev.c
>
> > It adds percpu-rwsem.o to lib-y, so the CONFIG_BLOCK=n kernel will
> > avoid including the code altogether, methinks?
>
> I am going to add another user (uprobes), this was my motivation for
> this patch. And perhaps it will have more users.
>
> But I agree, CONFIG_PERCPU_RWSEM makes sense at least now, I'll send
> the patch.
>
> > > +#include <linux/percpu-rwsem.h>
> > > +#include <linux/rcupdate.h>
> > > +#include <linux/sched.h>
> >
> > This list is nowhere near sufficient to support this file's
> > requirements. atomic.h, percpu.h, rwsem.h, wait.h, errno.h and plenty
> > more. IOW, if it compiles, it was sheer luck.
>
> OK, thanks, I'll send
> send percpu_rw_semaphore-reimplement-to-not-block-the-readers-unnecessarily.fix
>
> > > +/*
> > > + * A writer takes ->writer_mutex to exclude other writers and to force the
> > > + * readers to switch to the slow mode, note the mutex_is_locked() check in
> > > + * update_fast_ctr().
> > > + *
> > > + * After that the readers can only inc/dec the slow ->slow_read_ctr counter,
> > > + * ->fast_read_ctr is stable. Once the writer moves its sum into the slow
> > > + * counter it represents the number of active readers.
> > > + *
> > > + * Finally the writer takes ->rw_sem for writing and blocks the new readers,
> > > + * then waits until the slow counter becomes zero.
> > > + */
> >
> > Some overview of how fast/slow_read_ctr are supposed to work would be
> > useful. This comment seems to assume that the reader already knew
> > that.
>
> I hate to say this, but I'll try to update this comment too ;)
>
> > > +void percpu_down_write(struct percpu_rw_semaphore *brw)
> > > +{
> > > + /* also blocks update_fast_ctr() which checks mutex_is_locked() */
> > > + mutex_lock(&brw->writer_mutex);
> > > +
> > > + /*
> > > + * 1. Ensures mutex_is_locked() is visible to any down_read/up_read
> > > + * so that update_fast_ctr() can't succeed.
> > > + *
> > > + * 2. Ensures we see the result of every previous this_cpu_add() in
> > > + * update_fast_ctr().
> > > + *
> > > + * 3. Ensures that if any reader has exited its critical section via
> > > + * fast-path, it executes a full memory barrier before we return.
> > > + */
> > > + synchronize_sched();
> >
> > Here's where I get horridly confused. Your patch completely deRCUifies
> > this code, yes? Yet here we're using an RCU primitive. And we seem to
> > be using it not as an RCU primitive but as a handy thing which happens
> > to have desirable side-effects. But the implementation of
> > synchronize_sched() differs considerably according to which rcu
> > flavor-of-the-minute you're using.
>
> It is documented that synchronize_sched() should play well with
> preempt_disable/enable. From the comment:
>
> Note that preempt_disable(),
> local_irq_disable(), and so on may be used in place of
> rcu_read_lock_sched().
>
> But I guess this needs more discussion, I see other emails in this
> thread...
>
> > And part 3 talks about the reader's critical section. The only
> > critical sections I can see on the reader side are already covered by
> > mutex_lock() and preempt_diable().
>
> Yes, but we need to ensure that if we take the lock for writing, we
> should see all memory modifications done under down_read/up_read().
>
> IOW. Suppose that the reader does
>
> percpu_down_read();
> STORE;
> percpu_up_read(); // no barriers in the fast path
>
> The writer should see the result of that STORE under percpu_down_write().
>
> Part 3 tries to say that at this point we should already see the result,
> so we should not worry about acquire/release semantics.
>
> > If this code isn't as brain damaged as it
> > initially appears then please,
>
> I hope ;)
>
> > go easy on us simpletons in the next
> > version?
>
> Well, I'll try to update the comments... but the code is simple, I do
> not think I can simplify it more. The nontrivial part is the barriers,
> but this is always nontrivial.
>
> Contrary, I am going to try to add some complications later, so that
> it can have more users. In particular, I think it can replace
> get_online_cpus/cpu_hotplug_begin, just we need
> percpu_down_write_but_dont_deadlock_with_recursive_readers().
I must confess that I am a bit concerned about possible scalability
bottlenecks in the current get_online_cpus(), so +1 from me on this one.
Thanx, Paul
next prev parent reply other threads:[~2012-11-09 17:44 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
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 [this message]
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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