linux-kernel - Re: [PATCH 1/2] brw_mutex: big read-write mutex

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Message-ID: <20121018162409.GA28504@redhat.com>
Date:	Thu, 18 Oct 2012 18:24:09 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anton Arapov <anton@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] brw_mutex: big read-write mutex

On 10/17, Paul E. McKenney wrote:
>
> On Wed, Oct 17, 2012 at 06:37:02PM +0200, Oleg Nesterov wrote:
> > On 10/16, Paul E. McKenney wrote:
> > >
> > > 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)
>
> But the preempt_disable() region only covers read acquisition.  So
> synchronize_sched() waits only for all the brw_start_read() calls to
> reach the preempt_enable()

Yes.

> -- it cannot wait for all the resulting
> readers to reach the corresponding brw_end_read().

Indeed.

> > > and thus never sleeps, and so is also not awakened?
> >
> > and why do we need wakeup in this case?
>
> To get the memory barriers required to keep the critical sections
> ordered -- to ensure that everyone sees the reader's critical section
> as ending before the writer's critical section starts.

And now I am starting to think I misunderstood your concern from
the very beginning.

I thought that you meant that without mb() brw_start_write() can
race with brw_end_read() and hang forever.

But probably you meant that we need the barriers to ensure that,
say, if the reader does

	brw_start_read();
	CONDITION = 1;
	brw_end_read();

then the writer must see CONDITION != 0 after brw_start_write() ?
(or vice-versa)


In this case we need the barrier, yes. Obviously brw_start_write()
can return right after this_cpu_dec() and before wake_up_all().

2/2 doesn't need this guarantee but I agree, this doesn't look
sane in gerenal...

Or I misunderstood you again?

Oleg.

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