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

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Date:	Thu, 18 Oct 2012 09:38:33 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.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 Thu, Oct 18, 2012 at 06:24:09PM +0200, Oleg Nesterov wrote:
> 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)

Yes, this is exactly my concern.

> 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 name it something not containing "lock".  And clearly document
the behavior and how it is to be used.  ;-)

Otherwise, someone will get confused and introduce bugs.

> Or I misunderstood you again?

No, this was indeed my concern.

							Thanx, Paul

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