linux-kernel - Re: [RFC PATCH-tip v2 5/6] locking/rwsem: Change RWSEM_WAITING

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Date:	Wed, 15 Jun 2016 23:57:16 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@....com>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	x86@...nel.org, linux-alpha@...r.kernel.org,
	linux-ia64@...r.kernel.org, linux-s390@...r.kernel.org,
	linux-arch@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>,
	Jason Low <jason.low2@...com>,
	Dave Chinner <david@...morbit.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [RFC PATCH-tip v2 5/6] locking/rwsem: Change RWSEM_WAITING_BIAS
 for better disambiguation

On Wed, Jun 15, 2016 at 03:31:51PM -0400, Waiman Long wrote:
> On 06/15/2016 01:43 PM, Peter Zijlstra wrote:
> >On Tue, Jun 14, 2016 at 06:48:08PM -0400, Waiman Long wrote:
> >>even the reduced maximum of about 16k (32-bit) or 1G (64-bit) should
> >>be more than enough for the foreseeable future.
> >So what happens if I manage to create 16k+ threads on my 32bit kernel
> >and get them all to do mmap() or so at the same time.
> >
> >That doesn't seem too far fetched.
> >
> >Then again, with double that (the current limit) that doesn't seem
> >impossible either.
> 
> To hit the limit, we need to have all the threads calling down_write() at
> exactly the same instance in time which, I think, is pretty hard to do.
> Also, I don't believe you will ever see a 16k-cpu massive SMP system running
> on 32-bit kernel. I can imagine such a system running on 64-bit kernel, but
> certainly not 32-bit.

Ah, so I thought we kept the WRITE_BIAS while blocking, which we don't.

But if they all get preempted before we undo the WRITE_BIAS then 1 CPU
will be able to trigger this. However utterly unlikely.