Date: Fri, 4 Jul 2014 09:49:01 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Jason Low <jason.low2@hp.com>
Cc: Waiman Long <waiman.long@hp.com>, Davidlohr Bueso <davidlohr@hp.com>,
        torvalds@linux-foundation.org, paulmck@linux.vnet.ibm.com,
        mingo@kernel.org, linux-kernel@vger.kernel.org, riel@redhat.com,
        akpm@linux-foundation.org, hpa@zytor.com, andi@firstfloor.org,
        James.Bottomley@hansenpartnership.com, rostedt@goodmis.org,
        tim.c.chen@linux.intel.com, aswin@hp.com, scott.norton@hp.com,
        chegu_vinod@hp.com
Subject: Re: [RFC] Cancellable MCS spinlock rework
Message-ID: <20140704074901.GX19379@twins.programming.kicks-ass.net>
References: <1404318070-2856-1-git-send-email-jason.low2@hp.com>
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 <1404320356.3170.12.camel@j-VirtualBox>
 <20140702172333.GQ19379@twins.programming.kicks-ass.net>
 <1404322203.3170.17.camel@j-VirtualBox>
 <20140703073107.GS19379@twins.programming.kicks-ass.net>
 <1404407389.2498.3.camel@buesod1.americas.hpqcorp.net>
 <1404412485.8764.33.camel@j-VirtualBox>
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On Thu, Jul 03, 2014 at 01:51:48PM -0700, Jason Low wrote:
> On Thu, 2014-07-03 at 16:35 -0400, Waiman Long wrote:
> > On 07/03/2014 02:34 PM, Jason Low wrote:
> > > On Thu, 2014-07-03 at 10:09 -0700, Davidlohr Bueso wrote:
> > >> On Thu, 2014-07-03 at 09:31 +0200, Peter Zijlstra wrote:
> > >>> On Wed, Jul 02, 2014 at 10:30:03AM -0700, Jason Low wrote:
> > >>>> Would potentially reducing the size of the rw semaphore structure =
by 32
> > >>>> bits (for all architectures using optimistic spinning) be a nice
> > >>>> benefit?
> > >>> Possibly, although I had a look at the mutex structure and we didn't
> > >>> have a hole to place it in, unlike what you found with the rwsem.
> > >> Yeah, and currently struct rw_semaphore is the largest lock we have =
in
> > >> the kernel. Shaving off space is definitely welcome.
> > > Right, especially if it could help things like xfs inode.
> > >
> >=20
> > I do see a point in reducing the size of the rwsem structure. However, =
I=20
> > don't quite understand the point of converting pointers in the=20
> > optimistic_spin_queue structure to atomic_t.
>=20
> Converting the pointers in the optimistic_spin_queue to atomic_t would
> mean we're fully operating on atomic operations instead of using the
> potentially racy cmpxchg + ACCESS_ONCE stores on the pointers.
>=20
> If we're in the process of using the CPU numbers in atomic_t, I thought
> we might as well fix that as well since it has actually been shown to
> result in lockups on some architectures. We can then avoid needing to
> implement the tricky architecture workarounds for optimistic spinning.
> Wouldn't that be a "nice-have"?

Nah, I think those archs are fundamentally broken at the moment, we
should not make code harder to read and or more complex just for them.

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