linux-kernel - Re: [PATCH v8 4/4] qrwlock: Use smp_store_release() in write

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Message-ID: <20140119080405.GB10038@linux.vnet.ibm.com>
Date:	Sun, 19 Jan 2014 00:04:05 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Matt Turner <mattst88@...il.com>,
	Waiman Long <waiman.long@...com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Ivan Kokshaysky <ink@...assic.park.msu.ru>,
	Daniel J Blueman <daniel@...ascale.com>,
	Richard Henderson <rth@...ddle.net>
Subject: Re: [PATCH v8 4/4] qrwlock: Use smp_store_release() in write_unlock()

On Sat, Jan 18, 2014 at 04:57:05PM -0800, Linus Torvalds wrote:
> On Sat, Jan 18, 2014 at 1:22 PM, Paul E. McKenney
> <paulmck@...ux.vnet.ibm.com> wrote:
> >
> > Yes, this requires that -all- updates to the fields in the machine word
> > in question use atomic rmw.  Which would not be pretty from a core-code
> > perspective.  Hence my suggestion of ceasing Linux-kernel support for
> > DEC Alpha CPUs that don't support byte operations.  Also need 16-bit
> > operations as well, of course...
> 
> I'm not seeing this.
> 
> Why the hell would you have byte- or halfword-sized versions of the
> store_release or load_acquire things on alpha anyway?
> 
> What it means is that data structures that do locking or atomics need
> to be "int" or "long" on alpha.  That has always been true. What do
> you claim has changed?

OK, another approach would be to never add "select ARCH_USE_QUEUE_RWLOCK"
on Alpha, at least if the queued rwlocks really do want to atomically
manipulate bytes.  After all, the Alpha systems that I know about don't
have enough CPUs to make queued rwlocks necessary anyway.

Much simpler solution!

Is this what you were getting at, or am I missing your point?

							Thanx, Paul

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