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

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Message-ID: <20140114234443.GY10038@linux.vnet.ibm.com>
Date:	Tue, 14 Jan 2014 15:44:43 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Richard Henderson <rth@...ddle.net>
Cc:	Matt Turner <mattst88@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Daniel J Blueman <daniel@...ascale.com>,
	Waiman Long <waiman.long@...com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Ivan Kokshaysky <ink@...assic.park.msu.ru>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v8 4/4] qrwlock: Use smp_store_release() in write_unlock()

On Tue, Jan 14, 2014 at 10:01:04AM -0800, Richard Henderson wrote:
> On 01/14/2014 09:08 AM, Matt Turner wrote:
> > On Tue, Jan 14, 2014 at 3:03 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> >> On Tue, Jan 14, 2014 at 10:28:23AM +0800, Daniel J Blueman wrote:
> >>>> Peter,
> >>>>
> >>>> I found out that the build failure was caused by the fact that the
> >>>> __native_word() macro (used internally by compiletime_assert_atomic())
> >>>> allows only a size of 4 or 8 for x86-64. The data type that I used is a
> >>>> byte. Is there a reason why byte and short are not considered native?
> >>>
> >>> It seems likely it was implemented like that since there was no existing
> >>> need; long can be relied on as the largest native type, so this should
> >>> suffice and works here:
> >>
> >> There's Alphas that cannot actually atomically adres a byte; I do not
> >> konw if Linux cares about them, but if it does, we cannot in fact rely
> >> on this in generic primitives like this.
> > 
> > That's right, and thanks for the heads-up. Alpha can only address 4
> > and 8 bytes atomically. (LDL_L, LDQ_L, STL_C, STQ_C).
> > 
> > The Byte-Word extension in EV56 doesn't add new atomics, so in fact no
> > Alphas can address < 4 bytes atomically.
> 
> Emulated with aligned 4 byte atomics, and masking.  The same is true for arm,
> ppc, mips which, depending on cpu, also lack < 4 byte atomics.

Which means that Alpha should be able to similarly emulate 1-byte and
2-byte atomics, correct?

							Thanx, Paul

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