Re: [PATCH v8 4/4] qrwlock: Use smp_store_release() in write_unlock()

[Waiman Long]

On 01/14/2014 01:01 PM, 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@xxxxxxxxxxxxx>  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.

I would like to know if the action of writing out a byte (e.g. *byte = 
0) is atomic in those architectures or is emulated by a 
compiler-generated software read-modify-write.

-Longman
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