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From: Richard Henderson <rth@twiddle.net>
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Date: Tue, 17 May 2016 15:12:40 -0700
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Subject: Re: [Qemu-devel] [PATCH v5 07/18] qemu-thread: add simple
 test-and-set spinlock
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To: Sergey Fedorov <serge.fdrv@gmail.com>, "Emilio G. Cota" <cota@braap.org>, QEMU Developers <qemu-devel@nongnu.org>, MTTCG Devel <mttcg@greensocs.com>
Cc: =?UTF-8?Q?Alex_Benn=c3=a9e?= <alex.bennee@linaro.org>, Paolo Bonzini <pbonzini@redhat.com>, Peter Crosthwaite <crosthwaite.peter@gmail.com>

On 05/17/2016 01:01 PM, Sergey Fedorov wrote:
>> Sorry, I can't see reading ARMv6 ARM that 1-byte access can't be atomic. What
>> I've found:
>>
>>     B2.4.1 Normal memory attribute
>>     (snip)
>>     Shared Normal memory
>>
>>         (snip)
>>         ... Reads to Shared Normal Memory that are aligned in memory to the
>>         size of the access must be atomic.
...
> Looks like GCC has no trouble generating __atomic_store_n() for 1-byte bool...

Not loads and stores, but other atomic ops like xchg.  The native atomic
operations are all 4 bytes long.

I suppose the compiler may well be able to synthesize sub-word atomic ops, but
it'll be 2 or 3 times the size of a word-sized atomic op, and for no good reason.


r~