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From: Alex =?utf-8?Q?Benn=C3=A9e?= <alex.bennee@linaro.org>
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Date: Mon, 03 Oct 2016 10:48:02 +0100
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Subject: Re: [Qemu-devel] [PATCH v3 13/15] tcg: ensure
 cpu_tb_exec/tb_gen_code use atomic_read/write
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-devel@nongnu.org, mttcg@greensocs.com, fred.konrad@greensocs.com, a.rigo@virtualopensystems.com, cota@braap.org, bobby.prani@gmail.com, nikunj@linux.vnet.ibm.com, mark.burton@greensocs.com, jan.kiszka@siemens.com, serge.fdrv@gmail.com, rth@twiddle.net, peter.maydell@linaro.org, claudio.fontana@huawei.com, Peter Crosthwaite <crosthwaite.peter@gmail.com>


Paolo Bonzini <pbonzini@redhat.com> writes:

> On 30/09/2016 23:31, Alex Bennée wrote:
>>      tb = atomic_rcu_read(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)]);
>> -    if (unlikely(!tb || tb->pc != pc || tb->cs_base != cs_base ||
>> -                 tb->flags != flags)) {
>> +    if (unlikely(!tb || atomic_read(&tb->pc) != pc || atomic_read(&tb->cs_base) != cs_base ||
>> +                 atomic_read(&tb->flags) != flags)) {
>
> This should not be necessary (and is responsible for the 64-on-32
> compilation failure).  The load of tb from the cache is an acquire
> operation, and synchronizes with the corresponding store in
> cpu->tb_jmp_cache.

Is the C11 spec happy with "plain" accesses after the acquire operation?
Unfortunately the sanitizer isn't able to see the indirect acquires
effect on the other accesses.

>
> Paolo


--
Alex Bennée