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From: alvise rigo <a.rigo@virtualopensystems.com>
Date: Thu, 2 Jun 2016 18:21:35 +0200
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Subject: Re: [Qemu-devel] [RFC 01/10] exec: Introduce tcg_exclusive_{lock,
 unlock}()
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To: Pranith Kumar <bobby.prani@gmail.com>
Cc: MTTCG Devel <mttcg@listserver.greensocs.com>, =?UTF-8?B?QWxleCBCZW5uw6ll?= <alex.bennee@linaro.org>, qemu-devel <qemu-devel@nongnu.org>, Jani Kokkonen <jani.kokkonen@huawei.com>, Claudio Fontana <claudio.fontana@huawei.com>, VirtualOpenSystems Technical Team <tech@virtualopensystems.com>, =?UTF-8?B?S09OUkFEIEZyw6lkw6lyaWM=?= <fred.konrad@greensocs.com>, Paolo Bonzini <pbonzini@redhat.com>, Richard Henderson <rth@twiddle.net>, Sergey Fedorov <serge.fdrv@gmail.com>, "Emilio G. Cota" <cota@braap.org>, Peter Maydell <peter.maydell@linaro.org>

Hi Pranith,

Thank you for the hint, I will keep this in mind for the next version.

Regards,
alvise

On Tue, May 31, 2016 at 5:03 PM, Pranith Kumar <bobby.prani@gmail.com> wrote:
> Hi Alvise,
>
> On Thu, May 26, 2016 at 12:35 PM, Alvise Rigo
> <a.rigo@virtualopensystems.com> wrote:
>> Add tcg_exclusive_{lock,unlock}() functions that will be used for making
>> the emulation of LL and SC instructions thread safe.
>>
>> Signed-off-by: Alvise Rigo <a.rigo@virtualopensystems.com>
>
> <snip>
>
>> +__thread bool cpu_have_exclusive_lock;
>> +QemuSpin cpu_exclusive_lock;
>> +inline void tcg_exclusive_lock(void)
>> +{
>> +    if (!cpu_have_exclusive_lock) {
>> +        qemu_spin_lock(&cpu_exclusive_lock);
>> +        cpu_have_exclusive_lock = true;
>> +    }
>> +}
>> +
>> +inline void tcg_exclusive_unlock(void)
>> +{
>> +    if (cpu_have_exclusive_lock) {
>> +        cpu_have_exclusive_lock = false;
>> +        qemu_spin_unlock(&cpu_exclusive_lock);
>> +    }
>> +}
>
> I think the unlock() here should have an assert if
> cpu_have_exclusive_lock is false. From what I can see, a thread should
> either take the exclusive lock or wait spinning for it in lock(). So
> unlock() should always see cpu_have_exclusive_lock as true. It is a
> good place to find locking bugs.
>
> --
> Pranith