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From: Pranith Kumar <bobby.prani@gmail.com>
Date: Tue, 31 May 2016 11:03:20 -0400
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Subject: Re: [Qemu-devel] [RFC 01/10] exec: Introduce tcg_exclusive_{lock,
 unlock}()
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To: Alvise Rigo <a.rigo@virtualopensystems.com>
Cc: MTTCG Devel <mttcg@listserver.greensocs.com>, =?UTF-8?B?QWxleCBCZW5uw6ll?= <alex.bennee@linaro.org>, qemu-devel <qemu-devel@nongnu.org>, jani.kokkonen@huawei.com, claudio.fontana@huawei.com, 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 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