Re: [Qemu-devel] [RFC 01/10] exec: Introduce tcg_exclusive_{lock, unlock}()

[Pranith Kumar]

Hi Alvise,

On Thu, May 26, 2016 at 12:35 PM, Alvise Rigo
<address@hidden> 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 <address@hidden>

<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