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From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <56376B57.3090806@redhat.com>
Date: Mon, 2 Nov 2015 14:55:35 +0100
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Subject: Re: [Qemu-devel] [PATCH 1/4] fifolock: create rfifolock_is_locked
 helper
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To: "Denis V. Lunev" <den@openvz.org>, Stefan Hajnoczi <stefanha@gmail.com>
Cc: qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>, qemu-stable@nongnu.org



On 02/11/2015 14:39, Denis V. Lunev wrote:
>> This is thread-safe:
>>
>> bool owner;
>>
>> qemu_mutex_lock(&r->lock);
>> owner = r->nesting > 0 && qemu_thread_is_self(&r->owner_thread);
>> qemu_mutex_unlock(&r->lock);
>>
>> return owner;
> yep, I know.
> 
> But I do not want to take the lock for check.

You can use a trylock.  If it fails, it is definitely safe to return false.

> IMHO it would be better to
> 
> @@ -68,11 +68,16 @@ void rfifolock_lock(RFifoLock *r)
>  void rfifolock_unlock(RFifoLock *r)
>  {
>      qemu_mutex_lock(&r->lock);
> -    assert(r->nesting > 0);
> -    assert(qemu_thread_is_self(&r->owner_thread));
> +    assert(rfifolock_is_owner(r));
>      if (--r->nesting == 0) {
> +        qemu_thread_clear(&r->owner_thread);
>          r->head++;
>          qemu_cond_broadcast(&r->cond);
>      }
>      qemu_mutex_unlock(&r->lock);
>  }
> +
> +bool rfifolock_is_owner(RFifoLock *r)
> +{
> +    return r->nesting > 0 && qemu_thread_is_self(&r->owner_thread);
> +}
> 
> which does not require lock and thread safe.

I think it requires memory barriers though.  But it can be simplified:
if you clear owner_thread, you do not need to check r->nesting in
rfifolock_is_owner.

Clearing owner_thread can be done with a simple memset.

Paolo