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From: Richard Henderson <rth@twiddle.net>
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Subject: Re: [Qemu-devel] [RFC v4 16/28] tcg: drop global lock during TCG
 code execution
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To: =?UTF-8?Q?Alex_Benn=c3=a9e?= <alex.bennee@linaro.org>, mttcg@greensocs.com, qemu-devel@nongnu.org, fred.konrad@greensocs.com, a.rigo@virtualopensystems.com, cota@braap.org, bobby.prani@gmail.com, nikunj@linux.vnet.ibm.com
Cc: peter.maydell@linaro.org, claudio.fontana@huawei.com, Peter Crosthwaite <crosthwaite.peter@gmail.com>, jan.kiszka@siemens.com, "Michael S. Tsirkin" <mst@redhat.com>, mark.burton@greensocs.com, Eduardo Habkost <ehabkost@redhat.com>, serge.fdrv@gmail.com, pbonzini@redhat.com

On 08/11/2016 08:24 AM, Alex Bennée wrote:
> +    if (mr->global_locking) {
> +        qemu_mutex_lock_iothread();
> +        locked = true;
> +    }
>      memory_region_dispatch_read(mr, physaddr, &val, 1 << SHIFT,
>                                  iotlbentry->attrs);
> +    if (locked) {
> +        qemu_mutex_unlock_iothread();
> +    }

I'm not keen on this pattern.

(1) Why not use recursive locks?

(2) If there's a good reason why not, then perhaps

   if (mr->global_locking) {
     qemu_mutex_lock_iothread();
     do_something;
     qemu_mutex_unlock_iothread();
   } else {
     do_something;
   }

is a better pattern to use.


r~