From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:55431)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <alex.bennee@linaro.org>) id 1Z2K0G-0003wk-7E
	for qemu-devel@nongnu.org; Tue, 09 Jun 2015 09:55:48 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <alex.bennee@linaro.org>) id 1Z2K0B-0006Xo-U7
	for qemu-devel@nongnu.org; Tue, 09 Jun 2015 09:55:44 -0400
Received: from static.88-198-71-155.clients.your-server.de
	([88.198.71.155]:44498 helo=socrates.bennee.com)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <alex.bennee@linaro.org>) id 1Z2K0B-0006XA-Nk
	for qemu-devel@nongnu.org; Tue, 09 Jun 2015 09:55:39 -0400
References: <1433514678-4464-1-git-send-email-fred.konrad@greensocs.com>
	<871thl8ctj.fsf@linaro.org>
From: Alex =?utf-8?Q?Benn=C3=A9e?= <alex.bennee@linaro.org>
In-reply-to: <871thl8ctj.fsf@linaro.org>
Date: Tue, 09 Jun 2015 14:55:58 +0100
Message-ID: <87pp556l5d.fsf@linaro.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 8bit
Subject: Re: [Qemu-devel] [RFC PATCH] Use atomic cmpxchg to atomically check
	the exclusive value in a STREX
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: fred.konrad@greensocs.com
Cc: mttcg@listserver.greensocs.com, peter.maydell@linaro.org, mark.burton@greensocs.com, qemu-devel@nongnu.org, agraf@suse.de, guillaume.delbergue@greensocs.com, pbonzini@redhat.com


Alex Bennée <alex.bennee@linaro.org> writes:

> fred.konrad@greensocs.com writes:
>
>> From: KONRAD Frederic <fred.konrad@greensocs.com>
>>
>> This mechanism replaces the existing load/store exclusive mechanism which seems
>> to be broken for multithread.
>> It follows the intention of the existing mechanism and stores the target address
>> and data values during a load operation and checks that they remain unchanged
>> before a store.
>>
>> In common with the older approach, this provides weaker semantics than required
>> in that it could be that a different processor writes the same value as a
>> non-exclusive write, however in practise this seems to be irrelevant.
>
<snip>
>>  
>> +/* Protect cpu_exclusive_* variable .*/
>> +__thread bool cpu_have_exclusive_lock;
>> +QemuMutex cpu_exclusive_lock;
>> +
>> +inline void arm_exclusive_lock(void)
>> +{
>> +    if (!cpu_have_exclusive_lock) {
>> +        qemu_mutex_lock(&cpu_exclusive_lock);
>> +        cpu_have_exclusive_lock = true;
>> +    }
>> +}
>> +
>> +inline void arm_exclusive_unlock(void)
>> +{
>> +    if (cpu_have_exclusive_lock) {
>> +        cpu_have_exclusive_lock = false;
>> +        qemu_mutex_unlock(&cpu_exclusive_lock);
>> +    }
>> +}
>
> I don't quite follow. If these locks are mean to be protecting access to
> variables then how do they do that? The lock won't block if another
> thread is currently messing with the protected values.

Having re-read after coffee I'm still wondering why we need the
per-thread bool? All the lock/unlock pairs are for critical sections so
don't we just want to serialise on the qemu_mutex_lock(), what do the
flags add apart from allowing you to next locks that shouldn't happen?


-- 
Alex Bennée