Subject: Re: [PATCH v3 03/12] locking/ww_mutex: Extract stamp comparison to
 __ww_mutex_stamp_after
To: zhoucm1 <david1.zhou@amd.com>, linux-kernel@vger.kernel.org
References: <1482346000-9927-1-git-send-email-nhaehnle@gmail.com>
 <1482346000-9927-4-git-send-email-nhaehnle@gmail.com>
 <585B3346.2020001@amd.com>
Cc: Maarten Lankhorst <dev@mblankhorst.nl>,
        =?UTF-8?Q?Nicolai_H=c3=a4hnle?= <Nicolai.Haehnle@amd.com>,
        Peter Zijlstra <peterz@infradead.org>, dri-devel@lists.freedesktop.org,
        Ingo Molnar <mingo@redhat.com>
From: =?UTF-8?Q?Nicolai_H=c3=a4hnle?= <nhaehnle@gmail.com>
Message-ID: <b34536f6-8e7b-d771-a10f-6c2388f4da7d@gmail.com>
Date: Thu, 22 Dec 2016 09:43:50 +0100
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On 22.12.2016 02:58, zhoucm1 wrote:
> On 2016年12月22日 02:46, Nicolai Hähnle wrote:
>> +static inline bool __sched
>> +__ww_ctx_stamp_after(struct ww_acquire_ctx *a, struct ww_acquire_ctx *b)
>> +{
>> +    return a->stamp - b->stamp <= LONG_MAX &&
>> +           (a->stamp != b->stamp || a > b);
> I want to ask a stupid question, why a can compare with b? They are
> pointers of structure. Isn't stamp enough for compare?

As far as I understand, the idea is to provide a tie-breaker to ensure 
that there is a strict order between contexts even if the stamp happens 
to be equal. Since we get stamps from atomic increments, this really 
only matters if (a) someone makes a mistake and confuses ww_classes 
(which CONFIG_DEBUG_MUTEXES would flag) or (b) the ww_class stamp 
counter wraps around fully during the lifetime of the acquire context. 
This is extremely unlikely of course.

Nicolai