linux-kernel - Re: [PATCH v3 03/12] locking/ww_mutex: Extract stamp comparison to __ww_mutex_stamp

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Date:   Thu, 22 Dec 2016 09:43:50 +0100
From:   Nicolai Hähnle <nhaehnle@...il.com>
To:     zhoucm1 <david1.zhou@....com>, linux-kernel@...r.kernel.org
Cc:     Maarten Lankhorst <dev@...ankhorst.nl>,
        Nicolai Hähnle <Nicolai.Haehnle@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        dri-devel@...ts.freedesktop.org, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v3 03/12] locking/ww_mutex: Extract stamp comparison to
 __ww_mutex_stamp_after

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