[PATCH v2 04/11] locking/ww_mutex: Set use_ww_ctx even when locking without a context

[Maarten Lankhorst]

Op 16-12-16 om 14:17 schreef Nicolai Hähnle:
> On 06.12.2016 16:25, Peter Zijlstra wrote:
>> On Thu, Dec 01, 2016 at 03:06:47PM +0100, Nicolai Hähnle wrote:
>>
>>> @@ -640,10 +640,11 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
>>>      struct mutex_waiter waiter;
>>>      unsigned long flags;
>>>      bool first = false;
>>> -    struct ww_mutex *ww;
>>>      int ret;
>>>
>>> -    if (use_ww_ctx) {
>>> +    if (use_ww_ctx && ww_ctx) {
>>> +        struct ww_mutex *ww;
>>> +
>>>          ww = container_of(lock, struct ww_mutex, base);
>>>          if (unlikely(ww_ctx == READ_ONCE(ww->ctx)))
>>>              return -EALREADY;
>>
>> So I don't see the point of removing *ww from the function scope, we can
>> still compute that container_of() even if !ww_ctx, right? That would
>> safe a ton of churn below, adding all those struct ww_mutex declarations
>> and container_of() casts.
>>
>> (and note that the container_of() is a fancy NO-OP because base is the
>> first member).
>
> Sorry for taking so long to get back to you.
>
> In my experience, the undefined behavior sanitizer in GCC for userspace programs complains about merely casting a pointer to the wrong type. I never went into the standards rabbit hole to figure out the details. It might be a C++ only thing (ubsan cannot tell the difference otherwise anyway), but that was the reason for doing the change in this more complicated way.
>
> Are you sure that this is defined behavior in C? If so, I'd be happy to go with the version that has less churn.
>
> I'll also get rid of those ww_mutex_lock* wrapper functions. 

ww_ctx = use_ww_ctx ? container_of : NULL ?