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.
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