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>> u32 cnt; >> } >> u64 id; >> }; >> >> static u64 get_cookie(struct unwind_task_info *info) >> { >> u32 cnt = READ_ONCE(info->id.cnt); >> u32 new_cnt; >> >> if (cnt & 1) >> return info->id; >> >> if (unlikely(!info->id.task_id)) { >> u32 task_id = local_clock(); >> >> cnt = 0; >> if (try_cmpxchg(&info->id.task_id, &cnt, task_id)) >> task_id = cnt; >> } >> >> new_cnt = cnt + 3; >> if (try_cmpxchg(&info->id, &cnt, new_cnt)) >> new_cnt = cnt; // try_cmpxchg() expects something >> >> return info->id; >> } > > Honestly I think this is way overkill. What I would do, is to have the > cookie saved in the event be 64 bit, but we can start with the > simple 32 bit solution keeping the top 32 bits zeros. If this does > indeed become an issue in the future, we could fix it with a 64 bit > number. By making sure all the exposed "cookies" are 64 bit, it should > not break anything. The cookie is just supposed to be a random unique > number that associates a request with its deferred user space stack > trace. > > With any exposed cookies to user space being 64 bits, this should not > be an issue to address in the future. FWIW, I liked your idea of making the cookie 64-bit with: - 32-bit cpu number, - 32-bit per-CPU free running counter. This is simple, works on 32-bit archs, and it does not overflow as often as time LSB because it counts execution contexts. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com