sched: fix wait_for_completion_timeout() spurious failure under heavy load

It seems that the current implementaton of wait_for_completion_timeout() has a small problem under very high load for the common pattern: if (!wait_for_completion_timeout(&done, timeout)) /* handle failure */ because the implementation very roughly does (lots of code deleted to show the basic flow): static inline long __sched do_wait_for_common(struct completion *x, long timeout, int state) { if (x->done) return timeout; do { timeout = schedule_timeout(timeout); if (!timeout) return timeout; } while (!x->done); return timeout; } so if the system is very busy and x->done is not set when do_wait_for_common() is entered, it is possible that the first call to schedule_timeout() returns 0 because the task doing wait_for_completion doesn't get rescheduled for a long time, even if it is woken up early enough. In this case, wait_for_completion_timeout() returns 0 without even checking x->done again, and the code above falls into its failure case purely for scheduler reasons, even if the hardware event or whatever was being waited for happened early enough. It would make sense to add an extra test to do_wait_for() in the timeout case and return 1 if x->done is actually set. A quick audit (not exhaustive) of wait_for_completion_timeout() callers seems to indicate that no one actually cares about the return value in the success case -- they just test for 0 (timed out) versus non-zero (wait succeeded).

RDRoland Dreier committed 18 years ago