um: fix os_idle_sleep() to not hang
Changing os_idle_sleep() to use pause() (I accidentally described it as an empty select() in the commit log because I had changed it from that to pause() in a later revision) exposed a race condition in the idle code. The following can happen: timer_settime(0, 0, {it_interval={tv_sec=0, tv_nsec=0}, it_value={tv_sec=0, tv_nsec=624017}}, NULL) = 0 ... <SIGALRM is delivered but we're already on the way to idle> pause() and we now hang forever. This was previously possible as well, but it could never cause UML to hang for more than a second since we could only sleep for that much, so at most you'd notice a "hiccup" in the UML. Obviously, any sort of external interrupt also "saves" it and interrupts pause(). Fix this by properly handling the race, rather than papering over it again: - first, block SIGALRM, and obtain the old signal set - check the timer - suspend, waiting for any signal out of the old set, if, and only if, the timer will fire in the future - restore the old signal mask This ensures race-free operation: as it's blocked, the signal won't be delivered while we're looking at the timer even if it were to be triggered right _after_ we've returned from timer_gettime() with a non-zero value (telling us the timer will trigger). Thus, despite getting to sigsuspend() because timer_gettime() told us we're still waiting, we'll not hang because sigsuspend() will return immediately due to the pending signal. Fixes: 49da38a3 ("um: Simplify os_idle_sleep() and sleep longer") Signed-off-by: Johannes Berg <johannes.berg@intel.com> Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com> Signed-off-by: Richard Weinberger <richard@nod.at>
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