From 9868c2081d071f7c309796c8dffc94364fc07582 Mon Sep 17 00:00:00 2001 From: Johannes Berg Date: Fri, 22 Jan 2021 21:40:23 +0100 Subject: [PATCH] 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 ... 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: 49da38a3ef33 ("um: Simplify os_idle_sleep() and sleep longer") Signed-off-by: Johannes Berg Acked-By: Anton Ivanov Signed-off-by: Richard Weinberger --- arch/um/os-Linux/time.c | 15 ++++++++++++++- 1 file changed, 14 insertions(+), 1 deletion(-) diff --git a/arch/um/os-Linux/time.c b/arch/um/os-Linux/time.c index a61cbf73a179..6c5041c5560b 100644 --- a/arch/um/os-Linux/time.c +++ b/arch/um/os-Linux/time.c @@ -104,5 +104,18 @@ long long os_nsecs(void) */ void os_idle_sleep(void) { - pause(); + struct itimerspec its; + sigset_t set, old; + + /* block SIGALRM while we analyze the timer state */ + sigemptyset(&set); + sigaddset(&set, SIGALRM); + sigprocmask(SIG_BLOCK, &set, &old); + + /* check the timer, and if it'll fire then wait for it */ + timer_gettime(event_high_res_timer, &its); + if (its.it_value.tv_sec || its.it_value.tv_nsec) + sigsuspend(&old); + /* either way, restore the signal mask */ + sigprocmask(SIG_UNBLOCK, &set, NULL); }