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authorJohannes Berg <johannes.berg@intel.com>2021-01-22 21:40:23 +0100
committerRichard Weinberger <richard@nod.at>2021-01-26 22:11:38 +0100
commit9868c2081d071f7c309796c8dffc94364fc07582 (patch)
tree5d43a7655f2f5ef701f8cfda3286f1f893f6e1cc /arch/um/os-Linux
parenta31e9c4e7247d182192e9b85abbea498d63dd850 (diff)
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: 49da38a3ef33 ("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>
Diffstat (limited to 'arch/um/os-Linux')
-rw-r--r--arch/um/os-Linux/time.c15
1 files changed, 14 insertions, 1 deletions
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);
}