This is a note to let you know that I've just added the patch titled
mac80211-hwsim: fix late beacon hrtimer handling
to the 5.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
mac80211-hwsim-fix-late-beacon-hrtimer-handling.patch
and it can be found in the queue-5.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit ee565d10cbbaaea87a44300be8155eb82cfd2e7a
Author: Johannes Berg <johannes.berg@xxxxxxxxx>
Date: Wed Sep 15 11:29:37 2021 +0200
mac80211-hwsim: fix late beacon hrtimer handling
[ Upstream commit 313bbd1990b6ddfdaa7da098d0c56b098a833572 ]
Thomas explained in https://lore.kernel.org/r/87mtoeb4hb.ffs@tglx
that our handling of the hrtimer here is wrong: If the timer fires
late (e.g. due to vCPU scheduling, as reported by Dmitry/syzbot)
then it tries to actually rearm the timer at the next deadline,
which might be in the past already:
1 2 3 N N+1
| | | ... | |
^ intended to fire here (1)
^ next deadline here (2)
^ actually fired here
The next time it fires, it's later, but will still try to schedule
for the next deadline (now 3), etc. until it catches up with N,
but that might take a long time, causing stalls etc.
Now, all of this is simulation, so we just have to fix it, but
note that the behaviour is wrong even per spec, since there's no
value then in sending all those beacons unaligned - they should be
aligned to the TBTT (1, 2, 3, ... in the picture), and if we're a
bit (or a lot) late, then just resume at that point.
Therefore, change the code to use hrtimer_forward_now() which will
ensure that the next firing of the timer would be at N+1 (in the
picture), i.e. the next interval point after the current time.
Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Reported-by: syzbot+0e964fad69a9c462bc1e@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 01e59e467ecf ("mac80211_hwsim: hrtimer beacon")
Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20210915112936.544f383472eb.I3f9712009027aa09244b65399bf18bf482a8c4f1@changeid
Signed-off-by: Johannes Berg <johannes.berg@xxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/drivers/net/wireless/mac80211_hwsim.c b/drivers/net/wireless/mac80211_hwsim.c
index 1033513d3d9d..07b070b14d75 100644
--- a/drivers/net/wireless/mac80211_hwsim.c
+++ b/drivers/net/wireless/mac80211_hwsim.c
@@ -1603,8 +1603,8 @@ mac80211_hwsim_beacon(struct hrtimer *timer)
bcn_int -= data->bcn_delta;
data->bcn_delta = 0;
}
- hrtimer_forward(&data->beacon_timer, hrtimer_get_expires(timer),
- ns_to_ktime(bcn_int * NSEC_PER_USEC));
+ hrtimer_forward_now(&data->beacon_timer,
+ ns_to_ktime(bcn_int * NSEC_PER_USEC));
return HRTIMER_RESTART;
}
