tree d7e357909f470c2fc601a3cbb839fa03f24c47eb
parent 01bf79e116c45cb8d5a8c49301e0d169439857d1
author Steven Rostedt (Google) <rostedt@goodmis.org> 1667769871 -0500
committer Steven Rostedt (Google) <rostedt@goodmis.org> 1667777262 -0500

timers: Add timer_shutdown_sync() and timer_shutdown() to be called before freeing timers

We are hitting a common bug were a timer is being triggered after it is
freed. This causes a corruption in the timer link list and crashes the
kernel. Unfortunately it is not easy to know what timer it was that was
freed. Looking at the code, it appears that there are several cases that
del_timer() is used when del_timer_sync() should have been.

Add a timer_shutdown_sync() that not only does a del_timer_sync() but will mark
the timer as terminated in case it gets rearmed, it will trigger a WARN_ON. The
timer_shutdown_sync() is more likely to be used by developers that are about to
free a timer, then using del_timer_sync() as the latter is not as obvious
to being needed for freeing. Having the word "shutdown" in the name of the
function will hopefully help developers know that that function needs to
be called before freeing.

The added bonus is the marking of the timer as being freed such that it
will trigger a warning if it gets rearmed. At least that way if the system
crashes on a freed timer, at least we may see which timer it was that was
freed.

There's some situations that already know that the timer is shutdown and
does not need to perform the synchronization (or can not due to its
context). For these locations there's timer_shutdown() that only shuts
down the timer (prevents it from being rearmed) but does not add checks if
the timer is currently running.

This code is taken from Thomas Gleixner's "untested" version from my
original patch and modified after testing and with some other comments
from Linus addressed. As well as some extra comments added.

Link: https://lore.kernel.org/all/87pmlrkgi3.ffs@tglx/
Link: https://lkml.kernel.org/r/20221106212702.363575800@goodmis.org
Link: https://lore.kernel.org/all/20221105060024.598488967@goodmis.org/

Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Anna-Maria Gleixner <anna-maria@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Julia Lawall <Julia.Lawall@inria.fr>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>