Re: [RFC][PATCH] timers: Add del_time_free() to be called before freeing timers

From: Guenter Roeck <linux@roeck-us.net>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	jstultz@google.com, Stephen Boyd <sboyd@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Marcel Holtmann <marcel@holtmann.org>,
	Johan Hedberg <johan.hedberg@gmail.com>,
	Luiz Augusto von Dentz <luiz.dentz@gmail.com>,
	Eric Dumazet <edumazet@google.com>
Subject: Re: [RFC][PATCH] timers: Add del_time_free() to be called before freeing timers
Date: Thu, 7 Apr 2022 14:58:02 -0700	[thread overview]
Message-ID: <20220407215802.GB186606@roeck-us.net> (raw)
In-Reply-To: <20220407161745.7d6754b3@gandalf.local.home>

Hi Steven,

On Thu, Apr 07, 2022 at 04:17:45PM -0400, Steven Rostedt wrote:
> [
>   This is an RFC patch. As we hit a few bugs were del_timer() is called
>   instead of del_timer_sync() before the timer is freed, and there could
>   be bugs where even del_timer_sync() is used, but the timer gets rearmed,
>   I decided to introduce a "del_timer_free()" function that can be used
>   instead. This will at least educate developers on what to call before they
>   free a structure that holds a timer.
> 
>   In this RFC, I modified hci_qca.c as a use case, even though that change
>   needs some work, because the workqueue could still rearm it (I'm looking
>   to see if I can trigger the warning).
> 
>   If this approach is acceptable, then I will remove the hci_qca.c portion
>   from this patch, and create a series of patches to use the
>   del_timer_free() in all the locations in the kernel that remove the timer
>   before freeing.
> ]
> 
> From: "Steven Rostedt (Google)" <rostedt@goodmis.org>
> 
> 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 del_timer_free() that not only does a del_timer_sync() but will mark

This limits the use case to situations where del_timer_sync() can actually
be called. There is, however, code where this is not possible.
Specifically, it doesn't work if the code triggered with the timer uses a
lock, and del_timer() is also called under that same lock. An example for
that is the code in sound/synth/emux/emux.c. How do you suggest to handle
that situation ?

Thanks,
Guenter