[<prev] [next>] [<thread-prev] [day] [month] [year] [list]
Message-ID: <20240905083139.34698d60@gandalf.local.home>
Date: Thu, 5 Sep 2024 08:31:39 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Tomas Glozar <tglozar@...hat.com>
Cc: linux-trace-kernel@...r.kernel.org, linux-kernel@...r.kernel.org,
jkacur@...hat.com, "Luis Claudio R. Goncalves" <lgoncalv@...hat.com>
Subject: Re: [PATCH] tracing/timerlat: Check tlat_var for NULL in
timerlat_fd_release
On Thu, 5 Sep 2024 12:38:37 +0200
Tomas Glozar <tglozar@...hat.com> wrote:
> st 4. 9. 2024 v 16:23 odesÃlatel Steven Rostedt <rostedt@...dmis.org> napsal:
> >
> > When running my tests, the second one would end up deadlocking and
> > triggering lockdep. I found a way to do basically the same thing with a
> > cpumask and no added locking. I'm currently testing it and will be sending
> > out a patch later today (if it passes the tests).
> >
> > -- Steve
> >
>
> Oh that's unfortunate. Your cpumask patch does protect from timerlat
> trying to stop a user workload thread via kthread_stop, but I don't
> think it prevents this race in the code from the other patch:
>
> static int timerlat_fd_release(struct inode *inode, struct file *file)
> {
> ...
> mutex_lock(&interface_lock);
> <-- this can now run at the same time as tlat_var_reset(), since
> the latter is not done under interface_lock in the latest version
> ...
> if (tlat_var->kthread)
> <-- if tlat_var is zeroed here, we still panic on NULL dereference
> hrtimer_cancel(&tlat_var->timer);
> ...
> }
Right, but my other patch was protecting the osn variable too, which I
don't think is necessary.
>
> Either way, the results are much better: before, the kernel panicked
> in <10 iterations of the while loop of the reproducer; with the
> cpumask patch and the tlat_var->kthread check patch, it has been
> running on my test VM for a few hours already with no panic.
I think the hrtimer change is missing this:
diff --git a/kernel/trace/trace_osnoise.c b/kernel/trace/trace_osnoise.c
index 8543d941b870..8e611dcee68a 100644
--- a/kernel/trace/trace_osnoise.c
+++ b/kernel/trace/trace_osnoise.c
@@ -259,6 +259,9 @@ static inline void tlat_var_reset(void)
{
struct timerlat_variables *tlat_var;
int cpu;
+
+ /* Synchronize with the timerlat interfaces */
+ mutex_lock(&interface_lock);
/*
* So far, all the values are initialized as 0, so
* zeroing the structure is perfect.
@@ -269,6 +272,7 @@ static inline void tlat_var_reset(void)
hrtimer_cancel(&tlat_var->timer);
memset(tlat_var, 0, sizeof(*tlat_var));
}
+ mutex_unlock(&interface_lock);
}
#else /* CONFIG_TIMERLAT_TRACER */
#define tlat_var_reset() do {} while (0)
I'll add it and start testing it. I haven't pushed to Linus yet.
-- Steve
Powered by blists - more mailing lists