Date:   Thu, 18 Nov 2021 09:18:52 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Holger =?iso-8859-1?Q?Hoffst=E4tte?= 
        <holger@applied-asynchrony.com>,
        Qi Zheng <zhengqi.arch@bytedance.com>,
        Kees Cook <keescook@chromium.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Justin Forbes <jmforbes@linuxtx.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Guenter Roeck <linux@roeck-us.net>,
        Shuah Khan <shuah@kernel.org>, patches@kernelci.org,
        lkft-triage@lists.linaro.org, Pavel Machek <pavel@denx.de>,
        Jon Hunter <jonathanh@nvidia.com>,
        Florian Fainelli <f.fainelli@gmail.com>,
        stable <stable@vger.kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: [PATCH 5.15 000/923] 5.15.3-rc3 review
Message-ID: <20211118081852.GM174730@worktop.programming.kicks-ass.net>
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On Thu, Nov 18, 2021 at 09:06:27AM +0100, Peter Zijlstra wrote:
> On Wed, Nov 17, 2021 at 03:50:17PM -0800, Linus Torvalds wrote:
> 
> > I really don't think the WCHAN code should use unwinders at all. It's
> > too damn fragile, and it's too easily triggered from user space.
> 
> On x86, esp. with ORC, it pretty much has to. The thing is, the ORC
> unwinder has been very stable so far. I'm guessing there's some really
> stupid thing going on, like for example trying to unwind a freed stack.
> 
> I *just* managed to reproduce, so let me go have a poke.

Confirmed, with the below it no longer reproduces. Now, let me go undo
that and fix the unwinder to not explode while trying to unwind nothing.


---
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 862af1db22ab..f810c5192cb9 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1978,7 +1978,7 @@ unsigned long get_wchan(struct task_struct *p)
 	raw_spin_lock_irq(&p->pi_lock);
 	state = READ_ONCE(p->__state);
 	smp_rmb(); /* see try_to_wake_up() */
-	if (state != TASK_RUNNING && state != TASK_WAKING && !p->on_rq)
+	if (state != TASK_RUNNING && state != TASK_WAKING && !p->on_rq && !(p->flags & PF_EXITING))
 		ip = __get_wchan(p);
 	raw_spin_unlock_irq(&p->pi_lock);