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From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Colin Ian King <colin.king@canonical.com>
Cc: linux-rt-users@vger.kernel.org
Subject: Re: RT kernel testing with stress-ng and scheduling while in atomic bugs.
Date: Mon, 14 Dec 2020 12:37:22 +0100	[thread overview]
Message-ID: <20201214113722.iman6afnqvhtgcw4@linutronix.de> (raw)
In-Reply-To: <b9317e8c-d5d9-e440-6c32-f9c26ad4ef10@canonical.com>

On 2020-12-07 12:43:47 [+0000], Colin Ian King wrote:
> Hi,
Hi,

> Questions:
> 
> 1. Are these issues expected?
I haven't seen it, it is not expected. It hasn't been reported so far.

> 2. Is there an official way to report my bug findings?

Report it to the list, keep me in Cc:, please.

> 3. I am keen to debug and fix these issues, have RT folk got some advice
> on how to start debugging these kind of issues?

Based on the backtrace (and please tell your client to not break lines
while pasting bactraces / logs since it makes them hard to read):

>  BUG: scheduling while atomic: stress-ng-fstat/47271/0x00000002
>  CPU: 4 PID: 47271 Comm: stress-ng-fstat Not tainted 5.10.0-6-realtime #7
…
>  Call Trace:
>  __schedule_bug.cold+0x4a/0x5b
>   __schedule+0x50d/0x6b0
>   ? task_blocks_on_rt_mutex+0x29a/0x390
>  preempt_schedule_lock+0x24/0x50
>  rt_spin_lock_slowlock_locked+0x11b/0x2c0
>  rt_spin_lock_slowlock+0x57/0x90
>   rt_spin_lock+0x30/0x40
>   alloc_pid+0x1bc/0x400

alloc_pid() acquired a spinlock_t somewhere while the context was
"atomic". The output seems to lack details. Like the "scheduling while
atomic" should also contain (somwhere) "atomic: x irqs disabled: x"
which is missing. Also I would expect to see preemption level in your
backtrace.

Anyway. Something made the context atomic (like preempt_disable()) and
then you attempted to acquire a lock at alloc_pid+0x1bc/0x400. While
looking at the code, alloc_pid() should be preemptible. On PREEMPT_RT
spin_lock() does not disable preemption so you should remain
preemtible().
 
Sebastian

      parent reply	other threads:[~2020-12-14 11:38 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-07 12:43 RT kernel testing with stress-ng and scheduling while in atomic bugs Colin Ian King
2020-12-14  6:07 ` Punit Agrawal
2020-12-14 11:37 ` Sebastian Andrzej Siewior [this message]

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