linux-kernel - [PATCH 0/2] Fix BUG: Invalid wait context in hrtimer

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Message-Id: <20210123233741.3614408-1-qais.yousef@arm.com>
Date:   Sat, 23 Jan 2021 23:37:39 +0000
From:   Qais Yousef <qais.yousef@....com>
To:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        John Ogness <john.ogness@...utronix.de>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Qais Yousef <qais.yousef@....com>
Subject: [PATCH 0/2] Fix BUG: Invalid wait context in hrtimer_interrupt()

I hit a pr_warn() inside hrtimer_interrupt() which lead to a BUG: Invalid wait
context splat.

The problem wasn't reproducible but I think the cause is obvious, printk can't
be called from interrupt context.

AFAICU printk_deferred() is safe from NMI, so I assumed it is safe to be called
from hrtimer_interrupt() too. Adding a pr_warn_once() inside
hrtimer_interrupt() in a location where it is always hit produces the BUG
splat. Replacing it with pr_warn_deferred_once() generates the printk warning
without any splat.

I added a new pr_*_deferred_once() variants to avoid open coding; but the name
ended not much shorter and I'm not sure if the wrappers are a win overall.
Since I've already done it, I'm sticking to it in this post. But will be happy
to drop it and just open code the printk_deferred_once(KERN_WARN, ...) in
hrtimer_interrupt() instead.

Thanks

Qais Yousef (2):
  printk: Add new pr_*_deferred_once() variants
  hrtimer: Use pr_warn_deferred_once() in hrtimer_interrupt()

 include/linux/printk.h | 24 ++++++++++++++++++++++++
 kernel/time/hrtimer.c  |  3 ++-
 2 files changed, 26 insertions(+), 1 deletion(-)

-- 
2.25.1