linux-kernel - [PATCH v2] printk: defer_console

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Message-ID: <87bkzj3t77.fsf@jogness.linutronix.de>
Date:   Mon, 07 Feb 2022 10:52:20 +0106
From:   John Ogness <john.ogness@...utronix.de>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH v2] printk: defer_console_output: use atomic update

The per-cpu @printk_pending variable can be updated from
sleepable contexts, such as:

  get_random_bytes()
    warn_unseeded_randomness()
      printk_deferred()
        defer_console_output()

and can be updated from interrupt contexts, such as:

  handle_irq_event_percpu()
    __irq_wake_thread()
      wake_up_process()
        try_to_wake_up()
          select_task_rq()
            select_fallback_rq()
              printk_deferred()
                defer_console_output()

and can be updated from NMI contexts, such as:

  vprintk()
    if (in_nmi()) defer_console_output()

Therefore this_cpu_or(), the atomic variant of __this_cpu_or(),
should be used to update the variable.

Reported-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Reviewed-by: Sergey Senozhatsky <senozhatsky@...omium.org>
Signed-off-by: John Ogness <john.ogness@...utronix.de>
---
 Change from v1:
 
 Sebastian comments made it clear that the v1 title and message were not
 correctly specifying the problem or the reason for the fix. I hope this
 text is clearer.

 kernel/printk/printk.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 82abfaf3c2aa..164ccdb9423e 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3262,7 +3262,7 @@ void defer_console_output(void)
 		return;
 
 	preempt_disable();
-	__this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
+	this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
 	irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
 	preempt_enable();
 }

base-commit: dfd42facf1e4ada021b939b4e19c935dcdd55566
-- 
2.30.2