linux-kernel - [PATCH v3] printk: use atomic updates for klogd work

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Message-ID: <87iltld4ue.fsf@jogness.linutronix.de>
Date:   Fri, 11 Feb 2022 12:29:37 +0106
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH v3] printk: use atomic updates for klogd work

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 the atomic variant of the updating functions must be used.

Replace __this_cpu_xchg() with this_cpu_xchg().
Replace __this_cpu_or() with this_cpu_or().

Reported-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Signed-off-by: John Ogness <john.ogness@...utronix.de>
---
 Changes:

 v3: also fix __this_cpu_xchg() usage

 v2: fix commit message

 kernel/printk/printk.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 82abfaf3c2aa..c7280b40de6c 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3228,7 +3228,7 @@ static DEFINE_PER_CPU(int, printk_pending);
 
 static void wake_up_klogd_work_func(struct irq_work *irq_work)
 {
-	int pending = __this_cpu_xchg(printk_pending, 0);
+	int pending = this_cpu_xchg(printk_pending, 0);
 
 	if (pending & PRINTK_PENDING_OUTPUT) {
 		/* If trylock fails, someone else is doing the printing */
@@ -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();
 }
-- 
2.30.2