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    Subject[PATCH 5.17 284/772] printk: wake waiters for safe and NMI contexts
    Date
    From: John Ogness <john.ogness@linutronix.de>

    [ Upstream commit 5341b93dea8c39d7612f7a227015d4b1d5cf30db ]

    When printk() is called from safe or NMI contexts, it will directly
    store the record (vprintk_store()) and then defer the console output.
    However, defer_console_output() only causes console printing and does
    not wake any waiters of new records.

    Wake waiters from defer_console_output() so that they also are aware
    of the new records from safe and NMI contexts.

    Fixes: 03fc7f9c99c1 ("printk/nmi: Prevent deadlock when accessing the main log buffer in NMI")
    Signed-off-by: John Ogness <john.ogness@linutronix.de>
    Reviewed-by: Petr Mladek <pmladek@suse.com>
    Signed-off-by: Petr Mladek <pmladek@suse.com>
    Link: https://lore.kernel.org/r/20220421212250.565456-6-john.ogness@linutronix.de
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    kernel/printk/printk.c | 28 ++++++++++++++++------------
    1 file changed, 16 insertions(+), 12 deletions(-)

    diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
    index 8964b1db2745..4f344e7b2878 100644
    --- a/kernel/printk/printk.c
    +++ b/kernel/printk/printk.c
    @@ -743,7 +743,7 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
    * prepare_to_wait_event() pairs with the full memory barrier
    * within wq_has_sleeper().
    *
    - * This pairs with wake_up_klogd:A.
    + * This pairs with __wake_up_klogd:A.
    */
    ret = wait_event_interruptible(log_wait,
    prb_read_valid(prb,
    @@ -1521,7 +1521,7 @@ static int syslog_print(char __user *buf, int size)
    * prepare_to_wait_event() pairs with the full memory barrier
    * within wq_has_sleeper().
    *
    - * This pairs with wake_up_klogd:A.
    + * This pairs with __wake_up_klogd:A.
    */
    len = wait_event_interruptible(log_wait,
    prb_read_valid(prb, seq, NULL)); /* LMM(syslog_print:A) */
    @@ -3267,7 +3267,7 @@ static void wake_up_klogd_work_func(struct irq_work *irq_work)
    static DEFINE_PER_CPU(struct irq_work, wake_up_klogd_work) =
    IRQ_WORK_INIT_LAZY(wake_up_klogd_work_func);

    -void wake_up_klogd(void)
    +static void __wake_up_klogd(int val)
    {
    if (!printk_percpu_data_ready())
    return;
    @@ -3284,22 +3284,26 @@ void wake_up_klogd(void)
    *
    * This pairs with devkmsg_read:A and syslog_print:A.
    */
    - if (wq_has_sleeper(&log_wait)) { /* LMM(wake_up_klogd:A) */
    - this_cpu_or(printk_pending, PRINTK_PENDING_WAKEUP);
    + if (wq_has_sleeper(&log_wait) || /* LMM(__wake_up_klogd:A) */
    + (val & PRINTK_PENDING_OUTPUT)) {
    + this_cpu_or(printk_pending, val);
    irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
    }
    preempt_enable();
    }

    -void defer_console_output(void)
    +void wake_up_klogd(void)
    {
    - if (!printk_percpu_data_ready())
    - return;
    + __wake_up_klogd(PRINTK_PENDING_WAKEUP);
    +}

    - preempt_disable();
    - this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
    - irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
    - preempt_enable();
    +void defer_console_output(void)
    +{
    + /*
    + * New messages may have been added directly to the ringbuffer
    + * using vprintk_store(), so wake any waiters as well.
    + */
    + __wake_up_klogd(PRINTK_PENDING_WAKEUP | PRINTK_PENDING_OUTPUT);
    }

    void printk_trigger_flush(void)
    --
    2.35.1


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