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authorJohn Stultz <john.stultz@linaro.org>2014-06-04 16:11:43 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2014-06-04 16:54:17 -0700
commit6d9bcb621b0b0a20604cbdb298c4487e44dd0da2 (patch)
treee6bb8032b8662be3551d4abcc04499c39c9e85df /Documentation/CodingStyle
parentc224815dac9c739b79050d3cc67443ff500bc478 (diff)
timekeeping: use printk_deferred when holding timekeeping seqlock
Jiri Bohac pointed out that there are rare but potential deadlock possibilities when calling printk while holding the timekeeping seqlock. This is due to printk() triggering console sem wakeup, which can cause scheduling code to trigger hrtimers which may try to read the time. Specifically, as Jiri pointed out, that path is: printk vprintk_emit console_unlock up(&console_sem) __up wake_up_process try_to_wake_up ttwu_do_activate ttwu_activate activate_task enqueue_task enqueue_task_fair hrtick_update hrtick_start_fair hrtick_start_fair get_time ktime_get --> endless loop on read_seqcount_retry(&timekeeper_seq, ...) This patch tries to avoid this issue by using printk_deferred (previously named printk_sched) which should defer printing via a irq_work_queue. Signed-off-by: John Stultz <john.stultz@linaro.org> Reported-by: Jiri Bohac <jbohac@suse.cz> Reviewed-by: Steven Rostedt <rostedt@goodmis.org> Cc: Jan Kara <jack@suse.cz> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Ingo Molnar <mingo@redhat.com> Cc: Steven Rostedt <rostedt@goodmis.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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