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Commit 5874af20 authored by Jan Kara's avatar Jan Kara Committed by Linus Torvalds
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printk: enable interrupts before calling console_trylock_for_printk() 

We need interrupts disabled when calling console_trylock_for_printk()
only so that cpu id we pass to can_use_console() remains valid (for
other things console_sem provides all the exclusion we need and
deadlocks on console_sem due to interrupts are impossible because we use
down_trylock()).  However if we are rescheduled, we are guaranteed to
run on an online cpu so we can easily just get the cpu id in
can_use_console().

We can lose a bit of performance when we enable interrupts in
vprintk_emit() and then disable them again in console_unlock() but OTOH
it can somewhat reduce interrupt latency caused by console_unlock().

We differ from (reverted) commit 939f04be in that we avoid calling
console_unlock() from vprintk_emit() with lockdep enabled as that has
unveiled quite some bugs leading to system freezes during boot (e.g.
  https://lkml.org/lkml/2014/5/30/242,
  https://lkml.org/lkml/2014/6/28/521

).

Signed-off-by: default avatarJan Kara <jack@suse.cz>
Tested-by: default avatarAndreas Bombe <aeb@debian.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 249771b8

kernel/printk/printk.c

+21 −10
Original line number Diff line number Diff line
@@ -1450,10 +1450,9 @@ static int have_callable_console(void) 
/*

 * Can we actually use the console at this time on this cpu?

 *

 * Console drivers may assume that per-cpu resources have

 * been allocated. So unless they're explicitly marked as

 * being able to cope (CON_ANYTIME) don't call them until

 * this CPU is officially up.

 * Console drivers may assume that per-cpu resources have been allocated. So

 * unless they're explicitly marked as being able to cope (CON_ANYTIME) don't

 * call them until this CPU is officially up.

 */

static inline int can_use_console(unsigned int cpu)

{
@@ -1466,8 +1465,10 @@ static inline int can_use_console(unsigned int cpu) 
 * console_lock held, and 'console_locked' set) if it

 * is successful, false otherwise.

 */

static int console_trylock_for_printk(unsigned int cpu)

static int console_trylock_for_printk(void)

{

	unsigned int cpu = smp_processor_id();



	if (!console_trylock())

		return 0;

	/*
@@ -1642,7 +1643,8 @@ asmlinkage int vprintk_emit(int facility, int level, 
		 */

		if (!oops_in_progress && !lockdep_recursing(current)) {

			recursion_bug = 1;

			goto out_restore_irqs;

			local_irq_restore(flags);

			return 0;

		}

		zap_locks();

	}
@@ -1750,21 +1752,30 @@ asmlinkage int vprintk_emit(int facility, int level, 


	logbuf_cpu = UINT_MAX;

	raw_spin_unlock(&logbuf_lock);

	lockdep_on();

	local_irq_restore(flags);



	/* If called from the scheduler, we can not call up(). */

	if (!in_sched) {

		lockdep_off();

		/*

		 * Disable preemption to avoid being preempted while holding

		 * console_sem which would prevent anyone from printing to

		 * console

		 */

		preempt_disable();



		/*

		 * Try to acquire and then immediately release the console

		 * semaphore.  The release will print out buffers and wake up

		 * /dev/kmsg and syslog() users.

		 */

		if (console_trylock_for_printk(this_cpu))

		if (console_trylock_for_printk())

			console_unlock();

		preempt_enable();

		lockdep_on();

	}



	lockdep_on();

out_restore_irqs:

	local_irq_restore(flags);

	return printed_len;

}

EXPORT_SYMBOL(vprintk_emit);