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Commit 939f04be 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()
especially since later in the patch series we will want to spin on
console_sem in console_trylock_for_printk().

Signed-off-by: default avatarJan Kara <jack@suse.cz>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent bd8d7cf5

kernel/printk/printk.c

+18 −11
Original line number Diff line number Diff line
@@ -1418,10 +1418,9 @@ 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)

{
@@ -1434,8 +1433,10 @@ 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;

	/*
@@ -1605,7 +1606,8 @@ 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();

	}
@@ -1708,17 +1710,22 @@ asmlinkage int vprintk_emit(int facility, int level, 


	logbuf_cpu = UINT_MAX;

	raw_spin_unlock(&logbuf_lock);

	lockdep_on();

	local_irq_restore(flags);



	/*

	 * 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();



	lockdep_on();

out_restore_irqs:

	local_irq_restore(flags);

	preempt_enable();



	return printed_len;

}