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printk: enable interrupts before calling console_trylock_for_printk()

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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 939f04bec1 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: Jan Kara <jack@suse.cz>
Tested-by: Andreas Bombe <aeb@debian.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Jan Kara 2014-08-06 16:09:10 -07:00 committed by Linus Torvalds
parent 249771b830
commit 5874af2003
1 changed files with 21 additions and 10 deletions
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31
kernel/printk/printk.c
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@ -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);