[PATCH] printk: enable interrupts before calling console_trylock_for_printk()

Jan Kara
2014-10-10T14:24:11Z

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 939f04bec1a4 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).

Tested-by: Andreas Bombe <aeb@debian.org>
Signed-off-by: Jan Kara <jack@suse.cz>
---
kernel/printk/printk.c | 31 +++++++++++++++++++++----------
1 file changed, 21 insertions(+), 10 deletions(-)

Andrew, can you please queue this patch? Thanks.

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 13e839dbca07..fe4154d83fe4 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1416,10 +1416,9 @@ 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)
{
@@ -1432,8 +1431,10 @@ 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;
/*
@@ -1608,7 +1609,8 @@ 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();
}
@@ -1716,21 +1718,30 @@ 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);