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: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 18 insertions, 11 deletions

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 5ba37f813723..4e22230f1f6c 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -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;
 }