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>

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 5ba37f813723b21a34b22eb4a9b6881ec9bfe473..4e22230f1f6c4d272a06c1fea958f9ef3c195ed6 100644 (file)
--- 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;
 }