printk: Fix console_sem vs logbuf_lock unlock race

Fix up the fallout from commit 0b5e1c5255 ("printk: Release
console_sem after logbuf_lock").

The reason for unlocking the console_sem under the logbuf_lock
is that a concurrent printk() might fill up the buffer but fail
to acquire the console sem, resulting in a missed write to the
console until a subsequent console_sem acquire/release cycle.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: efault@gmx.de
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/1308734409.1022.14.camel@twins
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 18 insertions, 2 deletions

diff --git a/kernel/printk.c b/kernel/printk.c
index 751e7b84e9e3..37dff3429adb 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -1244,7 +1244,7 @@ void console_unlock(void)
 {
         unsigned long flags;
         unsigned _con_start, _log_end;
-        unsigned wake_klogd = 0;
+        unsigned wake_klogd = 0, retry = 0;
 
         if (console_suspended) {
                 up(&console_sem);
@@ -1253,6 +1253,7 @@ void console_unlock(void)
 
         console_may_schedule = 0;
 
+again:
         for ( ; ; ) {
                 spin_lock_irqsave(&logbuf_lock, flags);
                 wake_klogd |= log_start - log_end;
@@ -1273,8 +1274,23 @@ void console_unlock(void)
         if (unlikely(exclusive_console))
                 exclusive_console = NULL;
 
-        spin_unlock_irqrestore(&logbuf_lock, flags);
+        spin_unlock(&logbuf_lock);
+
         up(&console_sem);
+
+        /*
+         * Someone could have filled up the buffer again, so re-check if there's
+         * something to flush. In case we cannot trylock the console_sem again,
+         * there's a new owner and the console_unlock() from them will do the
+         * flush, no worries.
+         */
+        spin_lock(&logbuf_lock);
+        if (con_start != log_end)
+                retry = 1;
+        spin_unlock_irqrestore(&logbuf_lock, flags);
+        if (retry && console_trylock())
+                goto again;
+
         if (wake_klogd)
                 wake_up_klogd();
 }