[PATCH 1/3] printk: Release console_sem after logbuf_lock

Peter Zijlstra
2011-06-09T13:14:24Z

[-- Attachment #1: printk-up-after-unlock.patch --]
[-- Type: text/plain, Size: 1545 bytes --]

Release console_sem after unlocking the logbuf_lock so that we don't
generate wakeups while holding logbuf_lock. This avoids some lock
inversion troubles once we remove the lockdep_off bits between
logbuf_lock and rq->lock (prints while holding rq->lock vs doing
wakeups while holding logbuf_lock).

There's of course still an actual deadlock where the printk()s under
rq->lock will issue a wakeup from the up() call.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
kernel/printk.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)

Index: linux-2.6/kernel/printk.c
===================================================================
--- linux-2.6.orig/kernel/printk.c
+++ linux-2.6/kernel/printk.c
@@ -782,7 +782,7 @@ static inline int can_use_console(unsign
static int console_trylock_for_printk(unsigned int cpu)
__releases(&logbuf_lock)
{
- int retval = 0;
+ int retval = 0, wake = 0;

if (console_trylock()) {
retval = 1;
@@ -795,12 +795,14 @@ static int console_trylock_for_printk(un
*/
if (!can_use_console(cpu)) {
console_locked = 0;
- up(&console_sem);
+ wake = 1;
retval = 0;
}
}
printk_cpu = UINT_MAX;
spin_unlock(&logbuf_lock);
+ if (wake)
+ up(&console_sem);
return retval;
}
static const char recursion_bug_msg [] =
@@ -1271,8 +1273,8 @@ void console_unlock(void)
if (unlikely(exclusive_console))
exclusive_console = NULL;

- up(&console_sem);
spin_unlock_irqrestore(&logbuf_lock, flags);
+ up(&console_sem);
if (wake_klogd)
wake_up_klogd();
}