Commit 0b5e1c52 authored Jun 07, 2011 by Peter Zijlstra Committed by Ingo Molnar Jun 07, 2011

printk: Release console_sem after logbuf_lock



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, but lockdep won't
warn about that since semaphores are not tracked.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/n/tip-j8swthl12u73h4znbvitljzd@git.kernel.org


Signed-off-by: Ingo Molnar <mingo@elte.hu>

parent 59c5f46f

kernel/printk.c

+5 −3

Original line number	Diff line number	Diff line
		@@ -782,7 +782,7 @@ static inline int can_use_console(unsigned int cpu)
		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(unsigned int cpu)
		*/
		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();
		}

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