Fix locking bug in "acquire_console_semaphore_for_printk()"

When I cleaned up printk() and split up the printk locking logic in
commit 266c2e0abeca649fa6667a1a427ad1da507c6375 ("Make printk() console
semaphore accesses sensible") I had incorrectly moved the call to
have_callable_console() outside of the console semaphore.

That was buggy.  The console semaphore protects the console_drivers list
that is used by have_callable_console().

Thanks go to Bongani Hlope who saw this as a hang on shutdown and reboot
and bisected the bug to the right commit, and tested this patch. See

	http://lkml.org/lkml/2008/4/11/315

Bisected-and-tested-by: Bongani Hlope <bonganilinux@mweb.co.za>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 15 insertions, 2 deletions

diff --git a/kernel/printk.c b/kernel/printk.c
index c46a20a19a15..bdd4ea8c3f2b 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -643,8 +643,21 @@ static int acquire_console_semaphore_for_printk(unsigned int cpu)
 {
         int retval = 0;
 
-        if (can_use_console(cpu))
-                retval = !try_acquire_console_sem();
+        if (!try_acquire_console_sem()) {
+                retval = 1;
+
+                /*
+                 * If we can't use the console, we need to release
+                 * the console semaphore by hand to avoid flushing
+                 * the buffer. We need to hold the console semaphore
+                 * in order to do this test safely.
+                 */
+                if (!can_use_console(cpu)) {
+                        console_locked = 0;
+                        up(&console_sem);
+                        retval = 0;
+                }
+        }
         printk_cpu = UINT_MAX;
         spin_unlock(&logbuf_lock);
         return retval;