[PATCH] vt: printk: Fix framebuffer console triggering might_sleep assertion

Reported by: Dave Jones

Whilst printk'ing to both console and serial console, I got this...
(2.6.18rc1)

BUG: sleeping function called from invalid context at kernel/sched.c:4438
in_atomic():0, irqs_disabled():1

Call Trace:
 [<ffffffff80271db8>] show_trace+0xaa/0x23d
 [<ffffffff80271f60>] dump_stack+0x15/0x17
 [<ffffffff8020b9f8>] __might_sleep+0xb2/0xb4
 [<ffffffff8029232e>] __cond_resched+0x15/0x55
 [<ffffffff80267eb8>] cond_resched+0x3b/0x42
 [<ffffffff80268c64>] console_conditional_schedule+0x12/0x14
 [<ffffffff80368159>] fbcon_redraw+0xf6/0x160
 [<ffffffff80369c58>] fbcon_scroll+0x5d9/0xb52
 [<ffffffff803a43c4>] scrup+0x6b/0xd6
 [<ffffffff803a4453>] lf+0x24/0x44
 [<ffffffff803a7ff8>] vt_console_print+0x166/0x23d
 [<ffffffff80295528>] __call_console_drivers+0x65/0x76
 [<ffffffff80295597>] _call_console_drivers+0x5e/0x62
 [<ffffffff80217e3f>] release_console_sem+0x14b/0x232
 [<ffffffff8036acd6>] fb_flashcursor+0x279/0x2a6
 [<ffffffff80251e3f>] run_workqueue+0xa8/0xfb
 [<ffffffff8024e5e0>] worker_thread+0xef/0x122
 [<ffffffff8023660f>] kthread+0x100/0x136
 [<ffffffff8026419e>] child_rip+0x8/0x12

This can occur when release_console_sem() is called but the log
buffer still has contents that need to be flushed. The console drivers
are called while the console_may_schedule flag is still true. The
might_sleep() is triggered when fbcon calls console_conditional_schedule().

Fix by setting console_may_schedule to zero earlier, before the call to the
console drivers.

Signed-off-by: Antonino Daplas <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/printk.c

@@ -799,6 +799,9 @@ void release_console_sem(void)
 		up(&secondary_console_sem);
 		return;
 	}
+
+	console_may_schedule = 0;
+
 	for ( ; ; ) {
 		spin_lock_irqsave(&logbuf_lock, flags);
 		wake_klogd |= log_start - log_end;
@@ -812,7 +815,6 @@ void release_console_sem(void)
 		local_irq_restore(flags);
 	}
 	console_locked = 0;
-	console_may_schedule = 0;
 	up(&console_sem);
 	spin_unlock_irqrestore(&logbuf_lock, flags);
 	if (wake_klogd && !oops_in_progress && waitqueue_active(&log_wait)) {