[PATCH] vt: don't call unblank at irq time

This patch removes the call to unblank() from printk, and avoids calling
unblank at irq() time _unless_ oops_in_progress is 1.  I also export
oops_in_progress() so drivers who care like radeonfb can test it and know
what to do.  I audited call sites of unblank_screen(), console_unblank(),
etc...  and I _hope_ I got them all, the patch includes a small patch to
the s390 bust_spinlocks code that sets oops_in_progress back to 0 _after_
unblanking for example.

I added a few might_sleep() to help us catch possible remaining callers.

I'll soon write a document explaining fbdev locking.  The current situation
after this patch is that:

- All callbacks have console_semaphore held (fbdev's are fully
  serialised).

- Everything is called in schedule'able context, except the cfb_*
  rendering operations and cursor operations, with the special case of
  unblank who can be called at any time when "oops_in_progress" is true.  A
  driver that needs to sleep in it's unblank implementation is welcome to
  test that variable and use a fallback path (or just do nothing if it's
  not simple).

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 13 insertions, 5 deletions

diff --git a/kernel/printk.c b/kernel/printk.c
index 5d5754964bf4..1498689548d1 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -54,7 +54,12 @@ int console_printk[4] = {
 
 EXPORT_SYMBOL(console_printk);
 
+/*
+ * Low lever drivers may need that to know if they can schedule in
+ * their unblank() callback or not. So let's export it.
+ */
 int oops_in_progress;
+EXPORT_SYMBOL(oops_in_progress);
 
 /*
  * console_sem protects the console_drivers list, and also
@@ -751,12 +756,15 @@ void console_unblank(void)
 	struct console *c;
 
 	/*
-	 * Try to get the console semaphore. If someone else owns it
-	 * we have to return without unblanking because console_unblank
-	 * may be called in interrupt context.
+	 * console_unblank can no longer be called in interrupt context unless
+	 * oops_in_progress is set to 1..
 	 */
-	if (down_trylock(&console_sem) != 0)
-		return;
+	if (oops_in_progress) {
+		if (down_trylock(&console_sem) != 0)
+			return;
+	} else
+		acquire_console_sem();
+
 	console_locked = 1;
 	console_may_schedule = 0;
 	for (c = console_drivers; c != NULL; c = c->next)