[PATCH V4] firmware: google: Test spinlock on panic path to avoid lockups

[Guilherme G. Piccoli]

Currently the gsmi driver registers a panic notifier as well as
reboot and die notifiers. The callbacks registered are called in
atomic and very limited context - for instance, panic disables
preemption and local IRQs, also all secondary CPUs (not executing
the panic path) are shutdown.

With that said, taking a spinlock in this scenario is a dangerous
invitation for lockup scenarios. So, fix that by checking if the
spinlock is free to acquire in the panic notifier callback - if not,
bail-out and avoid a potential hang.

Fixes: 74c5b31c6618 ("driver: Google EFI SMI")
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Ard Biesheuvel <ardb@xxxxxxxxxx>
Cc: David Gow <davidgow@xxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Julius Werner <jwerner@xxxxxxxxxxxx>
Cc: Petr Mladek <pmladek@xxxxxxxx>
Reviewed-by: Evan Green <evgreen@xxxxxxxxxxxx>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@xxxxxxxxxx>
---


Hey folks, new version - V3 available at [0].

@Greg, I think the panic situation is well-described in the commit
message so I didn't rework that, lemme know if you want to add
something to the text.

@Evan, I kept your review tag even with the comment change, based on
the V3 discussion. Lemme know if you want it removed in case you
disagree with something!

V4:
* Reworked comment (thanks Andrew, for the suggestion!);
* Rebased against 6.0-rc4;
* Included more CCs (Andrew, Petr).

[0] https://lore.kernel.org/linux-kernel/20220819155059.451674-1-gpiccoli@xxxxxxxxxx/


 drivers/firmware/google/gsmi.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/firmware/google/gsmi.c b/drivers/firmware/google/gsmi.c
index adaa492c3d2d..4e2575dfeb90 100644
--- a/drivers/firmware/google/gsmi.c
+++ b/drivers/firmware/google/gsmi.c
@@ -681,6 +681,15 @@ static struct notifier_block gsmi_die_notifier = {
 static int gsmi_panic_callback(struct notifier_block *nb,
 			       unsigned long reason, void *arg)
 {
+
+	/*
+	 * Panic callbacks are executed with all other CPUs stopped,
+	 * so we must not attempt to spin waiting for gsmi_dev.lock
+	 * to be released.
+	 */
+	if (spin_is_locked(&gsmi_dev.lock))
+		return NOTIFY_DONE;
+
 	gsmi_shutdown_reason(GSMI_SHUTDOWN_PANIC);
 	return NOTIFY_DONE;
 }
-- 
2.37.2