efi_pstore: Avoid deadlock in non-blocking paths

[Issue]

There is a scenario which efi_pstore may hang up:

 - cpuA grabs efivars->lock
 - cpuB panics and calls smp_send_stop
 - smp_send_stop sends IRQ to cpuA
 - after 1 second, cpuB gives up on cpuA and sends an NMI instead
 - cpuA is now in an NMI handler while still holding efivars->lock
 - cpuB is deadlocked

This case may happen if a firmware has a bug and
cpuA is stuck talking with it.

[Solution]

This patch changes a spin_lock to a spin_trylock in non-blocking paths.
and if the spin_lock has already taken by another cpu,
it returns without accessing to a firmware to avoid the deadlock.

Signed-off-by: Seiji Aguchi <seiji.aguchi@hds.com>
Acked-by: Don Zickus <dzickus@redhat.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>

1 files changed, 10 insertions, 1 deletions

diff --git a/drivers/firmware/efivars.c b/drivers/firmware/efivars.c
index 7b1c37497c9a..ef5070d86f88 100644
--- a/drivers/firmware/efivars.c
+++ b/drivers/firmware/efivars.c
@@ -1209,7 +1209,16 @@ static int efi_pstore_write(enum pstore_type_id type,
         u64 storage_space, remaining_space, max_variable_size;
         efi_status_t status = EFI_NOT_FOUND;
 
-        spin_lock(&efivars->lock);
+        if (pstore_cannot_block_path(reason)) {
+                /*
+                 * If the lock is taken by another cpu in non-blocking path,
+                 * this driver returns without entering firmware to avoid
+                 * hanging up.
+                 */
+                if (!spin_trylock(&efivars->lock))
+                        return -EBUSY;
+        } else
+                spin_lock(&efivars->lock);
 
         /*
          * Check if there is a space enough to log.