stackdepot: fix stack_depot_save_flags() in NMI context

Per documentation, stack_depot_save_flags() was meant to be usable from
NMI context if STACK_DEPOT_FLAG_CAN_ALLOC is unset. However, it still
would try to take the pool_lock in an attempt to save a stack trace in the
current pool (if space is available).

This could result in deadlock if an NMI is handled while pool_lock is
already held. To avoid deadlock, only try to take the lock in NMI context
and give up if unsuccessful.

The documentation is fixed to clearly convey this.

Link: https://lkml.kernel.org/r/[e-mail]
Link: https://lkml.kernel.org/r/[e-mail]
Fixes: 4434a56ec209 ("stackdepot: make fast paths lock-less again")
Signed-off-by: Marco Elver <[e-mail]>
Reported-by: Sebastian Andrzej Siewior <[e-mail]>
Reviewed-by: Sebastian Andrzej Siewior <[e-mail]>
Cc: Alexander Potapenko <[e-mail]>
Cc: Andrey Konovalov <[e-mail]>
Cc: Dmitry Vyukov <[e-mail]>
Cc: Oscar Salvador <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/stackdepot.h

···
  * If the provided stack trace comes from the interrupt context, only the part
  * up to the interrupt entry is saved.
  *
- * Context: Any context, but setting STACK_DEPOT_FLAG_CAN_ALLOC is required if
+ * Context: Any context, but unsetting STACK_DEPOT_FLAG_CAN_ALLOC is required if
  *          alloc_pages() cannot be used from the current context. Currently
  *          this is the case for contexts where neither %GFP_ATOMIC nor
  *          %GFP_NOWAIT can be used (NMI, raw_spin_lock).
···
  */
 depot_stack_handle_t stack_depot_save_flags(unsigned long *entries,
 					    unsigned int nr_entries,
-					    gfp_t gfp_flags,
+					    gfp_t alloc_flags,
 					    depot_flags_t depot_flags);
 
 /**
···
  * Return: Handle of the stack trace stored in depot, 0 on failure
  */
 depot_stack_handle_t stack_depot_save(unsigned long *entries,
-				      unsigned int nr_entries, gfp_t gfp_flags);
+				      unsigned int nr_entries, gfp_t alloc_flags);
 
 /**
  * __stack_depot_get_stack_record - Get a pointer to a stack_record struct

lib/stackdepot.c

···
 			prealloc = page_address(page);
 	}
 
-	raw_spin_lock_irqsave(&pool_lock, flags);
+	if (in_nmi()) {
+		/* We can never allocate in NMI context. */
+		WARN_ON_ONCE(can_alloc);
+		/* Best effort; bail if we fail to take the lock. */
+		if (!raw_spin_trylock_irqsave(&pool_lock, flags))
+			goto exit;
+	} else {
+		raw_spin_lock_irqsave(&pool_lock, flags);
+	}
 	printk_deferred_enter();
 
 	/* Try to find again, to avoid concurrently inserting duplicates. */