[PATCH v2 2/2] efi_pstore: Avoid deadlock in non-blocking paths

[Seiji Aguchi]

[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@xxxxxxx>
---
 drivers/firmware/efivars.c |   11 ++++++++++-
 fs/pstore/platform.c       |    6 +++---
 include/linux/pstore.h     |    6 ++++++
 3 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/drivers/firmware/efivars.c b/drivers/firmware/efivars.c
index 52c5d89..25d464e 100644
--- a/drivers/firmware/efivars.c
+++ b/drivers/firmware/efivars.c
@@ -1210,7 +1210,16 @@ 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_is_non_blocking_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.
diff --git a/fs/pstore/platform.c b/fs/pstore/platform.c
index b87d1de..f959eba 100644
--- a/fs/pstore/platform.c
+++ b/fs/pstore/platform.c
@@ -96,7 +96,7 @@ static const char *get_reason_str(enum kmsg_dump_reason reason)
 	}
 }
 
-static bool is_non_blocking_path(enum kmsg_dump_reason reason)
+bool pstore_is_non_blocking_path(enum kmsg_dump_reason reason)
 {
 	/*
 	 * In case of NMI path, pstore shouldn't be blocked
@@ -134,7 +134,7 @@ static void pstore_dump(struct kmsg_dumper *dumper,
 
 	why = get_reason_str(reason);
 
-	if (is_non_blocking_path(reason)) {
+	if (pstore_is_non_blocking_path(reason)) {
 		is_locked = spin_trylock_irqsave(&psinfo->buf_lock, flags);
 		if (!is_locked) {
 			pr_err("pstore dump routine blocked in %s path, may corrupt error record\n"
@@ -165,7 +165,7 @@ static void pstore_dump(struct kmsg_dumper *dumper,
 		total += hsize + len;
 		part++;
 	}
-	if (is_non_blocking_path(reason)) {
+	if (pstore_is_non_blocking_path(reason)) {
 		if (is_locked)
 			spin_unlock_irqrestore(&psinfo->buf_lock, flags);
 	} else
diff --git a/include/linux/pstore.h b/include/linux/pstore.h
index 1788909..595a040 100644
--- a/include/linux/pstore.h
+++ b/include/linux/pstore.h
@@ -68,12 +68,18 @@ struct pstore_info {
 
 #ifdef CONFIG_PSTORE
 extern int pstore_register(struct pstore_info *);
+extern bool pstore_is_non_blocking_path(enum kmsg_dump_reason reason);
 #else
 static inline int
 pstore_register(struct pstore_info *psi)
 {
 	return -ENODEV;
 }
+static inline bool
+pstore_is_non_blocking_path(enum kmsg_dump_reason reason)
+{
+	return false;
+}
 #endif
 
 #endif /*_LINUX_PSTORE_H*/
-- 
1.7.1

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