[PATCH v2 1/2] pstore: Avoid deadlock in panic andemergency-restart path

From: Seiji Aguchi
Date: Mon Dec 17 2012 - 15:57:20 EST


When pstore is in panic and emergency-restart paths, it may be blocked
in those paths because it simply takes spin_lock.

This is an example scenario which pstore may hang up in a panic path:

- cpuA grabs psinfo->buf_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 buf_lock
- cpuB is deadlocked

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

Also, this is a similar scenario in an emergency-restart path:

- cpuA grabs psinfo->buf_lock and stucks in a firmware
- cpuB kicks emergency-restart via either sysrq-b or hangcheck timer.
And then, cpuB is deadlocked by taking psinfo->buf_lock again.


This patch avoids the deadlocking issues in both panic and emergency_restart
paths by introducing a function, is_non_blocking_path(), to check if a cpu
can be blocked in current path.

With this patch, pstore is not blocked even if another cpu has
taken a spin_lock, in those paths by changing from spin_lock_irqsave
to spin_trylock_irqsave.

In addition, according to a comment of emergency_restart() in kernel/sys.c,
spin_lock shouldn't be taken in an emergency_restart path to avoid
deadlock. This patch fits the comment below.

* emergency_restart - reboot the system
* Without shutting down any hardware or taking any locks
* reboot the system. This is called when we know we are in
* trouble so this is our best effort to reboot. This is
* safe to call in interrupt context.
void emergency_restart(void)

Signed-off-by: Seiji Aguchi <seiji.aguchi@xxxxxxx>
fs/pstore/platform.c | 34 ++++++++++++++++++++++++++++------
1 files changed, 28 insertions(+), 6 deletions(-)

diff --git a/fs/pstore/platform.c b/fs/pstore/platform.c
index 5ea2e77..b87d1de 100644
--- a/fs/pstore/platform.c
+++ b/fs/pstore/platform.c
@@ -96,6 +96,26 @@ static const char *get_reason_str(enum kmsg_dump_reason reason)

+static bool is_non_blocking_path(enum kmsg_dump_reason reason)
+ /*
+ * In case of NMI path, pstore shouldn't be blocked
+ * regardless of reason.
+ */
+ if (in_nmi())
+ return true;
+ switch (reason) {
+ /* In panic case, other cpus are stopped by smp_send_stop(). */
+ /* Emergency restart shouldn't be blocked by spin lock. */
+ return true;
+ default:
+ return false;
+ }
* callback from kmsg_dump. (s2,l2) has the most recently
* written bytes, older bytes are in (s1,l1). Save as much
@@ -114,10 +134,12 @@ static void pstore_dump(struct kmsg_dumper *dumper,

why = get_reason_str(reason);

- if (in_nmi()) {
- is_locked = spin_trylock(&psinfo->buf_lock);
- if (!is_locked)
- pr_err("pstore dump routine blocked in NMI, may corrupt error record\n");
+ if (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"
+ , in_nmi() ? "NMI" : why);
+ }
} else
spin_lock_irqsave(&psinfo->buf_lock, flags);
@@ -143,9 +165,9 @@ static void pstore_dump(struct kmsg_dumper *dumper,
total += hsize + len;
- if (in_nmi()) {
+ if (is_non_blocking_path(reason)) {
if (is_locked)
- spin_unlock(&psinfo->buf_lock);
+ spin_unlock_irqrestore(&psinfo->buf_lock, flags);
} else
spin_unlock_irqrestore(&psinfo->buf_lock, flags);

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