[RFC][PATCH] pstore: Skip spinlock when just one cpu is online

[Seiji Aguchi]

[Issue]

If one cpu ,which is taking a psinfo->buf_lock, 
receive NMI from a panicked cpu via smp_send_stop(),
the panicked cpu hangs up in pstore_dump() called by kmsg_dump(KMSG_DUMP_PANIC)
because the psinfo->buf_lock is taken again in it.

To avoid the deadlock, an easy solution is moving kmsg_dump above
smp_send_stop() in panic path.

But, it is not safe to kick pstore while multiple cpus are running in panic case,
because they may touch corrupted data/variables and unnecessary failures may happen.
In that case, we can't guarantee that a panicked cpu can log messages reliably
because it may have harmful effects due to the failures.

[Solution]

This patch skips taking a psinfo->buf_lock when just one cpu is online
because stopped cpus turn to offline via smp_send_stop()
in some architectures like x86, powerpc or arm64.

It may be a hack but solves my concern deadlocking in x86 architecture.

Signed-off-by: Seiji Aguchi <seiji.aguchi@xxxxxxx>
---
 fs/pstore/platform.c |   14 +++++++++++---
 1 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/fs/pstore/platform.c b/fs/pstore/platform.c
index 947fbe0..ca4d2ab 100644
--- a/fs/pstore/platform.c
+++ b/fs/pstore/platform.c
@@ -107,7 +107,7 @@ static void pstore_dump(struct kmsg_dumper *dumper,
 	unsigned long	total = 0;
 	const char	*why;
 	u64		id;
-	unsigned int	part = 1;
+	unsigned int	part = 1, cpu_num = num_online_cpus();
 	unsigned long	flags = 0;
 	int		is_locked = 0;
 	int		ret;
@@ -118,8 +118,14 @@ static void pstore_dump(struct kmsg_dumper *dumper,
 		is_locked = spin_trylock(&psinfo->buf_lock);
 		if (!is_locked)
 			pr_err("pstore dump routine blocked in NMI, may corrupt error record\n");
-	} else
+	} else if (cpu_num > 1) {
+		/*
+		 * Take a spin lock only when multiple cpus are online.
+		 */
 		spin_lock_irqsave(&psinfo->buf_lock, flags);
+	} else
+		local_irq_save(flags);
+
 	oopscount++;
 	while (total < kmsg_bytes) {
 		char *dst;
@@ -146,8 +152,10 @@ static void pstore_dump(struct kmsg_dumper *dumper,
 	if (in_nmi()) {
 		if (is_locked)
 			spin_unlock(&psinfo->buf_lock);
-	} else
+	} else if (cpu_num > 1) {
 		spin_unlock_irqrestore(&psinfo->buf_lock, flags);
+	} else
+		local_irq_restore(flags);
 }
 
 static struct kmsg_dumper pstore_dumper = {
-- 1.7.1
--
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