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Subject[RFC][PATCH v3 2/3] Skip spin_locks in panic case and add WARN_ON()
Patch Description:
- Skip spin_locks in panic case in both kmsg_dump() and pstore_dump() because they are
serialized via smp_send_stop

- Add WARN_ON() in "in_nmi() and !panic" case into kmsg_dump(). Currently, this case never
happens because only kmsg_dump(KMSG_DUMP_PANIC) is called in NMI case.
But if someone adds new kmsg_dump() into NMI path in the future, kmsg_dump() may deadlock.
We can trap it and complain with this WARN_ON().


With this patch, kmsg_dump()/pstore_dump() work as follows.
panic case (KMSG_DUMP_PANIC):
- don't take lock because they are serialized.

not panic case (KMSG_DUMP_OOPS/KMSG_DUMP_EMERG/KMSG_DUMP_RESTART/KMSG_DUMP_HALT):
- take locks normally

Regarding as NMI case,
- kmsg_dump()/pstore_dump() don't take locks, so deadlock issue will not happen
because kmsg_dump() is called in just panic case with current implementation.
- If someone adds new kmsg_dump() into NMI path, WARN_ON() is called.
So we can trap it and ask to fix it.

Signed-off-by: Seiji Aguchi <seiji.aguchi@hds.com>

---
fs/pstore/platform.c | 16 ++++++----------
kernel/printk.c | 13 +++++++++++--
2 files changed, 17 insertions(+), 12 deletions(-)
diff --git a/fs/pstore/platform.c b/fs/pstore/platform.c
index 57bbf90..823669e 100644
--- a/fs/pstore/platform.c
+++ b/fs/pstore/platform.c
@@ -90,18 +90,17 @@ static void pstore_dump(struct kmsg_dumper *dumper,
int hsize, ret;
unsigned int part = 1;
unsigned long flags = 0;
- int is_locked = 0;

if (reason < ARRAY_SIZE(reason_str))
why = reason_str[reason];
else
why = "Unknown";

- 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");
- } else
+ /*
+ * pstore_dump() is serialized in panic path.
+ * So, we don't need to take any locks.
+ */
+ if (reason != KMSG_DUMP_PANIC)
spin_lock_irqsave(&psinfo->buf_lock, flags);
oopscount++;
while (total < kmsg_bytes) {
@@ -131,10 +130,7 @@ static void pstore_dump(struct kmsg_dumper *dumper,
total += l1_cpy + l2_cpy;
part++;
}
- if (in_nmi()) {
- if (is_locked)
- spin_unlock(&psinfo->buf_lock);
- } else
+ if (reason != KMSG_DUMP_PANIC)
spin_unlock_irqrestore(&psinfo->buf_lock, flags);
}

diff --git a/kernel/printk.c b/kernel/printk.c
index 1455a0d..bc5ac61 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -1732,13 +1732,22 @@ void kmsg_dump(enum kmsg_dump_reason reason)
unsigned long l1, l2;
unsigned long flags;

+ WARN_ON(in_nmi() && reason != KMSG_DUMP_PANIC);
+
/* Theoretically, the log could move on after we do this, but
there's not a lot we can do about that. The new messages
will overwrite the start of what we dump. */
- raw_spin_lock_irqsave(&logbuf_lock, flags);
+
+ /*
+ * kmsg_dump(KMSG_DUMP_PANIC) is serialized.
+ * So, we don't need to take any locks.
+ */
+ if (reason != KMSG_DUMP_PANIC)
+ raw_spin_lock_irqsave(&logbuf_lock, flags);
end = log_end & LOG_BUF_MASK;
chars = logged_chars;
- raw_spin_unlock_irqrestore(&logbuf_lock, flags);
+ if (reason != KMSG_DUMP_PANIC)
+ raw_spin_unlock_irqrestore(&logbuf_lock, flags);

if (chars > end) {
s1 = log_buf + log_buf_len - chars + end;
--
1.7.1




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