linux-kernel - [RFC][PATCH v4 -next 2/4] Skip spin_locks in panic case and Add WARN

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Date:	Thu, 5 Jan 2012 12:38:08 -0500
From:	Seiji Aguchi <seiji.aguchi@....com>
To:	"Luck, Tony" <tony.luck@...el.com>, Don Zickus <dzickus@...hat.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Matthew Garrett <mjg@...hat.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	"Chen, Gong" <gong.chen@...el.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"Brown, Len" <len.brown@...el.com>,
	"'ying.huang@...el.com'" <'ying.huang@...el.com'>,
	"'ak@...ux.intel.com'" <'ak@...ux.intel.com'>,
	"'hughd@...omium.org'" <'hughd@...omium.org'>,
	"'mingo@...e.hu'" <'mingo@...e.hu'>,
	"jmorris@...ei.org" <jmorris@...ei.org>,
	"a.p.zijlstra@...llo.nl" <a.p.zijlstra@...llo.nl>,
	"namhyung@...il.com" <namhyung@...il.com>,
	"dle-develop@...ts.sourceforge.net" 
	<dle-develop@...ts.sourceforge.net>,
	Satoru Moriya <satoru.moriya@....com>
Subject: [RFC][PATCH v4 -next 2/4] Skip spin_locks in panic case and Add
 WARN_ON()

This patch does followings.

   - 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@....com>

---
 fs/pstore/platform.c |   16 ++++++----------
 kernel/printk.c      |   20 ++++++++++++++++++--
 2 files changed, 24 insertions(+), 12 deletions(-)

diff --git a/fs/pstore/platform.c b/fs/pstore/platform.c
index 9ec22d3..426e77d 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 13c0a11..5294426 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -1732,13 +1732,29 @@ void kmsg_dump(enum kmsg_dump_reason reason)
 	unsigned long l1, l2;
 	unsigned long flags;
 
+	/*
+	 * Currently, "in_nmi() && reason != KMSG_DUMP_PANIC" case never happens
+	 * because ,in NMI path, only kmsg_dump(KMSG_DUMP_PANIC) is called.
+	 * 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().
+	 */
+	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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