[tip:core/printk] ratelimit: Fix/allow use in atomic contexts

[tip-bot for Ingo Molnar]

Commit-ID:  edaac8e3167501cda336231d00611bf59c164346
Gitweb:     http://git.kernel.org/tip/edaac8e3167501cda336231d00611bf59c164346
Author:     Ingo Molnar <mingo@xxxxxxx>
AuthorDate: Tue, 22 Sep 2009 14:44:11 +0200
Committer:  Ingo Molnar <mingo@xxxxxxx>
CommitDate: Tue, 22 Sep 2009 14:05:48 +0200

ratelimit: Fix/allow use in atomic contexts

I'd like to use printk_ratelimit() in NMI context, but it's not
robust right now due to spinlock usage in lib/ratelimit.c. If an
NMI is unlucky enough to hit just that spot we might lock up trying
to take the spinlock again.

Fix that by using a trylock variant. If we contend on that lock we
can genuinely skip the message because the state is just being
accessed by another CPU (or by this CPU).

( We could use atomics for the suppressed messages field, but
  i doubt it matters in practice and it makes the code heavier. )

Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: David S. Miller <davem@xxxxxxxxxxxxx>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>


---
 lib/ratelimit.c |   10 +++++++++-
 1 files changed, 9 insertions(+), 1 deletions(-)

diff --git a/lib/ratelimit.c b/lib/ratelimit.c
index 0e2c28e..69bfcac 100644
--- a/lib/ratelimit.c
+++ b/lib/ratelimit.c
@@ -28,7 +28,15 @@ int __ratelimit(struct ratelimit_state *rs)
 	if (!rs->interval)
 		return 1;
 
-	spin_lock_irqsave(&rs->lock, flags);
+	/*
+	 * If we contend on this state's lock then almost
+	 * by definition we are too busy to print a message,
+	 * in addition to the one that will be printed by
+	 * the entity that is holding the lock already:
+	 */
+	if (!spin_trylock_irqsave(&rs->lock, flags))
+		return 1;
+
 	if (!rs->begin)
 		rs->begin = jiffies;
 
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