Patch "tracing: Fix stacktrace of latency tracers (irqsoff and friends)" has been added to the 3.3-stable tree

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This is a note to let you know that I've just added the patch titled

    tracing: Fix stacktrace of latency tracers (irqsoff and friends)

to the 3.3-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     tracing-fix-stacktrace-of-latency-tracers-irqsoff-and-friends.patch
and it can be found in the queue-3.3 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.


>From db4c75cbebd7e5910cd3bcb6790272fcc3042857 Mon Sep 17 00:00:00 2001
From: Steven Rostedt <srostedt@xxxxxxxxxx>
Date: Thu, 19 Apr 2012 10:31:47 -0400
Subject: tracing: Fix stacktrace of latency tracers (irqsoff and friends)

From: Steven Rostedt <srostedt@xxxxxxxxxx>

commit db4c75cbebd7e5910cd3bcb6790272fcc3042857 upstream.

While debugging a latency with someone on IRC (mirage335) on #linux-rt (OFTC),
we discovered that the stacktrace output of the latency tracers
(preemptirqsoff) was empty.

This bug was caused by the creation of the dynamic length stack trace
again (like commit 12b5da3 "tracing: Fix ent_size in trace output" was).

This bug is caused by the latency tracers requiring the next event
to determine the time between the current event and the next. But by
grabbing the next event, the iter->ent_size is set to the next event
instead of the current one. As the stacktrace event is the last event,
this makes the ent_size zero and causes nothing to be printed for
the stack trace. The dynamic stacktrace uses the ent_size to determine
how much of the stack can be printed. The ent_size of zero means
no stack.

The simple fix is to save the iter->ent_size before finding the next event.

Note, mirage335 asked to remain anonymous from LKML and git, so I will
not add the Reported-by and Tested-by tags, even though he did report
the issue and tested the fix.

Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 kernel/trace/trace_output.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/kernel/trace/trace_output.c
+++ b/kernel/trace/trace_output.c
@@ -650,6 +650,8 @@ int trace_print_lat_context(struct trace
 {
 	u64 next_ts;
 	int ret;
+	/* trace_find_next_entry will reset ent_size */
+	int ent_size = iter->ent_size;
 	struct trace_seq *s = &iter->seq;
 	struct trace_entry *entry = iter->ent,
 			   *next_entry = trace_find_next_entry(iter, NULL,
@@ -658,6 +660,9 @@ int trace_print_lat_context(struct trace
 	unsigned long abs_usecs = ns2usecs(iter->ts - iter->tr->time_start);
 	unsigned long rel_usecs;
 
+	/* Restore the original ent_size */
+	iter->ent_size = ent_size;
+
 	if (!next_entry)
 		next_ts = iter->ts;
 	rel_usecs = ns2usecs(next_ts - iter->ts);


Patches currently in stable-queue which might be from srostedt@xxxxxxxxxx are

queue-3.3/tracing-fix-stacktrace-of-latency-tracers-irqsoff-and-friends.patch
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