[3.11.y.z extended stable] Patch "tracing: Try again for saved cmdline if failed due to locking" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    tracing: Try again for saved cmdline if failed due to locking

to the linux-3.11.y-queue branch of the 3.11.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.11.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.11.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From fab9cca2a7c7ac608f754bdb81201f6155a3252c Mon Sep 17 00:00:00 2001
From: "Steven Rostedt (Red Hat)" <rostedt at goodmis.org>
Date: Fri, 30 May 2014 09:42:39 -0400
Subject: tracing: Try again for saved cmdline if failed due to locking

commit 379cfdac37923653c9d4242d10052378b7563005 upstream.

In order to prevent the saved cmdline cache from being filled when
tracing is not active, the comms are only recorded after a trace event
is recorded.

The problem is, a comm can fail to be recorded if the trace_cmdline_lock
is held. That lock is taken via a trylock to allow it to happen from
any context (including NMI). If the lock fails to be taken, the comm
is skipped. No big deal, as we will try again later.

But! Because of the code that was added to only record after an event,
we may not try again later as the recording is made as a oneshot per
event per CPU.

Only disable the recording of the comm if the comm is actually recorded.

Fixes: 7ffbd48d5cab "tracing: Cache comms only after an event occurred"
Signed-off-by: Steven Rostedt <rostedt at goodmis.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 kernel/trace/trace.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index f788ecfee618..56ecc310b818 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -1417,12 +1417,12 @@ static void tracing_stop_tr(struct trace_array *tr)

 void trace_stop_cmdline_recording(void);

-static void trace_save_cmdline(struct task_struct *tsk)
+static int trace_save_cmdline(struct task_struct *tsk)
 {
 	unsigned pid, idx;

 	if (!tsk->pid || unlikely(tsk->pid > PID_MAX_DEFAULT))
-		return;
+		return 0;

 	/*
 	 * It's not the end of the world if we don't get
@@ -1431,7 +1431,7 @@ static void trace_save_cmdline(struct task_struct *tsk)
 	 * so if we miss here, then better luck next time.
 	 */
 	if (!arch_spin_trylock(&trace_cmdline_lock))
-		return;
+		return 0;

 	idx = map_pid_to_cmdline[tsk->pid];
 	if (idx == NO_CMDLINE_MAP) {
@@ -1456,6 +1456,8 @@ static void trace_save_cmdline(struct task_struct *tsk)
 	memcpy(&saved_cmdlines[idx], tsk->comm, TASK_COMM_LEN);

 	arch_spin_unlock(&trace_cmdline_lock);
+
+	return 1;
 }

 void trace_find_cmdline(int pid, char comm[])
@@ -1497,9 +1499,8 @@ void tracing_record_cmdline(struct task_struct *tsk)
 	if (!__this_cpu_read(trace_cmdline_save))
 		return;

-	__this_cpu_write(trace_cmdline_save, false);
-
-	trace_save_cmdline(tsk);
+	if (trace_save_cmdline(tsk))
+		__this_cpu_write(trace_cmdline_save, false);
 }

 void
--
1.9.1