[3.19.y-ckt stable] Patch "arm64: kernel: pause/unpause function graph tracer in cpu_suspend()" has been added to staging queue

[Kamal Mostafa]

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

    arm64: kernel: pause/unpause function graph tracer in cpu_suspend()

to the linux-3.19.y-queue branch of the 3.19.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.19.y-queue

This patch is scheduled to be released in version 3.19.8-ckt12.

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

For more information about the 3.19.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From bf921c85248a3c75819e72c1fe3f8a748cc149dd Mon Sep 17 00:00:00 2001
From: Lorenzo Pieralisi <lorenzo.pieralisi at arm.com>
Date: Tue, 17 Nov 2015 11:50:51 +0000
Subject: arm64: kernel: pause/unpause function graph tracer in cpu_suspend()

commit de818bd4522c40ea02a81b387d2fa86f989c9623 upstream.

The function graph tracer adds instrumentation that is required to trace
both entry and exit of a function. In particular the function graph
tracer updates the "return address" of a function in order to insert
a trace callback on function exit.

Kernel power management functions like cpu_suspend() are called
upon power down entry with functions called "finishers" that are in turn
called to trigger the power down sequence but they may not return to the
kernel through the normal return path.

When the core resumes from low-power it returns to the cpu_suspend()
function through the cpu_resume path, which leaves the trace stack frame
set-up by the function tracer in an incosistent state upon return to the
kernel when tracing is enabled.

This patch fixes the issue by pausing/resuming the function graph
tracer on the thread executing cpu_suspend() (ie the function call that
subsequently triggers the "suspend finishers"), so that the function graph
tracer state is kept consistent across functions that enter power down
states and never return by effectively disabling graph tracer while they
are executing.

Fixes: 819e50e25d0c ("arm64: Add ftrace support")
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi at arm.com>
Reported-by: Catalin Marinas <catalin.marinas at arm.com>
Reported-by: AKASHI Takahiro <takahiro.akashi at linaro.org>
Suggested-by: Steven Rostedt <rostedt at goodmis.org>
Acked-by: Steven Rostedt <rostedt at goodmis.org>
Cc: Will Deacon <will.deacon at arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas at arm.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 arch/arm64/kernel/suspend.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/arm64/kernel/suspend.c b/arch/arm64/kernel/suspend.c
index 2d6b606..23e35de 100644
--- a/arch/arm64/kernel/suspend.c
+++ b/arch/arm64/kernel/suspend.c
@@ -1,3 +1,4 @@
+#include <linux/ftrace.h>
 #include <linux/percpu.h>
 #include <linux/slab.h>
 #include <asm/cacheflush.h>
@@ -92,6 +93,13 @@ int __cpu_suspend(unsigned long arg, int (*fn)(unsigned long))
 	local_dbg_save(flags);

 	/*
+	 * Function graph tracer state gets incosistent when the kernel
+	 * calls functions that never return (aka suspend finishers) hence
+	 * disable graph tracing during their execution.
+	 */
+	pause_graph_tracing();
+
+	/*
 	 * mm context saved on the stack, it will be restored when
 	 * the cpu comes out of reset through the identity mapped
 	 * page tables, so that the thread address space is properly
@@ -128,6 +136,8 @@ int __cpu_suspend(unsigned long arg, int (*fn)(unsigned long))
 			hw_breakpoint_restore(NULL);
 	}

+	unpause_graph_tracing();
+
 	/*
 	 * Restore pstate flags. OS lock and mdscr have been already
 	 * restored, so from this point onwards, debugging is fully
--
1.9.1