Patch "ARM: 8393/1: smp: Fix suspicious RCU usage with ipi tracepoints" has been added to the 4.1-stable tree

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

ARM: 8393/1: smp: Fix suspicious RCU usage with ipi tracepoints

to the 4.1-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:
arm-8393-1-smp-fix-suspicious-rcu-usage-with-ipi-tracepoints.patch
and it can be found in the queue-4.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Stephen Boyd <[email protected]>
Date: Fri, 19 Jun 2015 21:37:56 +0100
Subject: ARM: 8393/1: smp: Fix suspicious RCU usage with ipi tracepoints

From: Stephen Boyd <[email protected]>

commit 398f74569cebbf06bc6b069442bcd0e9616ca465 upstream.

John Stultz reports an RCU splat on boot with ARM ipi trace events enabled.

===============================
[ INFO: suspicious RCU usage. ]
4.1.0-rc7-00033-gb5bed2f #153 Not tainted
-------------------------------
include/trace/events/ipi.h:68 suspicious rcu_dereference_check() usage!

other info that might help us debug this:

RCU used illegally from idle CPU!
rcu_scheduler_active = 1, debug_locks = 0
RCU used illegally from extended quiescent state!
no locks held by swapper/0/0.

stack backtrace:
CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.1.0-rc7-00033-gb5bed2f #153
Hardware name: Qualcomm (Flattened Device Tree)
[<c0216b08>] (unwind_backtrace) from [<c02136e8>] (show_stack+0x10/0x14)
[<c02136e8>] (show_stack) from [<c075e678>] (dump_stack+0x70/0xbc)
[<c075e678>] (dump_stack) from [<c0215a80>] (handle_IPI+0x428/0x604)
[<c0215a80>] (handle_IPI) from [<c020942c>] (gic_handle_irq+0x54/0x5c)
[<c020942c>] (gic_handle_irq) from [<c0766604>] (__irq_svc+0x44/0x7c)
Exception stack(0xc09f3f48 to 0xc09f3f90)

3f40: 00000001 00000001 00000000 c09f73b8 c09f4528 c0a5de9c
3f60: c076b4f0 00000000 00000000 c09ef108 c0a5cec1 00000001 00000000 c09f3f90 3f80: c026bf60 c0210ab8 20000113 ffffffff
[<c0766604>] (__irq_svc) from [<c0210ab8>] (arch_cpu_idle+0x20/0x3c) [<c0210ab8>] (arch_cpu_idle) from [<c02647f0>] (cpu_startup_entry+0x2c0/0x5dc) [<c02647f0>] (cpu_startup_entry) from [<c099bc1c>] (start_kernel+0x358/0x3c4) [<c099bc1c>] (start_kernel) from [<8020807c>] (0x8020807c)

At this point in the IPI handling path we haven't called irq_enter() yet, so RCU doesn't know that we're about to exit idle and properly warns that we're using RCU from an idle CPU. Use trace_ipi_entry_rcuidle() instead of trace_ipi_entry() so that RCU is informed about our exit from idle.

---
 arch/arm/kernel/smp.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

@@ -576,7 +576,7 @@ void handle_IPI(int ipinr, struct pt_reg
 	struct pt_regs *old_regs = set_irq_regs(regs);
 
 	if ((unsigned)ipinr < NR_IPI) {
-		trace_ipi_entry(ipi_types[ipinr]);
+		trace_ipi_entry_rcuidle(ipi_types[ipinr]);
 		__inc_irq_stat(cpu, ipi_irqs[ipinr]);
 	}
 

@@ -635,7 +635,7 @@ void handle_IPI(int ipinr, struct pt_reg
 	}
 
 	if ((unsigned)ipinr < NR_IPI)
-		trace_ipi_exit(ipi_types[ipinr]);
+		trace_ipi_exit_rcuidle(ipi_types[ipinr]);
 	set_irq_regs(old_regs);
 }
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.1/arm-8393-1-smp-fix-suspicious-rcu-usage-with-ipi-tracepoints.patch queue-4.1/clk-qcom-use-parent-rate-when-set-rate-to-pixel-rcg-clock.patch queue-4.1/clk-fix-json-output-in-debugfs.patch queue-4.1/clk-ti-dra7-atl-clock-fix-possible-err_ptr-dereference.patch queue-4.1/arm64-smp-fix-suspicious-rcu-usage-with-ipi-tracepoints.patch