Re: Suspicious RCU usage at boot w/ arm ipi trace events?

From: Stephen Boyd
Date: Wed Jun 17 2015 - 21:49:54 EST

On 06/16/2015 09:46 PM, Paul E. McKenney wrote:
> On Tue, Jun 16, 2015 at 05:41:29PM -0700, Stephen Boyd wrote:
>> The tracepoint 'trace_ipi_entry' in handle_IPI() is using RCU and we
>> haven't called irq_enter() yet at the point. Does this tracepoint need
>> to have _rcuidle() added to it?
> Yes, I believe that would fix this problem.

Ok... here's the patch. I see the problem on my device and applying this
patch fixes it.


From: Stephen Boyd <sboyd@xxxxxxxxxxxxxx>
Subject: [PATCH] ARM: smp: Silence suspicious RCU usage with ipi tracepoints

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.

Reported-by: John Stultz <john.stultz@xxxxxxxxxx>
Fixes: 365ec7b17327 "ARM: add IPI tracepoints"
Signed-off-by: Stephen Boyd <sboyd@xxxxxxxxxxxxxx>
arch/arm/kernel/smp.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
index 13a91d390832..03eb8a446dca 100644
--- a/arch/arm/kernel/smp.c
+++ b/arch/arm/kernel/smp.c
@@ -589,7 +589,7 @@ void handle_IPI(int ipinr, struct pt_regs *regs)
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]);

@@ -648,7 +648,7 @@ void handle_IPI(int ipinr, struct pt_regs *regs)

if ((unsigned)ipinr < NR_IPI)
- trace_ipi_exit(ipi_types[ipinr]);
+ trace_ipi_exit_rcuidle(ipi_types[ipinr]);

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