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SubjectRe: [PATCH] tracing: Have stack tracer call rcu_nmi_enter() only when RCU is not watching
On Thu, Sep 21, 2017 at 01:10:38PM -0400, Steven Rostedt wrote:
>
> Testing some other code, this RCU splat triggered:
>
> =============================
> WARNING: suspicious RCU usage
> 4.13.0-rc7-test+ #117 Tainted: G W
> -----------------------------
> arch/x86/kernel/traps.c:305 entry code didn't wake RCU!
>
> other info that might help us debug this:
>
> RCU used illegally from idle CPU!
> rcu_scheduler_active = 2, debug_locks = 1
> RCU used illegally from extended quiescent state!
> no locks held by swapper/1/0.
>
> stack backtrace:
> CPU: 1 PID: 0 Comm: swapper/1 Tainted: G W 4.13.0-rc7-test+ #117
> Hardware name: Hewlett-Packard HP Compaq Pro 6300 SFF/339A, BIOS K01 v03.03 07/14/2016
> Call Trace:
> dump_stack+0x86/0xcf
> lockdep_rcu_suspicious+0xc5/0x100
> do_error_trap+0x125/0x130
> ? do_error_trap+0x5/0x130
> ? trace_hardirqs_off_thunk+0x1a/0x1c
> ? do_invalid_op+0x5/0x30
> do_invalid_op+0x20/0x30
> invalid_op+0x1e/0x30
> RIP: 0010:module_assert_mutex_or_preempt+0x34/0x40
> RSP: 0018:ffffc900006abc58 EFLAGS: 00010046
> RAX: 0000000000000000 RBX: ffffffffa000a077 RCX: 0000000000000002
> RDX: 0000000000000000 RSI: 00000000ffffffff RDI: 0000000000000046
> RBP: ffffc900006abc58 R08: ffffc900006abf40 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
> R13: 0000000000000000 R14: ffff8801188d8040 R15: ffffffff81ed5720
> ? 0xffffffffa000a077
> ? module_assert_mutex_or_preempt+0x30/0x40
> __module_address+0x2c/0xf0
> ? 0xffffffffa000a077
> __module_text_address+0x12/0x60
> ? 0xffffffffa000a077
> is_module_text_address+0x1f/0x50
> ? 0xffffffffa000a077
> __kernel_text_address+0x30/0x90
> unwind_get_return_address+0x1f/0x30
> __save_stack_trace+0x83/0xd0
> ? 0xffffffffa000a077
> ? rcu_dynticks_eqs_exit+0x5/0x40
> save_stack_trace+0x1b/0x20
> check_stack+0xf8/0x2f0
> ? rcu_dynticks_eqs_enter+0x30/0x30
> stack_trace_call+0x6e/0x80
> 0xffffffffa000a077
> ? ftrace_graph_caller+0x78/0xa8
> ? rcu_dynticks_eqs_exit+0x5/0x40
> rcu_dynticks_eqs_exit+0x5/0x40
> rcu_idle_exit+0xdf/0xf0
> ? rcu_dynticks_eqs_exit+0x5/0x40
> ? rcu_idle_exit+0xdf/0xf0
> do_idle+0x128/0x200
> cpu_startup_entry+0x1d/0x20
> start_secondary+0x108/0x130
> secondary_startup_64+0x9f/0x9f
>
> What happened, was that the CPU was coming out of idle, and
> rcu_dynticks_eqs_exit() was traced by the stack tracer. The stack was bigger
> than the current recorded max stack so the stack tracer triggered a stack
> trace calling save_stack_trace(). As save_stack_trace() performs RCU logic,
> the stack tracer called rcu_irq_enter() before calling save_stack_trace().
> There is two problems here.
>
> 1) rcu_irq_enter() is called unconditionally
>
> Talking with Paul McKenney, it should only be called if rcu_is_watching()
> returns false.
>
> 2) rcu_irq_enter() appears to not function well in this location of the RCU
> internal logic (tracing rcu_dynticks_eqs_exit()), and causes this bug.
>
> The solution is two fold.
>
> 1) check if RCU is watching, and only wake RCU if it is not
> (solves the first issue)
>
> 2) call rcu_nmi_enter() instead of rcu_irq_enter(). It makes more sense as
> tracing acts more like NMIs and not IRQs (can be called when interrupts
> are disabled. (solves the second issue).
>
> Suggested-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Looks good!

Acked-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

> ---
> kernel/trace/trace_stack.c | 10 ++++++++--
> 1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/trace/trace_stack.c b/kernel/trace/trace_stack.c
> index a4df67c..2462e11 100644
> --- a/kernel/trace/trace_stack.c
> +++ b/kernel/trace/trace_stack.c
> @@ -77,6 +77,7 @@ check_stack(unsigned long ip, unsigned long *stack)
> {
> unsigned long this_size, flags; unsigned long *p, *top, *start;
> static int tracer_frame;
> + bool no_rcu;
> int frame_size = ACCESS_ONCE(tracer_frame);
> int i, x;
>
> @@ -104,14 +105,18 @@ check_stack(unsigned long ip, unsigned long *stack)
> if (unlikely(rcu_irq_enter_disabled()))
> return;
>
> + no_rcu = !rcu_is_watching();
> +
> local_irq_save(flags);
> arch_spin_lock(&stack_trace_max_lock);
>
> /*
> * RCU may not be watching, make it see us.
> * The stack trace code uses rcu_sched.
> + * Note, tracing acts more like an NMI than an IRQ.
> */
> - rcu_irq_enter();
> + if (no_rcu)
> + rcu_nmi_enter();
>
> /* In case another CPU set the tracer_frame on us */
> if (unlikely(!frame_size))
> @@ -205,7 +210,8 @@ check_stack(unsigned long ip, unsigned long *stack)
> }
>
> out:
> - rcu_irq_exit();
> + if (no_rcu)
> + rcu_nmi_exit();
> arch_spin_unlock(&stack_trace_max_lock);
> local_irq_restore(flags);
> }
> --
> 2.9.5
>

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