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Date:   Wed, 5 Sep 2018 08:05:46 -0700
From:   "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Borislav Petkov <bp@alien8.de>, x86-ml <x86@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        lkml <linux-kernel@vger.kernel.org>, joel@joelfernandes.org
Subject: Re: [PATCH] tracing: Add back in rcu_irq_enter/exit_irqson() for
 rcuidle tracepoints
Reply-To: paulmck@linux.vnet.ibm.com
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On Wed, Sep 05, 2018 at 09:26:29AM -0400, Steven Rostedt wrote:
> On Wed, 5 Sep 2018 05:59:41 -0700
> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> 
> > On Wed, Sep 05, 2018 at 10:22:54AM +0200, Borislav Petkov wrote:
> > > On Tue, Sep 04, 2018 at 01:53:21PM -0700, Paul E. McKenney wrote:  
> > > > I must defer to Borislav on this one.  Assuming it has the desired
> > > > effect, I am good with it.  
> > > 
> > > It did survive a bunch of reboots (the WARN would fire after boot
> > > finishes, normally) so I guess we can run with it and see how it works
> > > out in the next couple of weeks.
> > > 
> > > Thanks guys!  
> > 
> > Woo-hoo!!!  Thank you for testing this!
> > 
> 
> Here's the official patch if you want to add an Ack/review/tested-by:
> 
> -- Steve
> 
> From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> 
> Borislav reported the following splat:
> 
>  =============================
>  WARNING: suspicious RCU usage
>  4.19.0-rc1+ #1 Not tainted
>  -----------------------------
>  ./include/linux/rcupdate.h:631 rcu_read_lock() used illegally while idle!
>  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!
>  1 lock held by swapper/0/0:
>   #0: 000000004557ee0e (rcu_read_lock){....}, at: perf_event_output_forward+0x0/0x130
> 
>  stack backtrace:
>  CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.19.0-rc1+ #1
>  Hardware name: LENOVO 2320CTO/2320CTO, BIOS G2ET86WW (2.06 ) 11/13/2012
>  Call Trace:
>   dump_stack+0x85/0xcb
>   perf_event_output_forward+0xf6/0x130
>   __perf_event_overflow+0x52/0xe0
>   perf_swevent_overflow+0x91/0xb0
>   perf_tp_event+0x11a/0x350
>   ? find_held_lock+0x2d/0x90
>   ? __lock_acquire+0x2ce/0x1350
>   ? __lock_acquire+0x2ce/0x1350
>   ? retint_kernel+0x2d/0x2d
>   ? find_held_lock+0x2d/0x90
>   ? tick_nohz_get_sleep_length+0x83/0xb0
>   ? perf_trace_cpu+0xbb/0xd0
>   ? perf_trace_buf_alloc+0x5a/0xa0
>   perf_trace_cpu+0xbb/0xd0
>   cpuidle_enter_state+0x185/0x340
>   do_idle+0x1eb/0x260
>   cpu_startup_entry+0x5f/0x70
>   start_kernel+0x49b/0x4a6
>   secondary_startup_64+0xa4/0xb0
> 
> This is due to the tracepoints moving to SRCU usage which does not require
> RCU to be "watching". But perf uses these tracepoints with RCU and expects
> it to be. Hence, we still need to add in the rcu_irq_enter/exit_irqson()
> calls for "rcuidle" tracepoints. This is a temporary fix until we have SRCU
> working in NMI context, and then perf can be converted to use that instead
> of normal RCU.
> 
> Link: http://lkml.kernel.org/r/20180904162611.6a120068@gandalf.local.home
> 
> Cc: x86-ml <x86@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> Reported-by: Borislav Petkov <bp@alien8.de>
> Fixes: e6753f23d961d ("tracepoint: Make rcuidle tracepoint callers use SRCU")
> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

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

> ---
>  include/linux/tracepoint.h | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> index 7f2e16e76ac4..041f7e56a289 100644
> --- a/include/linux/tracepoint.h
> +++ b/include/linux/tracepoint.h
> @@ -158,8 +158,10 @@ extern void syscall_unregfunc(void);
>  		 * For rcuidle callers, use srcu since sched-rcu	\
>  		 * doesn't work from the idle path.			\
>  		 */							\
> -		if (rcuidle)						\
> +		if (rcuidle) {						\
>  			idx = srcu_read_lock_notrace(&tracepoint_srcu);	\
> +			rcu_irq_enter_irqson();				\
> +		}							\
>  									\
>  		it_func_ptr = rcu_dereference_raw((tp)->funcs);		\
>  									\
> @@ -171,8 +173,10 @@ extern void syscall_unregfunc(void);
>  			} while ((++it_func_ptr)->func);		\
>  		}							\
>  									\
> -		if (rcuidle)						\
> +		if (rcuidle) {						\
> +			rcu_irq_exit_irqson();				\
>  			srcu_read_unlock_notrace(&tracepoint_srcu, idx);\
> +		}							\
>  									\
>  		preempt_enable_notrace();				\
>  	} while (0)
> -- 
> 2.13.6
>