Message-ID: <20120706203005.GF31508@n2100.arm.linux.org.uk>
Date:	Fri, 6 Jul 2012 21:30:05 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] ARM: smp: Fix suspicious RCU originating from cpu_die()

On Fri, Jul 06, 2012 at 11:39:09AM -0700, Stephen Boyd wrote:
> On 07/05/12 17:24, Paul E. McKenney wrote:
> > On Thu, Jul 05, 2012 at 04:45:58PM -0700, Stephen Boyd wrote:
> >> @@ -179,7 +184,7 @@ void __ref cpu_die(void)
> >>  	mb();
> >>
> >>  	/* Tell __cpu_die() that this CPU is now safe to dispose of */
> >> -	complete(&cpu_died);
> >> +	__this_cpu_write(cpu_state, CPU_DEAD);
> > Or you could do something like:
> >
> > 	RCU_NONIDLE(complete(&cpu_died));
> >
> > This would tell RCU that it needed to pay attention to this CPU for
> > the duration of the "complete()" function call despite the CPU's being
> > idle.  And might allow you to dispense with the rest of the patch.
> 
> Great! I like that more since we get to keep the completion mechanism
> instead of a busy wait.
> 
> Russell, which one would you prefer? Here's the other version

I think I prefer the version below.

> 
> ----->8-----8<-----
> Subject: [PATCH] ARM: smp: Fix suspicious RCU originating from cpu_die()
> 
> While running hotplug tests I ran into this RCU splat
> 
> ===============================
> [ INFO: suspicious RCU usage. ]
> 3.4.0 #3275 Tainted: G        W
> -------------------------------
> include/linux/rcupdate.h:729 rcu_read_lock() used illegally while idle!
> 
> 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!
> 4 locks held by swapper/2/0:
>  #0:  ((cpu_died).wait.lock){......}, at: [<c00ab128>] complete+0x1c/0x5c
>  #1:  (&p->pi_lock){-.-.-.}, at: [<c00b275c>] try_to_wake_up+0x2c/0x388
>  #2:  (&rq->lock){-.-.-.}, at: [<c00b2860>] try_to_wake_up+0x130/0x388
>  #3:  (rcu_read_lock){.+.+..}, at: [<c00abe5c>] cpuacct_charge+0x28/0x1f4
> 
> stack backtrace:
> [<c001521c>] (unwind_backtrace+0x0/0x12c) from [<c00abec8>] (cpuacct_charge+0x94/0x1f4)
> [<c00abec8>] (cpuacct_charge+0x94/0x1f4) from [<c00b395c>] (update_curr+0x24c/0x2c8)
> [<c00b395c>] (update_curr+0x24c/0x2c8) from [<c00b59c4>] (enqueue_task_fair+0x50/0x194)
> [<c00b59c4>] (enqueue_task_fair+0x50/0x194) from [<c00afea4>] (enqueue_task+0x30/0x34)
> [<c00afea4>] (enqueue_task+0x30/0x34) from [<c00b0908>] (ttwu_activate+0x14/0x38)
> [<c00b0908>] (ttwu_activate+0x14/0x38) from [<c00b28a8>] (try_to_wake_up+0x178/0x388)
> [<c00b28a8>] (try_to_wake_up+0x178/0x388) from [<c00a82a0>] (__wake_up_common+0x34/0x78)
> [<c00a82a0>] (__wake_up_common+0x34/0x78) from [<c00ab154>] (complete+0x48/0x5c)
> [<c00ab154>] (complete+0x48/0x5c) from [<c07db7cc>] (cpu_die+0x2c/0x58)
> [<c07db7cc>] (cpu_die+0x2c/0x58) from [<c000f954>] (cpu_idle+0x64/0xfc)
> [<c000f954>] (cpu_idle+0x64/0xfc) from [<80208160>] (0x80208160)
> 
> When a cpu is marked offline during its idle thread it calls
> cpu_die() during an RCU idle period. cpu_die() calls complete()
> to notify the killing process that the cpu has died. complete()
> calls into the scheduler code and eventually grabs an RCU read
> lock in cpuacct_charge().
> 
> Mark complete() as RCU_NONIDLE so that RCU pays attention to this
> CPU for the duration of the complete() function even though it's
> in idle.
> 
> Suggested-by: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> Signed-off-by: Stephen Boyd <sboyd@...eaurora.org>
> ---
>  arch/arm/kernel/smp.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
> index 2c7217d..aea74f5 100644
> --- a/arch/arm/kernel/smp.c
> +++ b/arch/arm/kernel/smp.c
> @@ -179,7 +179,7 @@ void __ref cpu_die(void)
>  	mb();
>  
>  	/* Tell __cpu_die() that this CPU is now safe to dispose of */
> -	complete(&cpu_died);
> +	RCU_NONIDLE(complete(&cpu_died));
>  
>  	/*
>  	 * actual CPU shutdown procedure is at least platform (if not
> 
> -- 
> Sent by an employee of the Qualcomm Innovation Center, Inc.
> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.
> 
> 
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> linux-arm-kernel@...ts.infradead.org
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