Re: [PATCH] ARM: smp: Fix suspicious RCU originating from cpu_die()

Stephen Boyd
2012-07-06T18:39:12Z

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

----->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@linux.vnet.ibm.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.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