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

Paul E. McKenney
2012-07-06T00:24:13Z

On Thu, Jul 05, 2012 at 04:45:58PM -0700, Stephen Boyd wrote:
> While running hotplug tests I ran into this RCU splat

One other possible alternative shown below for completeness.

Thanx, Paul

> ===============================
> [ 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 which sometimes grabs an RCU read
> lock in cpuacct_charge().
>
> To avoid this problem, copy what x86 is doing and have a per_cpu
> variable to track the cpu state and have the killing process poll
> that variable.
>
> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
> ---
> arch/arm/kernel/smp.c | 26 ++++++++++++++++----------
> 1 file changed, 16 insertions(+), 10 deletions(-)
>
> diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
> index 2c7217d..5430ea4 100644
> --- a/arch/arm/kernel/smp.c
> +++ b/arch/arm/kernel/smp.c
> @@ -59,6 +59,7 @@ enum ipi_msg_type {
> };
>
> static DECLARE_COMPLETION(cpu_running);
> +static DEFINE_PER_CPU(int, cpu_state) = { 0 };
>
> int __cpuinit __cpu_up(unsigned int cpu, struct task_struct *idle)
> {
> @@ -143,22 +144,26 @@ int __cpu_disable(void)
> return 0;
> }
>
> -static DECLARE_COMPLETION(cpu_died);
> -
> /*
> * called on the thread which is asking for a CPU to be shutdown -
> * waits until shutdown has completed, or it is timed out.
> */
> void __cpu_die(unsigned int cpu)
> {
> - if (!wait_for_completion_timeout(&cpu_died, msecs_to_jiffies(5000))) {
> - pr_err("CPU%u: cpu didn't die\n", cpu);
> - return;
> + unsigned int i;
> +
> + for (i = 0; i < 50; i++) {
> + if (per_cpu(cpu_state, cpu) == CPU_DEAD) {
> + if (platform_cpu_kill(cpu)) {
> + pr_notice("CPU%u: shutdown\n", cpu);
> + return;
> + } else {
> + break;
> + }
> + }
> + msleep(100);
> }
> - printk(KERN_NOTICE "CPU%u: shutdown\n", cpu);
> -
> - if (!platform_cpu_kill(cpu))
> - printk("CPU%u: unable to kill\n", cpu);
> + pr_err("CPU%u: unable to kill\n", cpu);
> }
>
> /*
> @@ -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.

>
> /*
> * actual CPU shutdown procedure is at least platform (if not
> @@ -258,6 +263,7 @@ asmlinkage void __cpuinit secondary_start_kernel(void)
> * before we continue - which happens after __cpu_up returns.
> */
> set_cpu_online(cpu, true);
> + per_cpu(cpu_state, smp_processor_id()) = CPU_ONLINE;
> complete(&cpu_running);
>
> /*
> --
> Sent by an employee of the Qualcomm Innovation Center, Inc.
> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.
>