[PATCH v2] drivers/perf: arm-pmu: fix RCU usage on pmu resume from low-power

[Paul E. McKenney]

On Thu, Apr 21, 2016 at 10:24:34AM +0100, Lorenzo Pieralisi wrote:
> Commit da4e4f18afe0 ("drivers/perf: arm_pmu: implement CPU_PM notifier")
> added code in the arm perf infrastructure that allows the kernel to
> save/restore perf counters whenever the CPU enters a low-power
> state. The kernel saves/restores the counters for each active event
> through the armpmu_{stop/start} ARM pmu API, so that the low-power state
> enter/exit cycle is emulated through pmu start/stop operations for each
> event in use.
> 
> However, calling armpmu_start() for each active event on power up
> executes code that requires RCU locking (perf_event_update_userpage())
> to be functional, so, given that the core may call the CPU_PM notifiers
> while running the idle thread in an quiescent RCU state this is not
> allowed as detected through the following splat when kernel is run with
> CONFIG_PROVE_LOCKING enabled:
> 
> [   49.293286]
> [   49.294761] ===============================
> [   49.298895] [ INFO: suspicious RCU usage. ]
> [   49.303031] 4.6.0-rc3+ #421 Not tainted
> [   49.306821] -------------------------------
> [   49.310956] include/linux/rcupdate.h:872 rcu_read_lock() used
> illegally while idle!
> [   49.318530]
> [   49.318530] other info that might help us debug this:
> [   49.318530]
> [   49.326451]
> [   49.326451] RCU used illegally from idle CPU!
> [   49.326451] rcu_scheduler_active = 1, debug_locks = 0
> [   49.337209] RCU used illegally from extended quiescent state!
> [   49.342892] 2 locks held by swapper/2/0:
> [   49.346768]  #0:  (cpu_pm_notifier_lock){......}, at:
> [<ffffff8008163c28>] cpu_pm_exit+0x18/0x80
> [   49.355492]  #1:  (rcu_read_lock){......}, at: [<ffffff800816dc38>]
> perf_event_update_userpage+0x0/0x260
> 
> This patch wraps the armpmu_start() call (that indirectly calls
> perf_event_update_userpage()) on CPU_PM notifier power state exit (or
> failed entry) within the RCU_NONIDLE() macro so that the RCU subsystem
> is made aware the calling cpu is not idle from an RCU perspective for
> the armpmu_start() call duration, therefore fixing the issue.
> 
> Fixes: da4e4f18afe0 ("drivers/perf: arm_pmu: implement CPU_PM notifier")
> Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi at arm.com>
> Reported-by: James Morse <james.morse at arm.com>
> Suggested-by: Kevin Hilman <khilman at baylibre.com>
> Cc: Ashwin Chaugule <ashwin.chaugule at linaro.org>
> Cc: Will Deacon <will.deacon at arm.com>
> Cc: Kevin Hilman <khilman at baylibre.com>
> Cc: Sudeep Holla <sudeep.holla at arm.com>
> Cc: Daniel Lezcano <daniel.lezcano at linaro.org>
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: "Paul E. McKenney" <paulmck at linux.vnet.ibm.com>
> Cc: Mathieu Poirier <mathieu.poirier at linaro.org>
> Cc: Mark Rutland <mark.rutland at arm.com>

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

> ---
> v1 -> v2
> 
> - Wrapped armpmu_start() code in CPU_PM notifier within RCU_NONIDLE
> - Dropped armpmu_event_set_period() refactoring
> - Updated patch subject, added required tags
> 
> v1: http://lists.infradead.org/pipermail/linux-arm-kernel/2016-April/423404.html
> 
>  drivers/perf/arm_pmu.c | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/perf/arm_pmu.c b/drivers/perf/arm_pmu.c
> index 32346b5..f700908 100644
> --- a/drivers/perf/arm_pmu.c
> +++ b/drivers/perf/arm_pmu.c
> @@ -737,8 +737,19 @@ static void cpu_pm_pmu_setup(struct arm_pmu *armpmu, unsigned long cmd)
>  			break;
>  		case CPU_PM_EXIT:
>  		case CPU_PM_ENTER_FAILED:
> -			 /* Restore and enable the counter */
> -			armpmu_start(event, PERF_EF_RELOAD);
> +			 /*
> +			  * Restore and enable the counter.
> +			  * armpmu_start() indirectly calls
> +			  *
> +			  * perf_event_update_userpage()
> +			  *
> +			  * that requires RCU read locking to be functional,
> +			  * wrap the call within RCU_NONIDLE to make the
> +			  * RCU subsystem aware this cpu is not idle from
> +			  * an RCU perspective for the armpmu_start() call
> +			  * duration.
> +			  */
> +			RCU_NONIDLE(armpmu_start(event, PERF_EF_RELOAD));
>  			break;
>  		default:
>  			break;
> -- 
> 2.6.4
>