Re: [PATCH 1/5] perf/arm-cci: Fix CPU hotplug race avoidance

[Suzuki K Poulose <suzuki.poulose@arm.com>]

Robin,

On 04/02/2019 17:09, Robin Murphy wrote:
> The arm-cci probe logic faces a cyclic dependency wherein it has to pick
> a valid CPU to associate with before registering the PMU device, has to
> have the PMU state initialised before handling hotplug events in case it
> must be migrated, but has to have the hotplug notifier registered before
> the chosen CPU may go offline lest things get out of sync. The present
> code has tried to solve the races by using get_cpu() to pick the current
> CPU and prevent it from disappearing while the other two registrations
> are performed, but that results in taking mutexes with preemption
> disabled, which makes certain configurations very unhappy:
> 
> [ 1.983337] BUG: sleeping function called from invalid context at kernel/locking/rtmutex.c:2004
> [ 1.983340] in_atomic(): 1, irqs_disabled(): 0, pid: 1, name: swapper/0
> [ 1.983342] Preemption disabled at:
> [ 1.983353] [<ffffff80089801f4>] cci_pmu_probe+0x1dc/0x488
> [ 1.983360] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.18.20-rt8-yocto-preempt-rt #1
> [ 1.983362] Hardware name: ZynqMP ZCU102 Rev1.0 (DT)
> [ 1.983364] Call trace:
> [ 1.983369] dump_backtrace+0x0/0x158
> [ 1.983372] show_stack+0x24/0x30
> [ 1.983378] dump_stack+0x80/0xa4
> [ 1.983383] ___might_sleep+0x138/0x160
> [ 1.983386] __might_sleep+0x58/0x90
> [ 1.983391] __rt_mutex_lock_state+0x30/0xc0
> [ 1.983395] _mutex_lock+0x24/0x30
> [ 1.983400] perf_pmu_register+0x2c/0x388
> [ 1.983404] cci_pmu_probe+0x2bc/0x488
> [ 1.983409] platform_drv_probe+0x58/0xa8
> 
> However, we don't actually mind being preempted or migrated at this
> point; all that really matters is that whichever CPU we pick does not
> get offlined before we're done. Thus, do the robust thing and instead
> take the lock to inhibit CPU hotplug for the duration. This also
> revealed an additional race in assigning the global pointer too late
> relative to the hotplug notifier, so that gets fixed in the process.
> 
> Reported-by: "Li, Meng" <Meng.Li@windriver.com>
> Signed-off-by: Robin Murphy <robin.murphy@arm.com>

Thanks for fixing the issues.

Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>