Re: [PATCH v2 1/2] perf/arm-cci: Remove broken race mitigation

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

On 03/04/2019 18:10, Robin Murphy wrote:
> Uncore PMU drivers face an awkward cyclic dependency wherein:
> 
>   - They have to pick a valid online CPU to associate with before
>     registering the PMU device, since it will get exposed to userspace
>     immediately.
>   - The PMU registration has to be be at least partly complete before
>     hotplug events can be handled, since trying to migrate an
>     uninitialised context would be bad.
>   - The hotplug handler has to be ready as soon as a CPU is chosen, lest
>     it go offline without the user-visible cpumask value getting updated.
> 
> The arm-cci driver has tried to solve this by using get_cpu() to pick
> the current CPU and prevent it from disappearing while both
> 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
> 
> It is not feasible to resolve all the possible races outside of the perf
> core itself, so address the immediate bug by following the example of
> nearly every other PMU driver and not even trying to do so. Registering
> the hotplug notifier first should minimise the window in which things
> can go wrong, so that's about as much as we can reasonably do here. This
> also revealed an additional race in assigning the global pointer too
> late relative to the hotplug notifier, which gets fixed in the process.
> 
> Reported-by: "Li, Meng" <Meng.Li@windriver.com>
> Reported-by: Corentin Labbe <clabbe.montjoie@gmail.com>
> Signed-off-by: Robin Murphy <robin.murphy@arm.com>

Thanks for fixing this.

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