Messages in this thread | | | Subject | Re: [PATCH v2 1/2] perf/arm-cci: Remove broken race mitigation | From | Suzuki K Poulose <> | Date | Thu, 4 Apr 2019 11:42:19 +0100 |
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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>
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