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