• Robin Murphy's avatar
    perf/arm-cci: Remove broken race mitigation · 0d2e2a82
    Robin Murphy authored
    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
     - 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: default avatarLi, Meng <Meng.Li@windriver.com>
    Tested-by: default avatarCorentin Labbe <clabbe.montjoie@gmail.com>
    Reviewed-by: default avatarSuzuki K Poulose <suzuki.poulose@arm.com>
    Signed-off-by: default avatarRobin Murphy <robin.murphy@arm.com>
    Signed-off-by: default avatarWill Deacon <will.deacon@arm.com>
arm-cci.c 48.1 KB