| Message ID | cf67c49058e3913540df1ebbb765c006f5205ec1.1554310292.git.robin.murphy@arm.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Fix Arm system PMU hotplug issues | expand |
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>
diff --git a/drivers/perf/arm-cci.c b/drivers/perf/arm-cci.c index bfd03e023308..7a29ed88bd4f 100644 --- a/drivers/perf/arm-cci.c +++ b/drivers/perf/arm-cci.c @@ -1684,19 +1684,17 @@ static int cci_pmu_probe(struct platform_device *pdev) raw_spin_lock_init(&cci_pmu->hw_events.pmu_lock); mutex_init(&cci_pmu->reserve_mutex); atomic_set(&cci_pmu->active_events, 0); - cci_pmu->cpu = get_cpu(); - - ret = cci_pmu_init(cci_pmu, pdev); - if (ret) { - put_cpu(); - return ret; - } + cci_pmu->cpu = raw_smp_processor_id(); + g_cci_pmu = cci_pmu; cpuhp_setup_state_nocalls(CPUHP_AP_PERF_ARM_CCI_ONLINE, "perf/arm/cci:online", NULL, cci_pmu_offline_cpu); - put_cpu(); - g_cci_pmu = cci_pmu; + + ret = cci_pmu_init(cci_pmu, pdev); + if (ret) + return ret; + pr_info("ARM %s PMU driver probed", cci_pmu->model->name); return 0; }
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> --- drivers/perf/arm-cci.c | 16 +++++++--------- 1 file changed, 7 insertions(+), 9 deletions(-)