On Thu, 21 Feb 2019 00:49:40 -0500
"Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx> wrote:
> Recently I added an RCU annotation check to rcu_assign_pointer(). All
> pointers assigned to RCU protected data are to be annotated with __rcu
> inorder to be able to use rcu_assign_pointer() similar to checks in
> other RCU APIs.
>
> This resulted in a sparse error: kernel//sched/cpufreq.c:41:9: sparse:
> error: incompatible types in comparison expression (different address
> spaces)
>
> Fix this by using the correct APIs for RCU accesses. This will
> potentially avoid any future bugs in the code. If it is felt that RCU
> protection is not needed here, then the rcu_assign_pointer call can be
> dropped and replaced with, say, WRITE_ONCE or smp_store_release. Or, may
> be we add a new API to do it. But calls rcu_assign_pointer seems an
> abuse of the RCU API unless RCU is being used.
This all looks broken, and this patch is papering over the issue, or
worse, hiding it.
>
> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> ---
> kernel/sched/cpufreq.c | 8 ++++++--
> kernel/sched/sched.h | 2 +-
> 2 files changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/cpufreq.c b/kernel/sched/cpufreq.c
> index 22bd8980f32f..c9aeb3bf5dc2 100644
> --- a/kernel/sched/cpufreq.c
> +++ b/kernel/sched/cpufreq.c
> @@ -7,7 +7,7 @@
> */
> #include "sched.h"
>
> -DEFINE_PER_CPU(struct update_util_data *, cpufreq_update_util_data);
> +DEFINE_PER_CPU(struct update_util_data __rcu *, cpufreq_update_util_data);
>
> /**
> * cpufreq_add_update_util_hook - Populate the CPU's update_util_data pointer.
> @@ -34,8 +34,12 @@ void cpufreq_add_update_util_hook(int cpu, struct update_util_data *data,
> if (WARN_ON(!data || !func))
> return;
>
> - if (WARN_ON(per_cpu(cpufreq_update_util_data, cpu)))
> + rcu_read_lock();
> + if (WARN_ON(rcu_dereference(per_cpu(cpufreq_update_util_data, cpu)))) {
> + rcu_read_unlock();
> return;
> + }
> + rcu_read_unlock();
>
> data->func = func;
> rcu_assign_pointer(per_cpu(cpufreq_update_util_data, cpu), data);
An rcu_assign_pointer() is to update something that is going to be read
under rcu_read_lock() elsewhere. But updates to an rcu variable are not
protected by rcu_read_lock() (hence the "read" in the name). Adding
rcu_read_lock() above does nothing, but perhaps hides an issue.
Writes usually have something else that protects against races. Thus,
the above shouldn't be switched to using a rcu_dereference(), but
perhaps a rcu_dereference_protected(), with whatever is protecting
updates?
Which doing a bit of investigating, looks to be the rwsem
"policy->rwsem", where policy comes from:
policy = cpufreq_cpu_get(cpu);
I would say the code as is, is not broken. But this patch isn't helping
anything.
-- Steve
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index d04530bf251f..2ab545d40381 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2166,7 +2166,7 @@ static inline u64 irq_time_read(int cpu)
> #endif /* CONFIG_IRQ_TIME_ACCOUNTING */
>
> #ifdef CONFIG_CPU_FREQ
> -DECLARE_PER_CPU(struct update_util_data *, cpufreq_update_util_data);
> +DECLARE_PER_CPU(struct update_util_data __rcu *, cpufreq_update_util_data);
>
> /**
> * cpufreq_update_util - Take a note about CPU utilization changes.
|