On 06/22, Peter Zijlstra wrote:
>
> The cpu hotplug lock is a rwsem with read-in-write and read-in-read
> recursion. Implement it as such.
And this patch fixes the problem afaics. Currently cpu_hotplug_begin()
can livelock because it doesn't stop the new readers. With this patch
this is no longer possible.
> -static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
> +static inline void _percpu_down_read(struct percpu_rw_semaphore *sem)
> {
> might_sleep();
>
> - rwsem_acquire_read(&sem->rw_sem.dep_map, 0, 0, _RET_IP_);
> -
> preempt_disable();
> /*
> * We are in an RCU-sched read-side critical section, so the writer
> @@ -46,6 +44,12 @@ static inline void percpu_down_read(stru
> */
> }
>
> +static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
> +{
> + rwsem_acquire_read(&sem->rw_sem.dep_map, 0, 0, _RET_IP_);
> + _percpu_down_read(sem);
> +}
...
> void get_online_cpus(void)
> {
> might_sleep();
> - if (cpu_hotplug.active_writer == current)
> +
> + /* read in write recursion */
> + if (cpu_hotplug.writer == current)
> + return;
> +
> + /* read in read recursion */
> + if (current->cpuhp_ref++)
> return;
> - cpuhp_lock_acquire_read();
> - mutex_lock(&cpu_hotplug.lock);
> - atomic_inc(&cpu_hotplug.refcount);
> - mutex_unlock(&cpu_hotplug.lock);
> +
> + lock_map_acquire_read(&cpu_hotplug.rwsem.rw_sem.dep_map);
> + _percpu_down_read(&cpu_hotplug.rwsem);
> }
Confused... Why do we need _percpu_down_read()? Can't get_online_cpus()
just use percpu_down_read() ?
Yes, percpu_down_read() is not recursive, like the normal down_read().
But this does not matter because we rely on ->cpuhp_ref anyway?
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1410,6 +1410,8 @@ static struct task_struct *copy_process(
> p->sequential_io_avg = 0;
> #endif
>
> + cpu_hotplug_init_task(p);
This is probably unnecessary, copy_process() should not be called under
get_online_cpus().
Oleg.
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