Messages in this thread | | | Date | Sun, 25 Mar 2012 19:42:10 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH v2.1 01/10] cpu: Introduce clear_tasks_mm_cpumask() helper |
| |
On 03/24, Anton Vorontsov wrote: > > Many architctures clear tasks' mm_cpumask like this: > > read_lock(&tasklist_lock); > for_each_process(p) { > if (p->mm) > cpumask_clear_cpu(cpu, mm_cpumask(p->mm)); > } > read_unlock(&tasklist_lock);
Namely arm, powerpc, and sh.
> The code above has several problems, such as: > > 1. Working with task->mm w/o getting mm or grabing the task lock is > dangerous as ->mm might disappear (exit_mm() assigns NULL under > task_lock(), so tasklist lock is not enough).
This is not actually true for arm and sh, afaics. They do not even need tasklist or rcu lock for for_each_process().
__cpu_disable() is called by __stop_machine(), we know that nobody can preempt us and other CPUs can do nothing.
> 2. Checking for process->mm is not enough because process' main > thread may exit or detach its mm via use_mm(), but other threads > may still have a valid mm.
Yes,
> Also, Per Peter Zijlstra's idea, now we don't grab tasklist_lock in > the new helper, instead we take the rcu read lock. We can do this > because the function is called after the cpu is taken down and marked > offline, so no new tasks will get this cpu set in their mm mask.
And only powerpc needs rcu_read_lock() and task_lock().
OTOH, I do not understand why powepc does this on CPU_DEAD... And probably CPU_UP_CANCELED doesn't need to clear mm_cpumask().
That said, personally I think these patches are fine, the common helper makes sense.
Oleg.
| |