Re: [PATCH 3/3] mm/memory-failure.c: support dedicated thread to handle SIGBUS(BUS_MCEERR_AO)

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 30 May 2014 02:51:10 -0400 Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> wrote:

> Currently memory error handler handles action optional errors in the deferred
> manner by default. And if a recovery aware application wants to handle it
> immediately, it can do it by setting PF_MCE_EARLY flag. However, such signal
> can be sent only to the main thread, so it's problematic if the application
> wants to have a dedicated thread to handler such signals.
> 
> So this patch adds dedicated thread support to memory error handler. We have
> PF_MCE_EARLY flags for each thread separately, so with this patch AO signal
> is sent to the thread with PF_MCE_EARLY flag set, not the main thread. If
> you want to implement a dedicated thread, you call prctl() to set PF_MCE_EARLY
> on the thread.
> 
> Memory error handler collects processes to be killed, so this patch lets it
> check PF_MCE_EARLY flag on each thread in the collecting routines.
> 
> No behavioral change for all non-early kill cases.
> 
> ...
>
> --- mmotm-2014-05-21-16-57.orig/mm/memory-failure.c
> +++ mmotm-2014-05-21-16-57/mm/memory-failure.c
> @@ -380,15 +380,44 @@ static void kill_procs(struct list_head *to_kill, int forcekill, int trapno,
>  	}
>  }
>  
> -static int task_early_kill(struct task_struct *tsk, int force_early)
> +/*
> + * Find a dedicated thread which is supposed to handle SIGBUS(BUS_MCEERR_AO)
> + * on behalf of the thread group. Return task_struct of the (first found)
> + * dedicated thread if found, and return NULL otherwise.
> + */
> +static struct task_struct *find_early_kill_thread(struct task_struct *tsk)
> +{
> +	struct task_struct *t;
> +	rcu_read_lock();
> +	for_each_thread(tsk, t)
> +		if ((t->flags & PF_MCE_PROCESS) && (t->flags & PF_MCE_EARLY))
> +			goto found;
> +	t = NULL;
> +found:
> +	rcu_read_unlock();
> +	return t;
> +}
> +
> +/*
> + * Determine whether a given process is "early kill" process which expects
> + * to be signaled when some page under the process is hwpoisoned.
> + * Return task_struct of the dedicated thread (main thread unless explicitly
> + * specified) if the process is "early kill," and otherwise returns NULL.
> + */
> +static struct task_struct *task_early_kill(struct task_struct *tsk,
> +					   int force_early)
>  {
> +	struct task_struct *t;
>  	if (!tsk->mm)
> -		return 0;
> +		return NULL;
>  	if (force_early)
> -		return 1;
> -	if (tsk->flags & PF_MCE_PROCESS)
> -		return !!(tsk->flags & PF_MCE_EARLY);
> -	return sysctl_memory_failure_early_kill;
> +		return tsk;
> +	t = find_early_kill_thread(tsk);
> +	if (t)
> +		return t;
> +	if (sysctl_memory_failure_early_kill)
> +		return tsk;
> +	return NULL;
>  }

The above two functions are to be called under
read_lock(tasklist_lock), which is rather important...

Given this requirement, did find_early_kill_thread() need rcu_read_lock()?


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