Re: [PATCH 3/3] introduce task_rcu_dereference()

[Kirill Tkhai <ktkhai@parallels.com>]

В Пн, 27/10/2014 в 20:54 +0100, Oleg Nesterov пишет:
> task_struct is only protected by RCU if it was found on a RCU protected
> list (say, for_each_process() or find_task_by_vpid()).
> 
> And as Kirill pointed out rq->curr isn't protected by RCU, the scheduler
> drops the (potentially) last reference without RCU gp, this means that we
> need to fix the code which uses foreign_rq->curr under rcu_read_lock().
> 
> Add a new helper which can be used to dereferene rq->curr or any other
> pointer to task_struct assuming that it should be cleared or updated
> before the final put_task_struct(). It returns non-NULL only if this
> task can't go away before rcu_read_unlock().
> 
> Suggested-by: Kirill Tkhai <ktkhai@parallels.com>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> ---
>  include/linux/sched.h |    1 +
>  kernel/exit.c         |   49 +++++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 50 insertions(+), 0 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 857ba40..0ba420e 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -2300,6 +2300,7 @@ extern void block_all_signals(int (*notifier)(void *priv), void *priv,
>  			      sigset_t *mask);
>  extern void unblock_all_signals(void);
>  extern void release_task(struct task_struct * p);
> +extern struct task_struct *task_rcu_dereference(struct task_struct **ptask);
>  extern int send_sig_info(int, struct siginfo *, struct task_struct *);
>  extern int force_sigsegv(int, struct task_struct *);
>  extern int force_sig_info(int, struct siginfo *, struct task_struct *);
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 32c58f7..d8b95c2 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -213,6 +213,55 @@ repeat:
>  		goto repeat;
>  }
>  
> +struct task_struct *task_rcu_dereference(struct task_struct **ptask)
> +{
> +	struct task_struct *task;
> +	struct sighand_struct *sighand;
> +
> +	/*
> +	 * We need to verify that release_task() was not called and thus
> +	 * delayed_put_task_struct() can't run and drop the last reference
> +	 * before rcu_read_unlock(). We check task->sighand != NULL, but
> +	 * we can read the already freed and reused memory.
> +	 */
> + retry:
> +	task = rcu_dereference(*ptask);
> +	if (!task)
> +		return NULL;
> +
> +	probe_slab_address(&task->sighand, sighand);
> +	/*
> +	 * Pairs with atomic_dec_and_test(usage) in put_task_struct(task).
> +	 * If this task was already freed we can not miss the preceding
> +	 * update of this pointer.
> +	 */
> +	smp_rmb();
> +	if (unlikely(task != ACCESS_ONCE(*ptask)))
> +		goto retry;
> +
> +	/*
> +	 * Either this is the same task and we can trust sighand != NULL, or
> +	 * its memory was re-instantiated as another instance of task_struct.
> +	 * In the latter case the new task can not go away until another rcu
> +	 * gp pass, so the only problem is that sighand == NULL can be false
> +	 * positive but we can pretend we got this NULL before it was freed.
> +	 */
> +	if (sighand)
> +		return task;
> +
> +	/*
> +	 * We could even eliminate the false positive mentioned above:
> +	 *
> +	 *	probe_slab_address(&task->sighand, sighand);
> +	 *	if (sighand)
> +	 *		goto retry;
> +	 *
> +	 * if sighand != NULL because we read the freed memory we should
> +	 * see the new pointer, otherwise we will likely return this task.
> +	 */
> +	return NULL;
> +}
> +
>  /*
>   * This checks not only the pgrp, but falls back on the pid if no
>   * satisfactory pgrp is found. I dunno - gdb doesn't work correctly

Reviewed-by: Kirill Tkhai <ktkhai@parallels.com>