Messages in this thread |  | | Date | Thu, 23 Oct 2014 00:23:30 +0200 | From | Oleg Nesterov <> | Subject | Re: introduce task_rcu_dereference? |
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Damn.
On 10/22, Oleg Nesterov wrote: > > +struct task_struct *task_rcu_dereference(struct task_struct **ptask) > +{ > + struct task_struct *task; > + struct sighand_struct *sighand; > + > + task = rcu_dereference(*ptask); > + if (!task) > + return NULL; > + > + /* If it fails the check below must fail too */ > + probe_slab_address(&task->sighand, sighand); > + /* > + * Pairs with atomic_dec_and_test() in put_task_struct(task). > + * If we have read the freed/reused memory, we must see that > + * the pointer was updated. The caller might want to retry in > + * this case. > + */ > + smp_rmb(); > + if (unlikely(task != ACCESS_ONCE(*ptask))) > + return ERR_PTR(-EAGAIN);
This is not exactly right. task == *ptask can be false positive.
It can be freed, then resused (so that sighand != NULL can be false positive), then freed again, and then reused again as task_struct.
This is not that bad, we still can safely use this task_struct, but the comment should be updated. Plus -EINVAL below can be wrong in this case although this minor.
Yeees, SLAB_DESTTROY_BY_RCU closes this race. Not sure why I'd like to avoid it, but I do ;)
Oleg.
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