Messages in this thread |  | | Date | Mon, 20 Oct 2014 18:56:14 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH v3] sched/numa: fix unsafe get_task_struct() in task_numa_assign() |
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On 10/20, Oleg Nesterov wrote: > > Again, perhaps we will need to change the lifetime rules for task_struct > anyway, if we have more problems like this. But until then this looks like > an overkill to me. Plus rq_curr_if_not_put() looks too subtle, and it is > not generic.
Yes... otoh, perhaps we can do something more generic? Something like
struct task_struct *xxx(struct task_struct **ptask) { struct task_struct *task; void *sighand; retry: task = ACCESS_ONCE(*ptask); if (!task) return NULL;
if (IS_ENABLED(CONFIG_DEBUG_PAGEALLOC)) { if (probe_kernel_read(&sighand, &task->sighand, sizeof(sighand))) goto retry; } else { sighand = task->sighand; }
if (!sighand) return NULL; /* * 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. */ smp_rmb(); if (task != ACCESS_ONCE(*ptask)) goto retry;
return task; }
task_numa_compare() can do cur = xxx(&rc->curr), but this helper can work with any "task_struct *" pointer assuming that somehow this pointer is cleared or changed before the final put_task_struct().
What do you think? Peter?
Oleg.
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