Messages in this thread | | | Date | Thu, 16 Mar 2017 15:54:36 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH 1/2] kthread: add barriers to set_kthread_struct() and to_kthread() |
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Hi Tejun,
On 03/15, Tejun Heo wrote: > > Until now, all to_kthread() users are interlocked with kthread > creation and there's no need to have explicit barriers when setting > the kthread pointer or dereferencing it. > > However, There is a race condition where userland can interfere with a > kthread while it's being initialized. To close it, to_kthread() needs > to be used from an unsynchronized context.
So this is preparation for 2/2... IIUC, the current code is not buggy, just you need to add kthread_initialized() which can't work without this change.
> static inline void set_kthread_struct(void *kthread) > { > + /* paired with smp_read_data_barrier_depends() in to_kthread() */ > + smp_wmb(); > + > /* > * We abuse ->set_child_tid to avoid the new member and because it > * can't be wrongly copied by copy_process(). We also rely on fact > @@ -67,8 +70,19 @@ static inline void set_kthread_struct(vo > > static inline struct kthread *to_kthread(struct task_struct *k) > { > + void *ptr; > + > WARN_ON(!(k->flags & PF_KTHREAD)); > - return (__force void *)k->set_child_tid; > + > + ptr = (__force void *)k->set_child_tid; > + > + /* > + * Paired with smp_wmb() in set_kthread_struct() and ensures that > + * the caller sees initialized content of the returned kthread. > + */ > + smp_read_barrier_depends(); > + > + return ptr;
This is almost off-topic, but I think lockless_dereference() will look better in to_kthread().
And perhaps we should add another helper, say,
#define lockless_assign_pointer(ptr, val) \ smp_store_release(&ptr, val)
for set_kthread_struct() ? it can have more users.
Not that I think you should change your patch, I am just asking.
Oleg.
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