Messages in this thread | | | Date | Fri, 17 Feb 2012 15:37:06 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH 1/4] introduce complete_vfork_done() |
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On 02/16, Andrew Morton wrote: > > On Thu, 16 Feb 2012 18:26:47 +0100 > Oleg Nesterov <oleg@redhat.com> wrote: > > > ... > > +void complete_vfork_done(struct task_struct *tsk) > > +{ > > + struct completion *vfork_done = tsk->vfork_done; > > + > > + tsk->vfork_done = NULL; > > + complete(vfork_done); > > +} > > + > > /* Please note the differences between mmput and mm_release. > > * mmput is called whenever we stop holding onto a mm_struct, > > * error success whatever. > > @@ -682,8 +690,6 @@ struct mm_struct *mm_access(struct task_struct *task, unsigned int mode) > > */ > > void mm_release(struct task_struct *tsk, struct mm_struct *mm) > > { > > - struct completion *vfork_done = tsk->vfork_done; > > - > > /* Get rid of any futexes when releasing the mm */ > > #ifdef CONFIG_FUTEX > > if (unlikely(tsk->robust_list)) { > > @@ -703,11 +709,8 @@ void mm_release(struct task_struct *tsk, struct mm_struct *mm) > > /* Get rid of any cached register state */ > > deactivate_mm(tsk, mm); > > > > - /* notify parent sleeping on vfork() */ > > - if (vfork_done) { > > - tsk->vfork_done = NULL; > > - complete(vfork_done); > > - } > > + if (tsk->vfork_done) > > + complete_vfork_done(tsk); > > This all looks somewhat smelly.
First of all, let me repeat that this patch changes nothing, justs move this code into the new helper.
> - Why do we zero tsk->vfork_done in this manner? It *looks* like > it's done to prevent the kernel from running complete() twice against > a single task
Yes,
> in a race situation.
No. More precisely, not before/after this patch.
"if (vfork_done) complete_vfork_done()" is called twice very often. A vforked child does exec and notifies its parent. It should clear ->vfork_done, otherwise it will do complete_vfork_done() again on exit when ->vfork_done points to nowhere.
The caller can never race with another user of ->vfork_done. It is the parent sleeping in do_fork(CLONE_VFORK). (I am ignoring the kernel threads created by kthread_create).
> We'd need external locking to firm that up > and I'm not seeing it.
After the next patch, parent/child can race with each other, that is why the next patch moves complete() under task_lock(). I'll write another email in reply to 2/4.
> - Moving the test for non-null tsk->vfork_done into > complete_vfork_done() would simplify things a bit?
Yes, perhaps this makes sense. After 3/4 mm_release() becomes the only caller and this microoptimization buys nothing, this helper will be static.
I like the explicit test a bit more, just because it looks more clear to me. But this is subjective, I can redo.
> - The complete_vfork_done() interface isn't wonderful. What prevents > tsk from getting freed? Presumably the caller must have pinned it in > some fashion? Or must hold some lock? Or it's always run against > `current',
Yes, it is always current,
> in which case it would be clearer to not pass the > task_struct arg at all?
Well, may be... But mm_release() already has the 'tsk' argument which is always current. It would be strange to not use it.
Oleg.
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