Re: [PATCH v2] ksm: fix conflict between mmput and scan_get_next_rmap_item

[zhouchengming]

On 2016/5/6 22:24, Andrea Arcangeli wrote:
> On Fri, May 06, 2016 at 11:27:36AM +0800, Zhou Chengming wrote:
> > @@ -1650,16 +1647,22 @@ next_mm:
> >   		 */
> >   		hash_del(&slot->link);
> >   		list_del(&slot->mm_list);
> > -		spin_unlock(&ksm_mmlist_lock);
> >
> >   		free_mm_slot(slot);
> >   		clear_bit(MMF_VM_MERGEABLE,&mm->flags);
> >   		up_read(&mm->mmap_sem);
> >   		mmdrop(mm);
> >   	} else {
> > -		spin_unlock(&ksm_mmlist_lock);
> >   		up_read(&mm->mmap_sem);
> >   	}
> > +	/*
> > +	 * up_read(&mm->mmap_sem) first because after
> > +	 * spin_unlock(&ksm_mmlist_lock) run, the "mm" may
> > +	 * already have been freed under us by __ksm_exit()
> > +	 * because the "mm_slot" is still hashed and
> > +	 * ksm_scan.mm_slot doesn't point to it anymore.
> > +	 */
> > +	spin_unlock(&ksm_mmlist_lock);
> >
> >   	/* Repeat until we've completed scanning the whole list */
> >   	slot = ksm_scan.mm_slot;
>
> Reviewed-by: Andrea Arcangeli<aarcange@xxxxxxxxxx>
>
> While the above patch is correct, I would however prefer if you could
> update it to keep releasing the ksm_mmlist_lock as before (I'm talking
> only about the quoted part, not the other one not quoted), because
> it's "strictier" and it better documents that it's only needed up
> until:
>
>    		hash_del(&slot->link);
>    		list_del(&slot->mm_list);
>
> It should be also a bit more scalable but to me this is just about
> keeping implicit documentation on the locking by keeping it strict.
>
> The fact up_read happens exactly after clear_bit also avoided me to
> overlook that it was really needed, same thing with the
> ksm_mmlist_lock after list_del, I'd like to keep it there and just
> invert the order of spin_unlock; up_read in the else branch.

Thanks a lot for your review and comment. It's my fault to misunderstand
your last reply. Yes it's better and more scalable to just invert the 
order of spin_unlock/up_read in the else branch. And it's also enough.

Thanks!
> That should be enough because after hash_del get_mm_slot will return
> NULL so the mmdrop will not happen anymore in __ksm_exit, this is
> further explicit by the code doing mmdrop itself just after
> up_read.
>
> The SMP race condition is fixed by just the two liner that reverse the
> order of spin_unlock; up_read without increasing the size of the
> spinlock critical section for the ksm_scan.address == 0 case. This is
> also why it wasn't reproducible because it's about 1 instruction window.
>
> Thanks!
> Andrea
>
> .