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

[Andrea Arcangeli <aarcange@redhat.com>]

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@redhat.com>

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.

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

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