Re: [PATCH] futex: Reduce the scope of lock_page, aka lockless futex_get_key()

[Davidlohr Bueso]

On Tue, 05 Jan 2016, Peter Zijlstra wrote:

> On Tue, Jan 05, 2016 at 12:23:55PM -0800, Davidlohr Bueso wrote:
> > +	if (unlikely(!mapping)) {
> > +		int shmem_swizzled;
> > +
> > +		/*
> > +		 * Page lock is required to identify which special case above
> > +		 * applies. If this is really a shmem page then the page lock
> > +		 * will prevent unexpected transitions.
> > +		 */
> > +		lock_page(page);
> > +		shmem_swizzled = PageSwapCache(page);
> >  		unlock_page(page);
> >  		put_page(page);
> > +		WARN_ON_ONCE(mapping);
>
> We've not re-loaded mapping, so how could this possibly be?

Yep, this wants to be page->mapping.

> > +		/*
> > +		 * Take a reference unless it is about to be freed. Previously
> > +		 * this reference was taken by ihold under the page lock
> > +		 * pinning the inode in place so i_lock was unnecessary. The
> > +		 * only way for this check to fail is if the inode was
> > +		 * truncated in parallel so warn for now if this happens.
> > +		 *
> > +		 * TODO: VFS and/or filesystem people should review this check
> > +		 * and see if there is a safer or more reliable way to do this.
> > +		 */
> > +		if (WARN_ON(!atomic_inc_not_zero(&inode->i_count))) {
> > +			rcu_read_unlock();
> > +			put_page(page);
> > +			goto again;
> > +		}
> > +
> > +		/*
> > +		 * get_futex_key() must imply MB (B) and we are not going to
> > +		 * call into get_futex_key_refs() at this point.
> > +		 */
> > +		smp_mb__after_atomic();
>
> I don't get this one, the above is a successful atomic op with return
> value, that _must_ imply a full barrier.

Ah sure, I was actually following convention of what we have for our plain atomic_inc,
but in this case we are returning a value, so yeah, it is not required. Will drop.

Thanks,
Davidlohr

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/