Re: [PATCH] ppp: fix refcount underflow on channel unbridge

[Guillaume Nault <gnault@redhat.com>]

On Tue, Jan 05, 2021 at 09:17:43PM +0000, Tom Parkin wrote:
>  err_unset:
>  	write_lock_bh(&pch->upl);
> -	RCU_INIT_POINTER(pch->bridge, NULL);
> +	/* Re-check pch->bridge with upl held since a racing unbridge might already
> +	 * have cleared it and dropped the reference on pch->bridge.
> +	 */
> +	if (rcu_dereference_protected(pch->bridge, lockdep_is_held(&pch->upl))) {
> +		RCU_INIT_POINTER(pch->bridge, NULL);
> +		drop_ref = true;
> +	}
>  	write_unlock_bh(&pch->upl);
>  	synchronize_rcu();
> +
> +	if (drop_ref)
> +		if (refcount_dec_and_test(&pchb->file.refcnt))
> +			ppp_destroy_channel(pchb);
> +

I think this works because ppp_mutex prevents pch->bridge from being
reassigned to another channel. However, this isn't obvious when reading
the code, and well, I prefer to not introduce new dependencies on
ppp_mutex (otherwise we'd better go with your original patch).

I think we could just save pch->bridge while we're under ->upl
protection, and then drop the reference of that channel (if non-NULL):

 err_unset:
 	write_lock_bh(&pch->upl);
+	/* Re-read pch->bridge in case it was modified concurrently */
+	pchb = rcu_dereference_protected(pch->bridge,
+					 lockdep_is_held(&pch->upl));
+	RCU_INIT_POINTER(pch->bridge, NULL);
 	write_unlock_bh(&pch->upl);
 	synchronize_rcu();
+
+	if (pchb)
+		if (refcount_dec_and_test(&pchb->file.refcnt))
+			ppp_destroy_channel(pchb);
+

 	return -EALREADY;
 }

This way we know that pchb is the channel we were pointing to when we
cleared pch->bridge. And this is also a bit simpler than using an extra
boolean.