netdev - Re: [PATCH] vhost/net: Replace wait_queue with completion in ubufs reference

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Message-Id: <20250718094836.1538136-1-kniv@yandex-team.ru>
Date: Fri, 18 Jul 2025 12:48:36 +0300
From: Nikolay Kuratov <kniv@...dex-team.ru>
To: linux-kernel@...r.kernel.org
Cc: netdev@...r.kernel.org,
	virtualization@...ts.linux.dev,
	kvm@...r.kernel.org,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Jason Wang <jasowang@...hat.com>,
	Eugenio Pérez <eperezma@...hat.com>,
	Lei Yang <leiyang@...hat.com>,
	Hillf Danton <hdanton@...a.com>
Subject: Re: [PATCH] vhost/net: Replace wait_queue with completion in ubufs reference

Yes, if multiple waiters call vhost_net_ubuf_put_and_wait() concurrently we
are screwed. Furthermore, it was not the case before this patch. While it
was explicitly mentioned in the commit message, now I changed my mind,
because amount of vhost_net_ubuf_put_and_wait() users may change when this
patch will be backported to older LTSes. In 6.6+ kernels there are
only two put_and_wait() callers, both are ensuring that there
is only one thread calling put_and_wait() at a time.

I think its better to preserve thread-safety of vhost_net_ubuf_put_and_wait()
and move reinit_completion() call to vhost_net_flush(). We don't need
reinit on free'ing path anyway.

I will send v2 with the fix. Thank you for noticing this.