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From:   Dexuan Cui <decui@microsoft.com>
To:     Michael Kelley <mikelley@microsoft.com>,
        kimbrownkd <kimbrownkd@gmail.com>,
        Long Li <longli@microsoft.com>,
        Sasha Levin <Alexander.Levin@microsoft.com>
CC:     KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        "devel@linuxdriverproject.org" <devel@linuxdriverproject.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH v2] Drivers: hv: vmbus: Expose counters for interrupts and
 full conditions
Thread-Topic: [PATCH v2] Drivers: hv: vmbus: Expose counters for interrupts
 and full conditions
Thread-Index: AQHUpLAXsstUN2kOgUOyNPgAu50eB6WhKi2AgAa6v7A=
Date:   Thu, 10 Jan 2019 03:58:34 +0000
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> From: Michael Kelley <mikelley@microsoft.com>
> Sent: Saturday, January 5, 2019 1:01 PM
> > From: Kimberly Brown <kimbrownkd@gmail.com>  Sent: Friday, January 4,
>  > 2019 8:35 PM
> >
> >  static inline void set_channel_pending_send_size(struct vmbus_channel =
*c,
> >  						 u32 size)
> >  {
> > +	if (size) {
> > +		++c->out_full_total;
> > +
> > +		if (!c->out_full_flag) {
> > +			++c->out_full_first;
> > +			c->out_full_flag =3D true;
> > +		}
> > +	} else {
> > +		c->out_full_flag =3D false;
> > +	}
> > +
> >  	c->outbound.ring_buffer->pending_send_sz =3D size;
> >  }
> >
>=20
> I think there may be an atomicity problem with the above code.  I looked
> in the hv_sock code, and didn't see any locks being held when
> set_channel_pending_send_size() is called.   The original code doesn't ne=
ed
> a lock because it is just storing a single value into pending_send_sz.
>
> In the similar code in hv_ringbuffer_write(), the ring buffer spin lock i=
s held
> while the counts are incremented and the out_full_flag is maintained, so =
all is
> good there.  But some locking may be needed here.  Dexuan knows the
> hv_sock
> code best and can comment on whether there is any higher level
> synchronization
> that prevents multiple threads from running the above code on the same
> channel.
> Even if there is such higher level synchronization, this code probably sh=
ouldn't
> depend on it for correctness.
>=20
> Michael

Yes, there is indeed a higher level per-"sock" lock, e.g. in the code path
vsock_stream_sendmsg() -> vsock_stream_has_space() ->=20
transport->stream_has_space() -> hvs_stream_has_space() ->=20
hvs_set_channel_pending_send_size(), we acquire the lock by
lock_sock(sk) at the beginning of vsock_stream_sendmsg(), and call
release_sock(sk) at the end of the function.

So we don't have an atomicity issue here for hv_sock, which is the only use=
r
of set_channel_pending_send_size(), so far.

Thanks,
-- Dexuan