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From:   Michael Kelley <mikelley@microsoft.com>
To:     kimbrownkd <kimbrownkd@gmail.com>, Long Li <longli@microsoft.com>,
        Sasha Levin <Alexander.Levin@microsoft.com>,
        Dexuan Cui <decui@microsoft.com>
CC:     KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        "devel@linuxdriverproject.org" <devel@linuxdriverproject.org>,
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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: AQHUpLAU1Q/2YfEbRkGAkXaUeYtNvqWhJZ5Q
Date:   Sat, 5 Jan 2019 21:01:07 +0000
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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 need
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 is =
held
while the counts are incremented and the out_full_flag is maintained, so al=
l 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 synchronizat=
ion
that prevents multiple threads from running the above code on the same chan=
nel.
Even if there is such higher level synchronization, this code probably shou=
ldn't
depend on it for correctness.

Michael