> -----Original Message----- > From: Michael Kelley [mailto:mhkel...@outlook.com] > Sent: Saturday, February 10, 2018 12:49 PM > To: gre...@linuxfoundation.org; linux-kernel@vger.kernel.org; > de...@linuxdriverproject.org; o...@aepfle.de; a...@canonical.com; > vkuzn...@redhat.com; jasow...@redhat.com; > leann.ogasaw...@canonical.com; marcelo.ce...@canonical.com; Stephen > Hemminger <sthem...@microsoft.com>; KY Srinivasan > <k...@microsoft.com> > Cc: Michael Kelley (EOSG) <michael.h.kel...@microsoft.com> > Subject: [PATCH char-misc 1/1] Drivers: hv: vmbus: Fix ring buffer signaling > > Fix bugs in signaling the Hyper-V host when freeing space in the > host->guest ring buffer: > > 1. The interrupt_mask must not be used to determine whether to signal > on the host->guest ring buffer > 2. The ring buffer write_index must be read (via hv_get_bytes_to_write) > *after* pending_send_sz is read in order to avoid a race condition > 3. Comparisons with pending_send_sz must treat the "equals" case as > not-enough-space > 4. Don't signal if the pending_send_sz feature is not present. Older > versions of Hyper-V that don't implement this feature will poll. > > Fixes: 03bad714a161 ("vmbus: more host signalling avoidance") > Signed-off-by: Michael Kelley <mhkel...@outlook.com> > --- > drivers/hv/ring_buffer.c | 24 ++++++++++++++++-------- > 1 file changed, 16 insertions(+), 8 deletions(-) > > diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c > index 50e0714..b64be18 100644 > --- a/drivers/hv/ring_buffer.c > +++ b/drivers/hv/ring_buffer.c > @@ -423,7 +423,11 @@ struct vmpacket_descriptor * > void hv_pkt_iter_close(struct vmbus_channel *channel) > { > struct hv_ring_buffer_info *rbi = &channel->inbound; > - u32 orig_write_sz = hv_get_bytes_to_write(rbi); > + u32 curr_write_sz; > + u32 delta = rbi->ring_buffer->read_index < rbi->priv_read_index ? > + (rbi->priv_read_index - rbi->ring_buffer- > >read_index) : > + (rbi->ring_datasize - rbi->ring_buffer->read_index + > + rbi->priv_read_index); > > /* > * Make sure all reads are done before we update the read index > since > @@ -446,27 +450,31 @@ void hv_pkt_iter_close(struct vmbus_channel > *channel) > */ > virt_mb(); > > - /* If host has disabled notifications then skip */ > - if (rbi->ring_buffer->interrupt_mask) > - return; > - > if (rbi->ring_buffer->feature_bits.feat_pending_send_sz) { > u32 pending_sz = READ_ONCE(rbi->ring_buffer- > >pending_send_sz); > > /* > + * Ensure the read of write_index in > hv_get_bytes_to_write() > + * happens after the read of pending_send_sz. > + */ > + virt_rmb(); We can avoid the read barrier by making the initialization of curr_write_sz conditional on pending_send_sz being non-zero. Indeed you can make all the signaling code conditional on pending_send_sz being non-zero.
> + curr_write_sz = hv_get_bytes_to_write(rbi); > + > + /* > * If there was space before we began iteration, > * then host was not blocked. Also handles case where > * pending_sz is zero then host has nothing pending > * and does not need to be signaled. > */ > - if (orig_write_sz > pending_sz) > + if (curr_write_sz - delta > pending_sz) > return; > > /* If pending write will not fit, don't give false hope. */ > - if (hv_get_bytes_to_write(rbi) < pending_sz) > + if (curr_write_sz <= pending_sz) > return; > + > + vmbus_setevent(channel); > } > > - vmbus_setevent(channel); > } > EXPORT_SYMBOL_GPL(hv_pkt_iter_close); > -- > 1.8.3.1
