From 3957d1ee272c2c2e8967b7c7da2d0bf09b0d3188 Mon Sep 17 00:00:00 2001
From: Sergio Lopez <slp@redhat.com>
Date: Tue, 10 Mar 2020 05:43:13 -0400
Subject: [PATCH] vhost_user_backend: call get_used_event from
 needs_notification

This change, combined with the compiler hint to inline get_used_event,
shortens the window between the memory read and the actual check by
calling get_used_event from needs_notification.

Without it, when putting enough pressure on the vring, it's possible
that a notification is wrongly omitted, causing the queue to stall.

Signed-off-by: Sergio Lopez <slp@redhat.com>
---
 src/bin/vhost_user_fs.rs      | 3 +--
 vhost_user_backend/src/lib.rs | 8 ++------
 vhost_user_block/src/lib.rs   | 3 +--
 3 files changed, 4 insertions(+), 10 deletions(-)

diff --git a/src/bin/vhost_user_fs.rs b/src/bin/vhost_user_fs.rs
index 01bcaaa25..5b666805f 100644
--- a/src/bin/vhost_user_fs.rs
+++ b/src/bin/vhost_user_fs.rs
@@ -126,8 +126,7 @@ impl<F: FileSystem + Send + Sync + 'static> VhostUserFsBackend<F> {
                 .map_err(Error::ProcessQueue)?;
             if self.event_idx {
                 if let Some(used_idx) = vring.mut_queue().add_used(mem, head_index, 0) {
-                    let used_event = vring.mut_queue().get_used_event(mem);
-                    if vring.needs_notification(Wrapping(used_idx), used_event) {
+                    if vring.needs_notification(&mem, Wrapping(used_idx)) {
                         vring.signal_used_queue().map_err(Error::SignalQueue)?;
                     }
                     used_any = true;
diff --git a/vhost_user_backend/src/lib.rs b/vhost_user_backend/src/lib.rs
index 6256e066f..10184a175 100644
--- a/vhost_user_backend/src/lib.rs
+++ b/vhost_user_backend/src/lib.rs
@@ -235,11 +235,7 @@ impl Vring {
         self.event_idx = enabled;
     }
 
-    pub fn needs_notification(
-        &mut self,
-        used_idx: Wrapping<u16>,
-        used_event: Option<Wrapping<u16>>,
-    ) -> bool {
+    pub fn needs_notification(&mut self, mem: &GuestMemoryMmap, used_idx: Wrapping<u16>) -> bool {
         if !self.event_idx {
             return true;
         }
@@ -247,7 +243,7 @@ impl Vring {
         let mut notify = true;
 
         if let Some(old_idx) = self.signalled_used {
-            if let Some(used_event) = used_event {
+            if let Some(used_event) = self.mut_queue().get_used_event(&mem) {
                 if (used_idx - used_event - Wrapping(1u16)) >= (used_idx - old_idx) {
                     notify = false;
                 }
diff --git a/vhost_user_block/src/lib.rs b/vhost_user_block/src/lib.rs
index c9671186b..395df9f44 100644
--- a/vhost_user_block/src/lib.rs
+++ b/vhost_user_block/src/lib.rs
@@ -197,8 +197,7 @@ impl VhostUserBlkBackend {
 
             if self.event_idx {
                 if let Some(used_idx) = vring.mut_queue().add_used(mem, head.index, len) {
-                    let used_event = vring.mut_queue().get_used_event(mem);
-                    if vring.needs_notification(Wrapping(used_idx), used_event) {
+                    if vring.needs_notification(&mem, Wrapping(used_idx)) {
                         debug!("signalling queue");
                         vring.signal_used_queue().unwrap();
                     } else {