From b04aa2d7eaa0e706792e3c19ec607959dc18fb1b Mon Sep 17 00:00:00 2001
From: He Sun <bochang.sh@antgroup.com>
Date: Tue, 11 Aug 2020 20:30:09 +0800
Subject: [PATCH] Fix the dequeuing order of items in a FutexBucket

---
 src/libos/src/process/do_futex.rs | 47 +++++++++++++++++--------------
 1 file changed, 26 insertions(+), 21 deletions(-)

diff --git a/src/libos/src/process/do_futex.rs b/src/libos/src/process/do_futex.rs
index 7fc6c475..85776bd7 100644
--- a/src/libos/src/process/do_futex.rs
+++ b/src/libos/src/process/do_futex.rs
@@ -75,7 +75,7 @@ pub fn futex_wait(
     futex_val: i32,
     timeout: &Option<timespec_t>,
 ) -> Result<()> {
-    info!(
+    debug!(
         "futex_wait addr: {:#x}, val: {}, timeout: {:?}",
         futex_addr as usize, futex_val, timeout
     );
@@ -130,6 +130,11 @@ pub fn futex_wait(
 
 /// Do futex wake
 pub fn futex_wake(futex_addr: *const i32, max_count: usize) -> Result<usize> {
+    debug!(
+        "futex_wake addr: {:#x}, max_count: {}",
+        futex_addr as usize, max_count
+    );
+
     // Get and lock the futex bucket
     let futex_key = FutexKey::new(futex_addr);
     let (_, futex_bucket_ref) = FUTEX_BUCKETS.get_bucket(futex_key);
@@ -266,29 +271,29 @@ impl FutexBucket {
         self.queue.push_back(item);
     }
 
+    // TODO: this is an O(N) operation. Try to make it more efficient
     pub fn dequeue_item(&mut self, futex_item: &FutexItem) -> Option<FutexItem> {
         let item_i = self.queue.iter().position(|item| *item == *futex_item);
         if item_i.is_none() {
             return None;
         }
-        self.queue.swap_remove_back(item_i.unwrap())
+        self.queue.remove(item_i.unwrap())
     }
 
     // TODO: consider using std::future to improve the readability
     pub fn dequeue_and_wake_items(&mut self, key: FutexKey, max_count: usize) -> usize {
         let mut count = 0;
-        let mut idx = 0;
         let mut items_to_wake = Vec::new();
-        while count < max_count && idx < self.queue.len() {
-            if key == self.queue[idx].key {
-                if let Some(item) = self.queue.swap_remove_back(idx) {
-                    items_to_wake.push(item);
-                    count += 1;
-                }
+
+        self.queue.retain(|item| {
+            if count >= max_count || key != item.key {
+                true
             } else {
-                idx += 1;
+                items_to_wake.push(item.clone());
+                count += 1;
+                false
             }
-        }
+        });
 
         FutexItem::batch_wake(&items_to_wake);
         count
@@ -315,18 +320,18 @@ impl FutexBucket {
         max_nrequeues: usize,
     ) -> () {
         let mut count = 0;
-        let mut idx = 0;
-        while count < max_nrequeues && idx < self.queue.len() {
-            if key == self.queue[idx].key {
-                if let Some(mut item) = self.queue.swap_remove_back(idx) {
-                    item.key = new_key;
-                    another.enqueue_item(item);
-                    count += 1;
-                }
+
+        self.queue.retain(|item| {
+            if count >= max_nrequeues || key != item.key {
+                true
             } else {
-                idx += 1;
+                let mut new_item = item.clone();
+                new_item.key = new_key;
+                another.enqueue_item(new_item);
+                count += 1;
+                false
             }
-        }
+        });
     }
 }