qht: return existing entry when qht_insert fails

The meaning of "existing" is now changed to "matches in hash and
ht->cmp result". This is saner than just checking the pointer value.

Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by:  Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

accel/tcg/translate-all.c

@@ -1242,7 +1242,7 @@ static void tb_link_page(TranslationBlock *tb, tb_page_addr_t phys_pc,
     /* add in the hash table */
     h = tb_hash_func(phys_pc, tb->pc, tb->flags, tb->cflags & CF_HASH_MASK,
                      tb->trace_vcpu_dstate);
-    qht_insert(&tb_ctx.htable, tb, h);
+    qht_insert(&tb_ctx.htable, tb, h, NULL);
 
 #ifdef CONFIG_USER_ONLY
     if (DEBUG_TB_CHECK_GATE) {

include/qemu/qht.h

@@ -70,6 +70,7 @@ void qht_destroy(struct qht *ht);
  * @ht: QHT to insert to
  * @p: pointer to be inserted
  * @hash: hash corresponding to @p
+ * @existing: address where the pointer to an existing entry can be copied to
  *
  * Attempting to insert a NULL @p is a bug.
  * Inserting the same pointer @p with different @hash values is a bug.
@@ -78,9 +79,11 @@ void qht_destroy(struct qht *ht);
  * inserted into the hash table.
  *
  * Returns true on success.
- * Returns false if the @p-@hash pair already exists in the hash table.
+ * Returns false if there is an existing entry in the table that is equivalent
+ * (i.e. ht->cmp matches and the hash is the same) to @p-@h. If @existing
+ * is !NULL, a pointer to this existing entry is copied to it.
  */
-bool qht_insert(struct qht *ht, void *p, uint32_t hash);
+bool qht_insert(struct qht *ht, void *p, uint32_t hash, void **existing);
 
 /**
  * qht_lookup_custom - Look up a pointer using a custom comparison function.

tests/qht-bench.c

@@ -163,7 +163,7 @@ static void do_rw(struct thread_info *info)
             bool written = false;
 
             if (qht_lookup(&ht, p, hash) == NULL) {
-                written = qht_insert(&ht, p, hash);
+                written = qht_insert(&ht, p, hash, NULL);
             }
             if (written) {
                 stats->in++;
@@ -322,7 +322,7 @@ static void htable_init(void)
             r = xorshift64star(r);
             p = &keys[r & (init_range - 1)];
             hash = h(*p);
-            if (qht_insert(&ht, p, hash)) {
+            if (qht_insert(&ht, p, hash, NULL)) {
                 break;
             }
             retries++;

tests/test-qht.c

@@ -27,11 +27,17 @@ static void insert(int a, int b)
 
     for (i = a; i < b; i++) {
         uint32_t hash;
+        void *existing;
+        bool inserted;
 
         arr[i] = i;
         hash = i;
 
-        qht_insert(&ht, &arr[i], hash);
+        inserted = qht_insert(&ht, &arr[i], hash, NULL);
+        g_assert_true(inserted);
+        inserted = qht_insert(&ht, &arr[i], hash, &existing);
+        g_assert_false(inserted);
+        g_assert_true(existing == &arr[i]);
     }
 }
 

util/qht.c

@@ -511,7 +511,7 @@ void *qht_lookup(struct qht *ht, const void *userp, uint32_t hash)
 }
 
 /* call with head->lock held */
-static bool qht_insert__locked(struct qht *ht, struct qht_map *map,
+static void *qht_insert__locked(struct qht *ht, struct qht_map *map,
                                 struct qht_bucket *head, void *p, uint32_t hash,
                                 bool *needs_resize)
 {
@@ -523,8 +523,9 @@ static bool qht_insert__locked(struct qht *ht, struct qht_map *map,
     do {
         for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
             if (b->pointers[i]) {
-                if (unlikely(b->pointers[i] == p)) {
-                    return false;
+                if (unlikely(b->hashes[i] == hash &&
+                             ht->cmp(b->pointers[i], p))) {
+                    return b->pointers[i];
                 }
             } else {
                 goto found;
@@ -553,7 +554,7 @@ static bool qht_insert__locked(struct qht *ht, struct qht_map *map,
     atomic_set(&b->hashes[i], hash);
     atomic_set(&b->pointers[i], p);
     seqlock_write_end(&head->sequence);
-    return true;
+    return NULL;
 }
 
 static __attribute__((noinline)) void qht_grow_maybe(struct qht *ht)
@@ -577,25 +578,31 @@ static __attribute__((noinline)) void qht_grow_maybe(struct qht *ht)
     qemu_mutex_unlock(&ht->lock);
 }
 
-bool qht_insert(struct qht *ht, void *p, uint32_t hash)
+bool qht_insert(struct qht *ht, void *p, uint32_t hash, void **existing)
 {
     struct qht_bucket *b;
     struct qht_map *map;
     bool needs_resize = false;
-    bool ret;
+    void *prev;
 
     /* NULL pointers are not supported */
     qht_debug_assert(p);
 
     b = qht_bucket_lock__no_stale(ht, hash, &map);
-    ret = qht_insert__locked(ht, map, b, p, hash, &needs_resize);
+    prev = qht_insert__locked(ht, map, b, p, hash, &needs_resize);
     qht_bucket_debug__locked(b);
     qemu_spin_unlock(&b->lock);
 
     if (unlikely(needs_resize) && ht->mode & QHT_MODE_AUTO_RESIZE) {
         qht_grow_maybe(ht);
     }
-    return ret;
+    if (likely(prev == NULL)) {
+        return true;
+    }
+    if (existing) {
+        *existing = prev;
+    }
+    return false;
 }
 
 static inline bool qht_entry_is_last(struct qht_bucket *b, int pos)