[Qemu-devel] [PATCH v4] XBZRLE: Fix qemu crash when resize the xbzrle cache

[Gonglei <arei.gonglei@huawei.com>]

Resizing the xbzrle cache during migration causes qemu-crash,
because the main-thread and migration-thread modify the xbzrle
cache size concurrently without lock-protection.

Signed-off-by: ChenLiang <chenliang88@huawei.com>
Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
---
Changes:
 - Resolve the problem of lock initialization on windows platform.

 arch_init.c | 52 +++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 49 insertions(+), 3 deletions(-)

diff --git a/arch_init.c b/arch_init.c
index fe17279..60c975d 100644
--- a/arch_init.c
+++ b/arch_init.c
@@ -164,8 +164,9 @@ static struct {
     uint8_t *encoded_buf;
     /* buffer for storing page content */
     uint8_t *current_buf;
-    /* Cache for XBZRLE */
+    /* Cache for XBZRLE, Protected by lock. */
     PageCache *cache;
+    QemuMutex lock;
 } XBZRLE = {
     .encoded_buf = NULL,
     .current_buf = NULL,
@@ -174,16 +175,52 @@ static struct {
 /* buffer used for XBZRLE decoding */
 static uint8_t *xbzrle_decoded_buf;

+static void XBZRLE_cache_lock(void)
+{
+    if (migrate_use_xbzrle())
+        qemu_mutex_lock(&XBZRLE.lock);
+}
+
+static void XBZRLE_cache_unlock(void)
+{
+    if (migrate_use_xbzrle())
+        qemu_mutex_unlock(&XBZRLE.lock);
+}
+
 int64_t xbzrle_cache_resize(int64_t new_size)
 {
+    PageCache *new_cache, *cache_to_free;
+
     if (new_size < TARGET_PAGE_SIZE) {
         return -1;
     }

+    /* no need to lock, the current thread holds qemu big lock */
     if (XBZRLE.cache != NULL) {
-        return cache_resize(XBZRLE.cache, new_size / TARGET_PAGE_SIZE) *
-            TARGET_PAGE_SIZE;
+        /* check XBZRLE.cache again later */
+        if (pow2floor(new_size) == migrate_xbzrle_cache_size()) {
+            return pow2floor(new_size);
+        }
+        new_cache = cache_init(new_size / TARGET_PAGE_SIZE,
+                                        TARGET_PAGE_SIZE);
+        if (!new_cache) {
+            DPRINTF("Error creating cache\n");
+            return -1;
+        }
+
+        XBZRLE_cache_lock();
+        /* the XBZRLE.cache may have be destroyed, check it again */
+        if (XBZRLE.cache != NULL) {
+            cache_to_free = XBZRLE.cache;
+            XBZRLE.cache = new_cache;
+        } else {
+            cache_to_free = new_cache;
+        }
+        XBZRLE_cache_unlock();
+
+        cache_fini(cache_to_free);
     }
+
     return pow2floor(new_size);
 }

@@ -539,6 +576,8 @@ static int ram_save_block(QEMUFile *f, bool last_stage)
             ret = ram_control_save_page(f, block->offset,
                                offset, TARGET_PAGE_SIZE, &bytes_sent);

+            XBZRLE_cache_lock();
+
             current_addr = block->offset + offset;
             if (ret != RAM_SAVE_CONTROL_NOT_SUPP) {
                 if (ret != RAM_SAVE_CONTROL_DELAYED) {
@@ -587,6 +626,7 @@ static int ram_save_block(QEMUFile *f, bool last_stage)
                 acct_info.norm_pages++;
             }

+            XBZRLE_cache_unlock();
             /* if page is unmodified, continue to the next */
             if (bytes_sent > 0) {
                 last_sent_block = block;
@@ -654,6 +694,7 @@ static void migration_end(void)
         migration_bitmap = NULL;
     }

+    XBZRLE_cache_lock();
     if (XBZRLE.cache) {
         cache_fini(XBZRLE.cache);
         g_free(XBZRLE.cache);
@@ -663,6 +704,7 @@ static void migration_end(void)
         XBZRLE.encoded_buf = NULL;
         XBZRLE.current_buf = NULL;
     }
+    XBZRLE_cache_unlock();
 }

 static void ram_migration_cancel(void *opaque)
@@ -693,13 +735,17 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
     dirty_rate_high_cnt = 0;

     if (migrate_use_xbzrle()) {
+        qemu_mutex_lock_iothread();
         XBZRLE.cache = cache_init(migrate_xbzrle_cache_size() /
                                   TARGET_PAGE_SIZE,
                                   TARGET_PAGE_SIZE);
         if (!XBZRLE.cache) {
+            qemu_mutex_unlock_iothread();
             DPRINTF("Error creating cache\n");
             return -1;
         }
+        qemu_mutex_init(&XBZRLE.lock);
+        qemu_mutex_unlock_iothread();

         /* We prefer not to abort if there is no memory */
         XBZRLE.encoded_buf = g_try_malloc0(TARGET_PAGE_SIZE);
-- 
1.7.12.4


Best regards,
-Gonglei