[Qemu-devel] [PATCH 1/1] Stop reinit of XBZRLE.lock

[Dr. David Alan Gilbert (git)]

From: "Dr. David Alan Gilbert" <address@hidden>

Markus Armbruster spotted that the XBZRLE.lock might get initalised
multiple times in the case of a second attempted migration, and
that's undefined behaviour for pthread_mutex_init.

This patch adds a flag to stop re-initialisation - finding somewhere
to cleanly destroy it where we can guarantee isn't happening
at the same place we might want to take the lock is tricky.

It also adds comments to explain the lock use more.

(I considered rewriting this lockless but can't justify it given
the simplicity of the fix).

Signed-off-by: Dr. David Alan Gilbert <address@hidden>
---
 arch_init.c | 30 ++++++++++++++++++++++++++----
 1 file changed, 26 insertions(+), 4 deletions(-)

diff --git a/arch_init.c b/arch_init.c
index 60c975d..16474b5 100644
--- a/arch_init.c
+++ b/arch_init.c
@@ -167,10 +167,13 @@ static struct {
     /* Cache for XBZRLE, Protected by lock. */
     PageCache *cache;
     QemuMutex lock;
+    bool lock_init; /* True once we've init'd lock */
 } XBZRLE = {
     .encoded_buf = NULL,
     .current_buf = NULL,
     .cache = NULL,
+    /* .lock is initialised in ram_save_setup */
+    .lock_init = false
 };
 /* buffer used for XBZRLE decoding */
 static uint8_t *xbzrle_decoded_buf;
@@ -187,6 +190,11 @@ static void XBZRLE_cache_unlock(void)
         qemu_mutex_unlock(&XBZRLE.lock);
 }
 
+/* called from qmp_migrate_set_cache_size in main thread, possibly while
+ * a migration is in progress.
+ * A running migration maybe using the cache and might finish during this
+ * call, hence changes to the cache are protected by XBZRLE.lock().
+ */
 int64_t xbzrle_cache_resize(int64_t new_size)
 {
     PageCache *new_cache, *cache_to_free;
@@ -195,9 +203,12 @@ int64_t xbzrle_cache_resize(int64_t new_size)
         return -1;
     }
 
-    /* no need to lock, the current thread holds qemu big lock */
+    /* The current thread holds qemu big lock, and we hold it while creating
+     * the cache in ram_save_setup, thus it's safe to test if the
+     * cache exists yet without it's own lock (which might not have been
+     * init'd yet)
+     */
     if (XBZRLE.cache != NULL) {
-        /* check XBZRLE.cache again later */
         if (pow2floor(new_size) == migrate_xbzrle_cache_size()) {
             return pow2floor(new_size);
         }
@@ -209,7 +220,10 @@ int64_t xbzrle_cache_resize(int64_t new_size)
         }
 
         XBZRLE_cache_lock();
-        /* the XBZRLE.cache may have be destroyed, check it again */
+        /* the migration might have finished between the check above and us
+         * taking the lock,  causing XBZRLE.cache to be destroyed
+         *   check it again
+         */
         if (XBZRLE.cache != NULL) {
             cache_to_free = XBZRLE.cache;
             XBZRLE.cache = new_cache;
@@ -744,7 +758,15 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
             DPRINTF("Error creating cache\n");
             return -1;
         }
-        qemu_mutex_init(&XBZRLE.lock);
+        /* mutex's can't be reinit'd without destroying them
+         * and we've not got a good place to destroy it that
+         * we can guarantee isn't being called when we might want
+         * to hold the lock.
+         */
+        if (!XBZRLE.lock_init) {
+            XBZRLE.lock_init = true;
+            qemu_mutex_init(&XBZRLE.lock);
+        }
         qemu_mutex_unlock_iothread();
 
         /* We prefer not to abort if there is no memory */
-- 
1.8.5.3