[Qemu-devel] [PATCH] Serialize qcow2 writes

[Qemu-devel] [PATCH] Serialize qcow2 writes

[Avi Kivity]

Commit 641636d ("qcow2 corruption: Fix alloc_cluster_link_l2"; 4df8f71 on
stable-0.10) exposes a bug with concurrent allocating qcow2 writes: the
writes will trigger a call to free_any_clusters() and corrupt the image.

As a temporary workaround until a real fix is written, this patch serializes
writes to avoid the issue.  With this, I can install Fedora 10 on a virtio
disk.

Signed-off-by: Avi Kivity <avi@redhat.com>
---
 block-qcow2.c |   27 ++++++++++++++++++++++++++-
 1 files changed, 26 insertions(+), 1 deletions(-)

diff --git a/block-qcow2.c b/block-qcow2.c
index 1f33125..6685915 100644
--- a/block-qcow2.c
+++ b/block-qcow2.c
@@ -157,6 +157,8 @@ typedef struct BDRVQcowState {
     int snapshots_size;
     int nb_snapshots;
     QCowSnapshot *snapshots;
+    int write_in_progress;
+    TAILQ_HEAD(QCow2DeferredWrites, QCowAIOCB) deferred_writes;
 } BDRVQcowState;
 
 static int decompress_cluster(BDRVQcowState *s, uint64_t cluster_offset);
@@ -371,6 +373,9 @@ static int qcow_open(BlockDriverState *bs, const char *filename, int flags)
     if (qcow_read_snapshots(bs) < 0)
         goto fail;
 
+    s->write_in_progress = 0;
+    TAILQ_INIT(&s->deferred_writes);
+
 #ifdef DEBUG_ALLOC
     check_refcounts(bs);
 #endif
@@ -1274,6 +1279,7 @@ typedef struct QCowAIOCB {
     QEMUIOVector hd_qiov;
     QEMUBH *bh;
     QCowL2Meta l2meta;
+    TAILQ_ENTRY(QCowAIOCB) deferred_writes_link;
 } QCowAIOCB;
 
 static void qcow_aio_read_cb(void *opaque, int ret);
@@ -1439,6 +1445,8 @@ static BlockDriverAIOCB *qcow_aio_readv(BlockDriverState *bs,
     return &acb->common;
 }
 
+static void fire_deferred_writes(BDRVQcowState *s);
+
 static void qcow_aio_write_cb(void *opaque, int ret)
 {
     QCowAIOCB *acb = opaque;
@@ -1509,6 +1517,21 @@ done:
         qemu_vfree(acb->orig_buf);
     acb->common.cb(acb->common.opaque, ret);
     qemu_aio_release(acb);
+
+    s->write_in_progress = 0;
+    fire_deferred_writes(s);
+}
+
+static void fire_deferred_writes(BDRVQcowState *s)
+{
+    QCowAIOCB *acb;
+
+    if (!s->write_in_progress && !TAILQ_EMPTY(&s->deferred_writes)) {
+        s->write_in_progress = 1;
+        acb = TAILQ_FIRST(&s->deferred_writes);
+        TAILQ_REMOVE(&s->deferred_writes, acb, deferred_writes_link);
+        qcow_aio_write_cb(acb, 0);
+    }
 }
 
 static BlockDriverAIOCB *qcow_aio_writev(BlockDriverState *bs,
@@ -1524,7 +1547,9 @@ static BlockDriverAIOCB *qcow_aio_writev(BlockDriverState *bs,
     if (!acb)
         return NULL;
 
-    qcow_aio_write_cb(acb, 0);
+    TAILQ_INSERT_TAIL(&s->deferred_writes, acb, deferred_writes_link);
+    fire_deferred_writes(s);
+
     return &acb->common;
 }
 
-- 
1.6.1.1