block: access write_gen with atomics

Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-Id: <20170605123908.18777-13-pbonzini@redhat.com>
Signed-off-by: NFam Zheng <famz@redhat.com>

block.c

@@ -3424,7 +3424,7 @@ int bdrv_truncate(BdrvChild *child, int64_t offset, Error **errp)
         ret = refresh_total_sectors(bs, offset >> BDRV_SECTOR_BITS);
         bdrv_dirty_bitmap_truncate(bs);
         bdrv_parent_cb_resize(bs);
-        ++bs->write_gen;
+        atomic_inc(&bs->write_gen);
     }
     return ret;
 }

block/io.c

@@ -1402,7 +1402,7 @@ static int coroutine_fn bdrv_aligned_pwritev(BdrvChild *child,
     }
     bdrv_debug_event(bs, BLKDBG_PWRITEV_DONE);
 
-    ++bs->write_gen;
+    atomic_inc(&bs->write_gen);
     bdrv_set_dirty(bs, start_sector, end_sector - start_sector);
 
     stat64_max(&bs->wr_highest_offset, offset + bytes);
@@ -2291,7 +2291,7 @@ int coroutine_fn bdrv_co_flush(BlockDriverState *bs)
         goto early_exit;
     }
 
-    current_gen = bs->write_gen;
+    current_gen = atomic_read(&bs->write_gen);
 
     /* Wait until any previous flushes are completed */
     while (bs->active_flush_req) {
@@ -2516,7 +2516,7 @@ int coroutine_fn bdrv_co_pdiscard(BlockDriverState *bs, int64_t offset,
     }
     ret = 0;
 out:
-    ++bs->write_gen;
+    atomic_inc(&bs->write_gen);
     bdrv_set_dirty(bs, req.offset >> BDRV_SECTOR_BITS,
                    req.bytes >> BDRV_SECTOR_BITS);
     tracked_request_end(&req);

include/block/block_int.h

@@ -612,7 +612,6 @@ struct BlockDriverState {
     QLIST_HEAD(, BdrvTrackedRequest) tracked_requests;
     CoQueue flush_queue;                  /* Serializing flush queue */
     bool active_flush_req;                /* Flush request in flight? */
-    unsigned int write_gen;               /* Current data generation */
     unsigned int flushed_gen;             /* Flushed write generation */
 
     QLIST_HEAD(, BdrvDirtyBitmap) dirty_bitmaps;
@@ -647,6 +646,7 @@ struct BlockDriverState {
 
     /* Accessed with atomic ops.  */
     int quiesce_counter;
+    unsigned int write_gen;               /* Current data generation */
 };
 
 struct BlockBackendRootState {