block/export: don't require AioContext lock around blk_exp_ref/unref()

The FUSE export calls blk_exp_ref/unref() without the AioContext lock.
Instead of fixing the FUSE export, adjust blk_exp_ref/unref() so they
work without the AioContext lock. This way it's less error-prone.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230516190238.8401-15-stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/export/export.c

@@ -204,11 +204,10 @@ BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)
     return NULL;
 }
 
-/* Callers must hold exp->ctx lock */
 void blk_exp_ref(BlockExport *exp)
 {
-    assert(exp->refcount > 0);
-    exp->refcount++;
+    assert(qatomic_read(&exp->refcount) > 0);
+    qatomic_inc(&exp->refcount);
 }
 
 /* Runs in the main thread */
@@ -231,11 +230,10 @@ static void blk_exp_delete_bh(void *opaque)
     aio_context_release(aio_context);
 }
 
-/* Callers must hold exp->ctx lock */
 void blk_exp_unref(BlockExport *exp)
 {
-    assert(exp->refcount > 0);
-    if (--exp->refcount == 0) {
+    assert(qatomic_read(&exp->refcount) > 0);
+    if (qatomic_fetch_dec(&exp->refcount) == 1) {
         /* Touch the block_exports list only in the main thread */
         aio_bh_schedule_oneshot(qemu_get_aio_context(), blk_exp_delete_bh,
                                 exp);
@@ -343,7 +341,8 @@ void qmp_block_export_del(const char *id,
     if (!has_mode) {
         mode = BLOCK_EXPORT_REMOVE_MODE_SAFE;
     }
-    if (mode == BLOCK_EXPORT_REMOVE_MODE_SAFE && exp->refcount > 1) {
+    if (mode == BLOCK_EXPORT_REMOVE_MODE_SAFE &&
+        qatomic_read(&exp->refcount) > 1) {
         error_setg(errp, "export '%s' still in use", exp->id);
         error_append_hint(errp, "Use mode='hard' to force client "
                           "disconnect\n");

block/export/vduse-blk.c

@@ -44,9 +44,7 @@ static void vduse_blk_inflight_inc(VduseBlkExport *vblk_exp)
 {
     if (qatomic_fetch_inc(&vblk_exp->inflight) == 0) {
         /* Prevent export from being deleted */
-        aio_context_acquire(vblk_exp->export.ctx);
         blk_exp_ref(&vblk_exp->export);
-        aio_context_release(vblk_exp->export.ctx);
     }
 }
 
@@ -57,9 +55,7 @@ static void vduse_blk_inflight_dec(VduseBlkExport *vblk_exp)
         aio_wait_kick();
 
         /* Now the export can be deleted */
-        aio_context_acquire(vblk_exp->export.ctx);
         blk_exp_unref(&vblk_exp->export);
-        aio_context_release(vblk_exp->export.ctx);
     }
 }
 

include/block/export.h

@@ -57,6 +57,8 @@ struct BlockExport {
      * Reference count for this block export. This includes strong references
      * both from the owner (qemu-nbd or the monitor) and clients connected to
      * the export.
+     *
+     * Use atomics to access this field.
      */
     int refcount;