block: remove AioContext locking

This is the big patch that removes
aio_context_acquire()/aio_context_release() from the block layer and
affected block layer users.

There isn't a clean way to split this patch and the reviewers are likely
the same group of people, so I decided to do it in one patch.

Signed-off-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Kevin Wolf <[email protected]>
Reviewed-by: Paul Durrant <[email protected]>
Message-ID: <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>

Author: stefanhaRH

block.c

@@ -429,7 +429,6 @@ BlockBackend *blk_new_open(const char *filename, const char *reference,
 {
     BlockBackend *blk;
     BlockDriverState *bs;
-    AioContext *ctx;
     uint64_t perm = 0;
     uint64_t shared = BLK_PERM_ALL;
 
@@ -459,23 +458,18 @@ BlockBackend *blk_new_open(const char *filename, const char *reference,
         shared = BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE_UNCHANGED;
     }
 
-    aio_context_acquire(qemu_get_aio_context());
     bs = bdrv_open(filename, reference, options, flags, errp);
-    aio_context_release(qemu_get_aio_context());
     if (!bs) {
         return NULL;
     }
 
     /* bdrv_open() could have moved bs to a different AioContext */
-    ctx = bdrv_get_aio_context(bs);
     blk = blk_new(bdrv_get_aio_context(bs), perm, shared);
     blk->perm = perm;
     blk->shared_perm = shared;
 
-    aio_context_acquire(ctx);
     blk_insert_bs(blk, bs, errp);
     bdrv_unref(bs);
-    aio_context_release(ctx);
 
     if (!blk->root) {
         blk_unref(blk);
@@ -577,13 +571,9 @@ void blk_remove_all_bs(void)
     GLOBAL_STATE_CODE();
 
     while ((blk = blk_all_next(blk)) != NULL) {
-        AioContext *ctx = blk_get_aio_context(blk);
-
-        aio_context_acquire(ctx);
         if (blk->root) {
             blk_remove_bs(blk);
         }
-        aio_context_release(ctx);
     }
 }
 
@@ -2736,20 +2726,16 @@ int blk_commit_all(void)
     GRAPH_RDLOCK_GUARD_MAINLOOP();
 
     while ((blk = blk_all_next(blk)) != NULL) {
-        AioContext *aio_context = blk_get_aio_context(blk);
         BlockDriverState *unfiltered_bs = bdrv_skip_filters(blk_bs(blk));
 
-        aio_context_acquire(aio_context);
         if (blk_is_inserted(blk) && bdrv_cow_child(unfiltered_bs)) {
             int ret;
 
             ret = bdrv_commit(unfiltered_bs);
             if (ret < 0) {
-                aio_context_release(aio_context);
                 return ret;
             }
         }
-        aio_context_release(aio_context);
     }
     return 0;
 }

block/block-backend.c

@@ -412,7 +412,6 @@ static int cbw_open(BlockDriverState *bs, QDict *options, int flags,
     int64_t cluster_size;
     g_autoptr(BlockdevOptions) full_opts = NULL;
     BlockdevOptionsCbw *opts;
-    AioContext *ctx;
     int ret;
 
     full_opts = cbw_parse_options(options, errp);
@@ -435,15 +434,11 @@ static int cbw_open(BlockDriverState *bs, QDict *options, int flags,
 
     GRAPH_RDLOCK_GUARD_MAINLOOP();
 
-    ctx = bdrv_get_aio_context(bs);
-    aio_context_acquire(ctx);
-
     if (opts->bitmap) {
         bitmap = block_dirty_bitmap_lookup(opts->bitmap->node,
                                            opts->bitmap->name, NULL, errp);
         if (!bitmap) {
-            ret = -EINVAL;
-            goto out;
+            return -EINVAL;
         }
     }
     s->on_cbw_error = opts->has_on_cbw_error ? opts->on_cbw_error :
@@ -461,24 +456,21 @@ static int cbw_open(BlockDriverState *bs, QDict *options, int flags,
     s->bcs = block_copy_state_new(bs->file, s->target, bitmap, errp);
     if (!s->bcs) {
         error_prepend(errp, "Cannot create block-copy-state: ");
-        ret = -EINVAL;
-        goto out;
+        return -EINVAL;
     }
 
     cluster_size = block_copy_cluster_size(s->bcs);
 
     s->done_bitmap = bdrv_create_dirty_bitmap(bs, cluster_size, NULL, errp);
     if (!s->done_bitmap) {
-        ret = -EINVAL;
-        goto out;
+        return -EINVAL;
     }
     bdrv_disable_dirty_bitmap(s->done_bitmap);
 
     /* s->access_bitmap starts equal to bcs bitmap */
     s->access_bitmap = bdrv_create_dirty_bitmap(bs, cluster_size, NULL, errp);
     if (!s->access_bitmap) {
-        ret = -EINVAL;
-        goto out;
+        return -EINVAL;
     }
     bdrv_disable_dirty_bitmap(s->access_bitmap);
     bdrv_dirty_bitmap_merge_internal(s->access_bitmap,
@@ -487,11 +479,7 @@ static int cbw_open(BlockDriverState *bs, QDict *options, int flags,
 
     qemu_co_mutex_init(&s->lock);
     QLIST_INIT(&s->frozen_read_reqs);
-
-    ret = 0;
-out:
-    aio_context_release(ctx);
-    return ret;
+    return 0;
 }
 
 static void cbw_close(BlockDriverState *bs)

block/copy-before-write.c

@@ -114,7 +114,6 @@ BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)
     }
 
     ctx = bdrv_get_aio_context(bs);
-    aio_context_acquire(ctx);
 
     if (export->iothread) {
         IOThread *iothread;
@@ -133,8 +132,6 @@ BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)
         set_context_errp = fixed_iothread ? errp : NULL;
         ret = bdrv_try_change_aio_context(bs, new_ctx, NULL, set_context_errp);
         if (ret == 0) {
-            aio_context_release(ctx);
-            aio_context_acquire(new_ctx);
             ctx = new_ctx;
         } else if (fixed_iothread) {
             goto fail;
@@ -191,16 +188,13 @@ BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)
     assert(exp->blk != NULL);
 
     QLIST_INSERT_HEAD(&block_exports, exp, next);
-
-    aio_context_release(ctx);
     return exp;
 
 fail:
     if (blk) {
         blk_set_dev_ops(blk, NULL, NULL);
         blk_unref(blk);
     }
-    aio_context_release(ctx);
     if (exp) {
         g_free(exp->id);
         g_free(exp);
@@ -218,9 +212,6 @@ void blk_exp_ref(BlockExport *exp)
 static void blk_exp_delete_bh(void *opaque)
 {
     BlockExport *exp = opaque;
-    AioContext *aio_context = exp->ctx;
-
-    aio_context_acquire(aio_context);
 
     assert(exp->refcount == 0);
     QLIST_REMOVE(exp, next);
@@ -230,8 +221,6 @@ static void blk_exp_delete_bh(void *opaque)
     qapi_event_send_block_export_deleted(exp->id);
     g_free(exp->id);
     g_free(exp);
-
-    aio_context_release(aio_context);
 }
 
 void blk_exp_unref(BlockExport *exp)
@@ -249,32 +238,23 @@ void blk_exp_unref(BlockExport *exp)
  * connections and other internally held references start to shut down. When
  * the function returns, there may still be active references while the export
  * is in the process of shutting down.
- *
- * Acquires exp->ctx internally. Callers must *not* hold the lock.
  */
 void blk_exp_request_shutdown(BlockExport *exp)
 {
-    AioContext *aio_context = exp->ctx;
-
-    aio_context_acquire(aio_context);
-
     /*
      * If the user doesn't own the export any more, it is already shutting
      * down. We must not call .request_shutdown and decrease the refcount a
      * second time.
      */
     if (!exp->user_owned) {
-        goto out;
+        return;
     }
 
     exp->drv->request_shutdown(exp);
 
     assert(exp->user_owned);
     exp->user_owned = false;
     blk_exp_unref(exp);
-
-out:
-    aio_context_release(aio_context);
 }
 
 /*

block/export/export.c

@@ -294,16 +294,13 @@ static void bdrv_co_drain_bh_cb(void *opaque)
     BlockDriverState *bs = data->bs;
 
     if (bs) {
-        AioContext *ctx = bdrv_get_aio_context(bs);
-        aio_context_acquire(ctx);
         bdrv_dec_in_flight(bs);
         if (data->begin) {
             bdrv_do_drained_begin(bs, data->parent, data->poll);
         } else {
             assert(!data->poll);
             bdrv_do_drained_end(bs, data->parent);
         }
-        aio_context_release(ctx);
     } else {
         assert(data->begin);
         bdrv_drain_all_begin();
@@ -320,8 +317,6 @@ static void coroutine_fn bdrv_co_yield_to_drain(BlockDriverState *bs,
 {
     BdrvCoDrainData data;
     Coroutine *self = qemu_coroutine_self();
-    AioContext *ctx = bdrv_get_aio_context(bs);
-    AioContext *co_ctx = qemu_coroutine_get_aio_context(self);
 
     /* Calling bdrv_drain() from a BH ensures the current coroutine yields and
      * other coroutines run if they were queued by aio_co_enter(). */
@@ -340,29 +335,13 @@ static void coroutine_fn bdrv_co_yield_to_drain(BlockDriverState *bs,
         bdrv_inc_in_flight(bs);
     }
 
-    /*
-     * Temporarily drop the lock across yield or we would get deadlocks.
-     * bdrv_co_drain_bh_cb() reaquires the lock as needed.
-     *
-     * When we yield below, the lock for the current context will be
-     * released, so if this is actually the lock that protects bs, don't drop
-     * it a second time.
-     */
-    if (ctx != co_ctx) {
-        aio_context_release(ctx);
-    }
     replay_bh_schedule_oneshot_event(qemu_get_aio_context(),
                                      bdrv_co_drain_bh_cb, &data);
 
     qemu_coroutine_yield();
     /* If we are resumed from some other event (such as an aio completion or a
      * timer callback), it is a bug in the caller that should be fixed. */
     assert(data.done);
-
-    /* Reacquire the AioContext of bs if we dropped it */
-    if (ctx != co_ctx) {
-        aio_context_acquire(ctx);
-    }
 }
 
 static void bdrv_do_drained_begin(BlockDriverState *bs, BdrvChild *parent,
@@ -478,13 +457,12 @@ static bool bdrv_drain_all_poll(void)
     GLOBAL_STATE_CODE();
     GRAPH_RDLOCK_GUARD_MAINLOOP();
 
-    /* bdrv_drain_poll() can't make changes to the graph and we are holding the
-     * main AioContext lock, so iterating bdrv_next_all_states() is safe. */
+    /*
+     * bdrv_drain_poll() can't make changes to the graph and we hold the BQL,
+     * so iterating bdrv_next_all_states() is safe.
+     */
     while ((bs = bdrv_next_all_states(bs))) {
-        AioContext *aio_context = bdrv_get_aio_context(bs);
-        aio_context_acquire(aio_context);
         result |= bdrv_drain_poll(bs, NULL, true);
-        aio_context_release(aio_context);
     }
 
     return result;
@@ -525,11 +503,7 @@ void bdrv_drain_all_begin_nopoll(void)
     /* Quiesce all nodes, without polling in-flight requests yet. The graph
      * cannot change during this loop. */
     while ((bs = bdrv_next_all_states(bs))) {
-        AioContext *aio_context = bdrv_get_aio_context(bs);
-
-        aio_context_acquire(aio_context);
         bdrv_do_drained_begin(bs, NULL, false);
-        aio_context_release(aio_context);
     }
 }
 
@@ -588,11 +562,7 @@ void bdrv_drain_all_end(void)
     }
 
     while ((bs = bdrv_next_all_states(bs))) {
-        AioContext *aio_context = bdrv_get_aio_context(bs);
-
-        aio_context_acquire(aio_context);
         bdrv_do_drained_end(bs, NULL);
-        aio_context_release(aio_context);
     }
 
     assert(qemu_get_current_aio_context() == qemu_get_aio_context());
@@ -2368,15 +2338,10 @@ int bdrv_flush_all(void)
     }
 
     for (bs = bdrv_first(&it); bs; bs = bdrv_next(&it)) {
-        AioContext *aio_context = bdrv_get_aio_context(bs);
-        int ret;
-
-        aio_context_acquire(aio_context);
-        ret = bdrv_flush(bs);
+        int ret = bdrv_flush(bs);
         if (ret < 0 && !result) {
             result = ret;
         }
-        aio_context_release(aio_context);
     }
 
     return result;