Comment 3 for bug 1839428

Xujun Ma (maxujun) wrote :

I found the commit that introduced this regression.

commit 57830a499f7c815bb0cb325c94a3d8c910d13cfa (HEAD)
Author: Denis Plotnikov <email address hidden>
Date: Fri Feb 15 16:03:25 2019 +0300

    block: don't set the same context

    Adds a fast path on aio context setting preventing
    unnecessary context setting routine.
    Also, it prevents issues with cyclic walk of child
    bds-es appeared because of registering aio walking
    notifiers:

    Call stack:

    0 __GI_raise
    1 __GI_abort
    2 __assert_fail_base
    3 __GI___assert_fail
    4 bdrv_detach_aio_context (bs=0x55f54d65c000) <<<
    5 bdrv_detach_aio_context (bs=0x55f54fc8a800)
    6 bdrv_set_aio_context (bs=0x55f54fc8a800, ...)
    7 block_job_attached_aio_context
    8 bdrv_attach_aio_context (bs=0x55f54d65c000, ...) <<<
    9 bdrv_set_aio_context (bs=0x55f54d65c000)
    10 blk_set_aio_context
    11 virtio_blk_data_plane_stop
    12 virtio_bus_stop_ioeventfd
    13 virtio_vmstate_change
    14 vm_state_notify (running=0, state=RUN_STATE_SHUTDOWN)
    15 do_vm_stop (state=RUN_STATE_SHUTDOWN, send_stop=true)
    16 vm_stop (state=RUN_STATE_SHUTDOWN)
    17 main_loop_should_exit
    18 main_loop
    19 main

    This can happen because of "new" context attachment to VM disk bds.
    When attaching a new context the corresponding aio context handler is
    called for each of aio_notifiers registered on the VM disk bds context.
    Among those handlers, there is the block_job_attached_aio_context handler
    which sets a new aio context for the block job bds. When doing so,
    the old context is detached from all the block job bds children and one of
    them is the VM disk bds, serving as backing store for the blockjob bds,
    although the VM disk bds is actually the initializer of that process.
    Since the VM disk bds is protected with walking_aio_notifiers flag
    from double processing in recursive calls, the assert fires.

    Signed-off-by: Denis Plotnikov <email address hidden>
    Signed-off-by: Kevin Wolf <email address hidden>

diff --git a/block.c b/block.c
index 4ad0e90d7e..0c12632661 100644
--- a/block.c
+++ b/block.c
@@ -5265,6 +5265,10 @@ void bdrv_set_aio_context(BlockDriverState *bs, AioContext *new_context)
 {
     AioContext *ctx = bdrv_get_aio_context(bs);

+ if (ctx == new_context) {
+ return;
+ }
+
     aio_disable_external(ctx);
     bdrv_parent_drained_begin(bs, NULL, false);
     bdrv_drain(bs); /* ensure there are no in-flight requests */