The following stack exhaustion was reported in
https://bugzilla.redhat.com/show_bug.cgi?id=2186181:
...
#51 0x000055884fca7451 aio_poll (qemu-kvm + 0x9d6451)
#52 0x000055884fab9cbd bdrv_poll_co (qemu-kvm + 0x7e8cbd)
#53 0x000055884fab654b blk_io_plug (qemu-kvm + 0x7e554b)
#54 0x000055884f927fef virtio_blk_handle_vq (qemu-kvm + 0x656fef)
#55 0x000055884f96d384 virtio_queue_host_notifier_aio_poll_ready (qemu-kvm +
0x69c384)
#56 0x000055884fca671b aio_dispatch_handler (qemu-kvm + 0x9d571b)
#57 0x000055884fca7451 aio_poll (qemu-kvm + 0x9d6451)
#58 0x000055884fab9cbd bdrv_poll_co (qemu-kvm + 0x7e8cbd)
#59 0x000055884fab654b blk_io_plug (qemu-kvm + 0x7e554b)
#60 0x000055884f927fef virtio_blk_handle_vq (qemu-kvm + 0x656fef)
#61 0x000055884f96d384 virtio_queue_host_notifier_aio_poll_ready (qemu-kvm +
0x69c384)
#62 0x000055884fca671b aio_dispatch_handler (qemu-kvm + 0x9d571b)
#63 0x000055884fca7451 aio_poll (qemu-kvm + 0x9d6451)
...
This happens because some block layer APIs in QEMU 8.0 run in coroutines in
order to take the graph rdlock. Existing virtqueue handler functions weren't
written with this in mind.
A simplified example of the problem is:
void my_fd_handler(void *opaque)
{
do_something();
event_notifier_test_and_clear(opaque);
do_something_else();
}
When do_something() calls aio_poll(), my_fd_handler() will be entered again
immediately because the fd is still readable and stack exhaustion will occur.
When do_something_else() calls aio_poll(), there is no stack exhaustion since
the event notifier has been cleared and the fd is not readable.
The actual bug is more involved. The handler in question is a poll handler, not
an fd handler, but the principle is the same.
I haven't been able to reproduce the bug, but I have included a test case that
demonstrates the problem.
Stefan Hajnoczi (2):
aio-posix: do not nest poll handlers
tested: add test for nested aio_poll() in poll handlers
tests/unit/test-nested-aio-poll.c | 130 ++++++++++++++++++++++++++++++
util/aio-posix.c | 11 +++
tests/unit/meson.build | 1 +
3 files changed, 142 insertions(+)
create mode 100644 tests/unit/test-nested-aio-poll.c
--
2.40.1
