| Message ID | 20230502184134.534703-1-stefanha@redhat.com |
|---|---|
| Headers | show |
| Series | aio-posix: do not nest poll handlers | expand |
Am 02.05.2023 um 20:41 hat Stefan Hajnoczi geschrieben: > 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. Thanks, applied to the block branch. Kevin
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