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virtio-blk: On restart, process queued requests in the proper context
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On restart, we were scheduling a BH to process queued requests, which
would run before starting up the data plane, leading to those requests
being assigned and started on coroutines on the main context.

This could cause requests to be wrongly processed in parallel from
different threads (the main thread and the iothread managing the data
plane), potentially leading to multiple issues.

For example, stopping and resuming a VM multiple times while the guest
is generating I/O on a virtio_blk device can trigger a crash with a
stack tracing looking like this one:

<------>
 Thread 2 (Thread 0x7ff736765700 (LWP 1062503)):
 #0  0x00005567a13b99d6 in iov_memset
     (iov=0x6563617073206f4e, iov_cnt=1717922848, offset=516096, fillc=0, bytes=7018105756081554803)
     at util/iov.c:69
 CTSRD-CHERI#1  0x00005567a13bab73 in qemu_iovec_memset
     (qiov=0x7ff73ec99748, offset=516096, fillc=0, bytes=7018105756081554803) at util/iov.c:530
 CTSRD-CHERI#2  0x00005567a12f411c in qemu_laio_process_completion (laiocb=0x7ff6512ee6c0) at block/linux-aio.c:86
 CTSRD-CHERI#3  0x00005567a12f42ff in qemu_laio_process_completions (s=0x7ff7182e8420) at block/linux-aio.c:217
 CTSRD-CHERI#4  0x00005567a12f480d in ioq_submit (s=0x7ff7182e8420) at block/linux-aio.c:323
 CTSRD-CHERI#5  0x00005567a12f43d9 in qemu_laio_process_completions_and_submit (s=0x7ff7182e8420)
     at block/linux-aio.c:236
 CTSRD-CHERI#6  0x00005567a12f44c2 in qemu_laio_poll_cb (opaque=0x7ff7182e8430) at block/linux-aio.c:267
 CTSRD-CHERI#7  0x00005567a13aed83 in run_poll_handlers_once (ctx=0x5567a2b58c70, timeout=0x7ff7367645f8)
     at util/aio-posix.c:520
 CTSRD-CHERI#8  0x00005567a13aee9f in run_poll_handlers (ctx=0x5567a2b58c70, max_ns=16000, timeout=0x7ff7367645f8)
     at util/aio-posix.c:562
 CTSRD-CHERI#9  0x00005567a13aefde in try_poll_mode (ctx=0x5567a2b58c70, timeout=0x7ff7367645f8)
     at util/aio-posix.c:597
 CTSRD-CHERI#10 0x00005567a13af115 in aio_poll (ctx=0x5567a2b58c70, blocking=true) at util/aio-posix.c:639
 CTSRD-CHERI#11 0x00005567a109acca in iothread_run (opaque=0x5567a2b29760) at iothread.c:75
 CTSRD-CHERI#12 0x00005567a13b2790 in qemu_thread_start (args=0x5567a2b694c0) at util/qemu-thread-posix.c:519
 CTSRD-CHERI#13 0x00007ff73eedf2de in start_thread () at /lib64/libpthread.so.0
 CTSRD-CHERI#14 0x00007ff73ec10e83 in clone () at /lib64/libc.so.6

 Thread 1 (Thread 0x7ff743986f00 (LWP 1062500)):
 #0  0x00005567a13b99d6 in iov_memset
     (iov=0x6563617073206f4e, iov_cnt=1717922848, offset=516096, fillc=0, bytes=7018105756081554803)
     at util/iov.c:69
 CTSRD-CHERI#1  0x00005567a13bab73 in qemu_iovec_memset
     (qiov=0x7ff73ec99748, offset=516096, fillc=0, bytes=7018105756081554803) at util/iov.c:530
 CTSRD-CHERI#2  0x00005567a12f411c in qemu_laio_process_completion (laiocb=0x7ff6512ee6c0) at block/linux-aio.c:86
 CTSRD-CHERI#3  0x00005567a12f42ff in qemu_laio_process_completions (s=0x7ff7182e8420) at block/linux-aio.c:217
 CTSRD-CHERI#4  0x00005567a12f480d in ioq_submit (s=0x7ff7182e8420) at block/linux-aio.c:323
 CTSRD-CHERI#5  0x00005567a12f4a2f in laio_do_submit (fd=19, laiocb=0x7ff5f4ff9ae0, offset=472363008, type=2)
     at block/linux-aio.c:375
 CTSRD-CHERI#6  0x00005567a12f4af2 in laio_co_submit
     (bs=0x5567a2b8c460, s=0x7ff7182e8420, fd=19, offset=472363008, qiov=0x7ff5f4ff9ca0, type=2)
     at block/linux-aio.c:394
 CTSRD-CHERI#7  0x00005567a12f1803 in raw_co_prw
     (bs=0x5567a2b8c460, offset=472363008, bytes=20480, qiov=0x7ff5f4ff9ca0, type=2)
     at block/file-posix.c:1892
 CTSRD-CHERI#8  0x00005567a12f1941 in raw_co_pwritev
     (bs=0x5567a2b8c460, offset=472363008, bytes=20480, qiov=0x7ff5f4ff9ca0, flags=0)
     at block/file-posix.c:1925
 CTSRD-CHERI#9  0x00005567a12fe3e1 in bdrv_driver_pwritev
     (bs=0x5567a2b8c460, offset=472363008, bytes=20480, qiov=0x7ff5f4ff9ca0, qiov_offset=0, flags=0)
     at block/io.c:1183
 CTSRD-CHERI#10 0x00005567a1300340 in bdrv_aligned_pwritev
     (child=0x5567a2b5b070, req=0x7ff5f4ff9db0, offset=472363008, bytes=20480, align=512, qiov=0x7ff72c0425b8, qiov_offset=0, flags=0) at block/io.c:1980
 CTSRD-CHERI#11 0x00005567a1300b29 in bdrv_co_pwritev_part
     (child=0x5567a2b5b070, offset=472363008, bytes=20480, qiov=0x7ff72c0425b8, qiov_offset=0, flags=0)
     at block/io.c:2137
 CTSRD-CHERI#12 0x00005567a12baba1 in qcow2_co_pwritev_task
     (bs=0x5567a2b92740, file_cluster_offset=472317952, offset=487305216, bytes=20480, qiov=0x7ff72c0425b8, qiov_offset=0, l2meta=0x0) at block/qcow2.c:2444
 CTSRD-CHERI#13 0x00005567a12bacdb in qcow2_co_pwritev_task_entry (task=0x5567a2b48540) at block/qcow2.c:2475
 CTSRD-CHERI#14 0x00005567a13167d8 in aio_task_co (opaque=0x5567a2b48540) at block/aio_task.c:45
 CTSRD-CHERI#15 0x00005567a13cf00c in coroutine_trampoline (i0=738245600, i1=32759) at util/coroutine-ucontext.c:115
 CTSRD-CHERI#16 0x00007ff73eb622e0 in __start_context () at /lib64/libc.so.6
 CTSRD-CHERI#17 0x00007ff6626f1350 in  ()
 CTSRD-CHERI#18 0x0000000000000000 in  ()
<------>

This is also known to cause crashes with this message (assertion
failed):

 aio_co_schedule: Co-routine was already scheduled in 'aio_co_schedule'

RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1812765
Signed-off-by: Sergio Lopez <slp@redhat.com>
Message-Id: <20200603093240.40489-3-slp@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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@slp @kevmw
slp authored and kevmw committed Jun 17, 2020
1 parent 7aa1c24 commit 49b4454
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Showing 3 changed files with 21 additions and 7 deletions.
8 changes: 8 additions & 0 deletions hw/block/dataplane/virtio-blk.c
View file
Expand Up @@ -220,6 +220,9 @@ int virtio_blk_data_plane_start(VirtIODevice *vdev)
goto fail_guest_notifiers;
}

/* Process queued requests before the ones in vring */
virtio_blk_process_queued_requests(vblk, false);

/* Kick right away to begin processing requests already in vring */
for (i = 0; i < nvqs; i++) {
VirtQueue *vq = virtio_get_queue(s->vdev, i);
Expand All @@ -239,6 +242,11 @@ int virtio_blk_data_plane_start(VirtIODevice *vdev)
return 0;

fail_guest_notifiers:
/*
* If we failed to set up the guest notifiers queued requests will be
* processed on the main context.
*/
virtio_blk_process_queued_requests(vblk, false);
vblk->dataplane_disabled = true;
s->starting = false;
vblk->dataplane_started = true;
Expand Down
18 changes: 12 additions & 6 deletions hw/block/virtio-blk.c
View file
Expand Up @@ -819,7 +819,7 @@ static void virtio_blk_handle_output(VirtIODevice *vdev, VirtQueue *vq)
virtio_blk_handle_output_do(s, vq);
}

void virtio_blk_process_queued_requests(VirtIOBlock *s)
void virtio_blk_process_queued_requests(VirtIOBlock *s, bool is_bh)
{
VirtIOBlockReq *req = s->rq;
MultiReqBuffer mrb = {};
Expand Down Expand Up @@ -847,7 +847,9 @@ void virtio_blk_process_queued_requests(VirtIOBlock *s)
if (mrb.num_reqs) {
virtio_blk_submit_multireq(s->blk, &mrb);
}
blk_dec_in_flight(s->conf.conf.blk);
if (is_bh) {
blk_dec_in_flight(s->conf.conf.blk);
}
aio_context_release(blk_get_aio_context(s->conf.conf.blk));
}

Expand All @@ -858,21 +860,25 @@ static void virtio_blk_dma_restart_bh(void *opaque)
qemu_bh_delete(s->bh);
s->bh = NULL;

virtio_blk_process_queued_requests(s);
virtio_blk_process_queued_requests(s, true);
}

static void virtio_blk_dma_restart_cb(void *opaque, int running,
RunState state)
{
VirtIOBlock *s = opaque;
BusState *qbus = BUS(qdev_get_parent_bus(DEVICE(s)));
VirtioBusState *bus = VIRTIO_BUS(qbus);

if (!running) {
return;
}

if (!s->bh) {
/* FIXME The data plane is not started yet, so these requests are
* processed in the main thread. */
/*
* If ioeventfd is enabled, don't schedule the BH here as queued
* requests will be processed while starting the data plane.
*/
if (!s->bh && !virtio_bus_ioeventfd_enabled(bus)) {
s->bh = aio_bh_new(blk_get_aio_context(s->conf.conf.blk),
virtio_blk_dma_restart_bh, s);
blk_inc_in_flight(s->conf.conf.blk);
Expand Down
2 changes: 1 addition & 1 deletion include/hw/virtio/virtio-blk.h
View file
Expand Up @@ -86,6 +86,6 @@ typedef struct MultiReqBuffer {
} MultiReqBuffer;

bool virtio_blk_handle_vq(VirtIOBlock *s, VirtQueue *vq);
void virtio_blk_process_queued_requests(VirtIOBlock *s);
void virtio_blk_process_queued_requests(VirtIOBlock *s, bool is_bh);

#endif

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