Re: [PATCH] block/copy-before-write: wait for conflicts when read locking to avoid assertion failure

[Vladimir Sementsov-Ogievskiy]

On 11.07.24 16:36, Fiona Ebner wrote:
> There is no protection against two callers of cbw_snapshot_read_lock()
> calling reqlist_init_req() with overlapping ranges, and
> reqlist_init_req() asserts that there are no conflicting requests.
>
> In particular, two cbw_co_snapshot_block_status() callers can race,
> with the second calling reqlist_init_req() before the first one
> finishes and removes its conflicting request, leading to an assertion
> failure.
>
> Reproducer script [0] and backtrace [1] are attached below.


Understand. But seems in case of CBW read-lock, nothing bad in intersecting 
read requests?

reqlist is shared with backup, where we care to avoid intersecting requests in 
the list. What about just move the assertion to block_copy_task_create() ? And 
add comment somewhere that we support intersecting reads in frozen_read_reqs.


> [0]:
>
> > #!/bin/bash -e
> > dd if=/dev/urandom of=/tmp/disk.raw bs=1M count=1024
> > ./qemu-img create /tmp/fleecing.raw -f raw 1G
> > (
> > ./qemu-system-x86_64 --qmp stdio \
> > --blockdev raw,node-name=node0,file.driver=file,file.filename=/tmp/disk.raw \
> > --blockdev raw,node-name=node1,file.driver=file,file.filename=/tmp/fleecing.raw 
> > \
> > <<EOF
> > {"execute": "qmp_capabilities"}
> > {"execute": "blockdev-add", "arguments": { "driver": "copy-before-write", "file": "node0", "target": 
> > "node1", "node-name": "node3" } }
> > {"execute": "blockdev-add", "arguments": { "driver": "snapshot-access", "file": "node3", 
> > "node-name": "snap0" } }
> > {"execute": "nbd-server-start", "arguments": {"addr": { "type": "unix", "data": { "path": 
> > "/tmp/nbd.socket" } } } }
> > {"execute": "block-export-add", "arguments": {"id": "exp0", "node-name": "snap0", "type": "nbd", 
> > "name": "exp0"}}
> > EOF
> > ) &
> > sleep 5
> > while true; do
> > ./qemu-nbd -d /dev/nbd0
> > ./qemu-nbd -c /dev/nbd0 nbd:unix:/tmp/nbd.socket:exportname=exp0 -f raw -r
> > nbdinfo --map 'nbd+unix:///exp0?socket=/tmp/nbd.socket'
> > done
>
> [1]:
>
> > #5  0x000071e5f0088eb2 in __GI___assert_fail (...) at ./assert/assert.c:101
> > #6  0x0000615285438017 in reqlist_init_req (...) at ../block/reqlist.c:23
> > #7  0x00006152853e2d98 in cbw_snapshot_read_lock (...) at 
> > ../block/copy-before-write.c:237
> > #8  0x00006152853e3068 in cbw_co_snapshot_block_status (...) at 
> > ../block/copy-before-write.c:304
> > #9  0x00006152853f4d22 in bdrv_co_snapshot_block_status (...) at 
> > ../block/io.c:3726
> > #10 0x000061528543a63e in snapshot_access_co_block_status (...) at 
> > ../block/snapshot-access.c:48
> > #11 0x00006152853f1a0a in bdrv_co_do_block_status (...) at ../block/io.c:2474
> > #12 0x00006152853f2016 in bdrv_co_common_block_status_above (...) at 
> > ../block/io.c:2652
> > #13 0x00006152853f22cf in bdrv_co_block_status_above (...) at ../block/io.c:2732
> > #14 0x00006152853d9a86 in blk_co_block_status_above (...) at 
> > ../block/block-backend.c:1473
> > #15 0x000061528538da6c in blockstatus_to_extents (...) at ../nbd/server.c:2374
> > #16 0x000061528538deb1 in nbd_co_send_block_status (...) at ../nbd/server.c:2481
> > #17 0x000061528538f424 in nbd_handle_request (...) at ../nbd/server.c:2978
> > #18 0x000061528538f906 in nbd_trip (...) at ../nbd/server.c:3121
> > #19 0x00006152855a7caf in coroutine_trampoline (...) at 
> > ../util/coroutine-ucontext.c:175
>
> Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
> ---
>   block/copy-before-write.c | 1 +
>   1 file changed, 1 insertion(+)
>
> diff --git a/block/copy-before-write.c b/block/copy-before-write.c
> index 853e01a1eb..376ff3f3e1 100644
> --- a/block/copy-before-write.c
> +++ b/block/copy-before-write.c
> @@ -234,6 +234,7 @@ cbw_snapshot_read_lock(BlockDriverState *bs, int64_t 
> offset, int64_t bytes,
>           *req = (BlockReq) {.offset = -1, .bytes = -1};
>           *file = s->target;
>       } else {
> +        reqlist_wait_all(&s->frozen_read_reqs, offset, bytes, &s->lock);
>           reqlist_init_req(&s->frozen_read_reqs, req, offset, bytes);
>           *file = bs->file;
>       }

--
Best regards,
Vladimir