On 9/8/26 16:55, Denis V. Lunev wrote:
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> Applying I/O limits to a disk that is already serving requests from an
> iothread can kill QEMU:
>
> #0 throttle_group_next_tgm (tgm=0x0) at block/throttle-groups.c:185
> #1 next_throttle_token (tgm=..., direction=THROTTLE_READ)
> #2 throttle_group_co_io_limits_intercept (bytes=8192, ...)
> #3 blk_co_do_preadv_part (blk=..., bytes=8192, ...)
> #4 blk_aio_read_entry (opaque=...)
> #5 coroutine_trampoline (i0=..., i1=...)
>
> throttle_group_register_tgm() publishes tgm->throttle_state before it
> takes tg->lock, and the group it points at is only filled in afterwards.
> A request that samples the pointer in between and wins tg->lock finds
> tg->tokens[] still NULL. The ordering is as old as the throttle group
> code, 76f4afb40f ("throttle: Add throttle group support").
>
> This was found from a production core. The group in it was brand new -
> name matching the disk, refcount 1, empty member list, both tokens NULL,
> a zeroed config because throttle_group_config() had not run yet, and
> tg->lock owned by the crashing iothread while the main thread sat
> blocked on the same lock inside throttle_group_register_tgm(). The
> trigger was libvirt applying blkdeviotune right after attaching a
> volume, while the guest was probing the new disk.
>
> The window is a couple of instructions wide. To see the bug on demand,
> widen it by hand:
>
> --- a/block/throttle-groups.c
> +++ b/block/throttle-groups.c
> @@ -584,6 +584,7 @@ void throttle_group_register_tgm(...)
> tgm->throttle_state = ts;
> tgm->aio_context = ctx;
> qatomic_set(&tgm->restart_pending, 0);
>
> + g_usleep(1000);
> QEMU_LOCK_GUARD(&tg->lock);
>
> With that in place, any I/O against the disk during a
> block_set_io_throttle that creates the group crashes at once, and the
> patch below makes it stop.
>
> Cc: Kevin Wolf <[email protected]>
> Cc: Hanna Reitz <[email protected]>
> Cc: Alberto Garcia <[email protected]>
> Signed-off-by: Denis V. Lunev <[email protected]>
>
> Denis V. Lunev (1):
> block/throttle-groups: fix crash when enabling I/O limits on a busy
> disk
>
> block/block-backend.c | 12 ++++++------
> block/throttle-groups.c | 4 +++-
> 2 files changed, 9 insertions(+), 7 deletions(-)
>
ping
