On Fri, Apr 26, 2024 at 11:20:34AM -0300, Fabiano Rosas wrote:
> We're enabling using the fdset interface to pass file descriptors for
> use in the migration code. Since migrations can happen more than once
> during the VMs lifetime, we need a way to remove an fd from the fdset
> at the end of migration.
>
> The current code only removes an fd from the fdset if the VM is
> running. This causes a QMP call to "remove-fd" to not actually remove
> the fd if the VM happens to be stopped.
>
> While the fd would eventually be removed when monitor_fdset_cleanup()
> is called again, the user request should be honored and the fd
> actually removed. Calling remove-fd + query-fdset shows a recently
> removed fd still present.
>
> The runstate_is_running() check was introduced by commit ebe52b592d
> ("monitor: Prevent removing fd from set during init"), which by the
> shortlog indicates that they were trying to avoid removing an
> yet-unduplicated fd too early.
>
> I don't see why an fd explicitly removed with qmp_remove_fd() should
> be under runstate_is_running(). I'm assuming this was a mistake when
> adding the parenthesis around the expression.
>
> Move the runstate_is_running() check to apply only to the
> QLIST_EMPTY(dup_fds) side of the expression and ignore it when
> mon_fdset_fd->removed has been explicitly set.
I am confused on why the fdset removal is as complicated. I'm also
wondering here whether it's dropped because we checked against
"mon_refcount == 0", and maybe monitor_fdset_cleanup() is simply called
_before_ a monitor is created? Why do we need such check on the first
place?
I'm thinking one case where the only QMP monitor got (for some reason)
disconnected, and reconnected again during VM running. Won't current code
already lead to unwanted removal of mostly all fds due to mon_refcount==0?
I also am confused why ->removed flags is ever needed, and why we can't
already remove the fdsets fds if found matching.
Copy Corey, Eric and Kevin.
>
> Signed-off-by: Fabiano Rosas <faro...@suse.de>
> ---
> monitor/fds.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/monitor/fds.c b/monitor/fds.c
> index d86c2c674c..4ec3b7eea9 100644
> --- a/monitor/fds.c
> +++ b/monitor/fds.c
> @@ -173,9 +173,9 @@ static void monitor_fdset_cleanup(MonFdset *mon_fdset)
> MonFdsetFd *mon_fdset_fd_next;
>
> QLIST_FOREACH_SAFE(mon_fdset_fd, &mon_fdset->fds, next,
> mon_fdset_fd_next) {
> - if ((mon_fdset_fd->removed ||
> - (QLIST_EMPTY(&mon_fdset->dup_fds) && mon_refcount == 0)) &&
> - runstate_is_running()) {
> + if (mon_fdset_fd->removed ||
> + (QLIST_EMPTY(&mon_fdset->dup_fds) && mon_refcount == 0 &&
> + runstate_is_running())) {
> close(mon_fdset_fd->fd);
> g_free(mon_fdset_fd->opaque);
> QLIST_REMOVE(mon_fdset_fd, next);
> --
> 2.35.3
>
--
Peter Xu
