Cédric Le Goater <c...@redhat.com> writes: > On 2/8/24 14:57, Fabiano Rosas wrote: >> Cédric Le Goater <c...@redhat.com> writes: >> >>> On 2/8/24 14:07, Fabiano Rosas wrote: >>>> Cédric Le Goater <c...@redhat.com> writes: >>>> >>>>> close_return_path_on_source() retrieves the migration error from the >>>>> the QEMUFile '->to_dst_file' to know if a shutdown is required. This >>>>> shutdown is required to exit the return-path thread. However, in >>>>> migrate_fd_cleanup(), '->to_dst_file' is cleaned up before calling >>>>> close_return_path_on_source() and the shutdown is never performed, >>>>> leaving the source and destination waiting for an event to occur. >>>>> >>>>> Avoid relying on '->to_dst_file' and use migrate_has_error() instead. >>>>> >>>>> Suggested-by: Peter Xu <pet...@redhat.com> >>>>> Signed-off-by: Cédric Le Goater <c...@redhat.com> >>>>> --- >>>>> migration/migration.c | 3 +-- >>>>> 1 file changed, 1 insertion(+), 2 deletions(-) >>>>> >>>>> diff --git a/migration/migration.c b/migration/migration.c >>>>> index >>>>> d5f705ceef4c925589aa49335969672c0d761fa2..5f55af3d7624750ca416c4177781241b3e291e5d >>>>> 100644 >>>>> --- a/migration/migration.c >>>>> +++ b/migration/migration.c >>>>> @@ -2372,8 +2372,7 @@ static bool >>>>> close_return_path_on_source(MigrationState *ms) >>>>> * cause it to unblock if it's stuck waiting for the destination. >>>>> */ >>>>> WITH_QEMU_LOCK_GUARD(&ms->qemu_file_lock) { >>>>> - if (ms->to_dst_file && ms->rp_state.from_dst_file && >>>>> - qemu_file_get_error(ms->to_dst_file)) { >>>>> + if (migrate_has_error(ms) && ms->rp_state.from_dst_file) { >>>>> qemu_file_shutdown(ms->rp_state.from_dst_file); >>>>> } >>>>> } >>>> >>>> Hm, maybe Peter can help defend this, but this assumes that every >>>> function that takes an 'f' and sets the file error also sets >>>> migrate_set_error(). I'm not sure we have determined that, have we? >>> >>> How could we check all the code path ? I agree it is difficult when >>> looking at the code :/ >> >> It would help if the thing wasn't called 'f' for the most part of the >> code to begin with. >> >> Whenever there's a file error at to_dst_file there's the chance that the >> rp_state.from_dst_file got stuck. So we cannot ignore the file error. >> >> Would it work if we checked it earlier during cleanup as you did >> previously and then set the migration error? > > Do you mean doing something similar to what is done in > source_return_path_thread() ? > > if (qemu_file_get_error(s->to_dst_file)) { > qemu_file_get_error_obj(s->to_dst_file, &err); > if (err) { > migrate_set_error(ms, err); > error_free(err); > ... > > Yes. That would be safer I think.
Yes, something like that. I wish we could make that return path cleanup more deterministic, but currently it's just: "if something hangs, call shutdown()". We don't have a way to detect a hang, we just look at the file error and hope it works. A crucial aspect here is that calling qemu_file_shutdown() itself sets the file error. So there's not even a guarantee that an error is actually an error. > > > Nevertheless, I am struggling to understand how qemu_file_set_error() > and migrate_set_error() fit together. I was expecting some kind of > synchronization routine but there isn't it seems. Are they completely > orthogonal ? when should we use these routines and when not ? We're trying to phase out the QEMUFile usage altogether. One thing that is getting in the way is this dependency on the qemu_file_*_error functions. While we're not there yet, a good pattern is to find a qemu_file_set|get_error() pair and replace it with migrate_set|has_error(). Unfortunately the return path does not fit in this, because we don't have a matching qemu_file_set_error, it could be anywhere. As I said above, we're using that error as a heuristic for: "a recvmsg() might be hanging". > > My initial goal was to modify some of the memory handlers (log_global*) > and migration handlers to propagate errors at the QMP level and them > report to the management layer. This is growing in something bigger > and currently, I don't find a good approach to the problem. > > The last two patches of this series try to fix the return-path thread > termination. Let's keep that for after. I'll try to figure that out. I see you provided a reproducer. > > Thanks, > > C.