On 29.06.20 17:41, Dr. David Alan Gilbert wrote: > * Max Reitz (mre...@redhat.com) wrote: >> Hi, >> >> In an iotest, I’m trying to quit qemu immediately after a migration has >> failed. Unfortunately, that doesn’t seem to be possible in a clean way: >> migrate_fd_cleanup() runs only at some point after the migration state >> is already “failed”, so if I just wait for that “failed” state and >> immediately quit, some cleanup functions may not have been run yet. > > Yeh this is hard; I always take the end of migrate_fd_cleanup to be the > real end.
Yes, unfortunately I don’t seem to have a way to look for that end. :( > It always happens on the main thread I think (it's done as a bh in some > cases). > >> This is a problem with dirty bitmap migration at least, because it >> increases the refcount on all block devices that are to be migrated, so >> if we don’t call the cleanup function before quitting, the refcount will >> stay elevated and bdrv_close_all() will hit an assertion because those >> block devices are still around after blk_remove_all_bs() and >> blockdev_close_all_bdrv_states(). >> >> In practice this particular issue might not be that big of a problem, >> because it just means qemu aborts when the user intended to let it quit >> anyway. But on one hand I could imagine that there are other clean-up >> paths that should definitely run before qemu quits (although I don’t >> know), and on the other, it’s a problem for my test. > > 'quit' varies - there are a lot of incoming failures that just assert; > very few of them cause a clean exit (I think there are more clean ones > after Peter's work on restartable postcopy a year or two ago). Well, my problem is about the source side, where there is still a VM running that I would expect to be in a sane state even after a failed migration. > I do see the end of migrate_fd_cleanup calls the notifier list; but it's > not clear to me that it's alwyas going to see the first transition to > 'failed' at that point. What exactly do you mean? It appears to me that both query-status and the MIGRATION events signal the failed state before migrate_fd_cleanup() is invoked. If you mean I could add a notifier to that list to do something™, I’m not sure what exactly it is I’d so. My test can’t do it, because it’s an iotest, and even if it could, I suppose I’d want to wait until even after all notifiers have been invoked (which isn’t guaranteed if I’d add a notifier myself). >> I tried working around the problem for my test by waiting on “Unable to >> write” appearing on stderr, because that indicates that >> migrate_fd_cleanup()’s error_report_err() has been reached. But on one >> hand, that isn’t really nice, and on the other, it doesn’t even work >> when the failure is on the source side (because then there is no >> s->error for migrate_fd_cleanup() to report). >> >> In all, I’m asking: >> (1) Is there a nice solution for me now to delay quitting qemu until the >> failed migration has been fully resolved, including the clean-up? > > In vl.c, I added a call to migration_shutdown in qemu_cleanup - although > that seems to be mostly about cleaning up the *outgoing* side; you could > add some incoming cleanup there. So you mean waiting until migrate_fd_cleanup() has run? Maybe I’ll try that tomorrow, although I’d hoped I could get this done without having to modify the code base... (I.e., I’d hoped there would be some QMP-queriable flag somewhere that could tell me whether the migrate_fd_cleanup() has run) >> (2) Isn’t it a problem if qemu crashes when you issue “quit” via QMP at >> the wrong time? Like, maybe lingering subprocesses when using “exec”? > > Yeh that should be cleaner, but isn't. :( OK then. Thanks for your insights! Max
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