Re: lock order reversals - what do they mean?
Robert Watson <[email protected]>
| Newsgroups | gmane.os.freebsd.current,gmane.os.freebsd.devel.mobile |
|---|---|
| Message-ID | <[email protected]> |
On Sun, 14 Dec 2003, C. Kukulies wrote: > I'm getting > lock order reversal > 1st 0xc09524c0 UMA lock (UMA lock) @ /u/src/sys/vm/uma_core.c:1200 > 2nd 0xc1431100 system map (system map) @ /u/src/sys/vm/vm_map.c:2210 > Stack backtrace: > > This happenend while doing a find . something on my notebook hd. These warnings are generated by Witness, a run-time lock diagnostic system found in FreeBSD 5-CURRENT kernels (but removed in releases). You can read more about Witness in the WITNESS(4) man page, which talks about its capabilities. Among other things, Witness performs run-time lock order verification using a combination of hard coded lock orders, and run-time detected lock orders, and generates console warnings when lock orders are violated. The intent of this is to detect the potential for deadlocks due to lock order violations; it's worth observing that Witness is actually slightly conservative, and so it's possible to get false positives. In the event that Witness is accurately reporting a lock order problem, it's basically saying "If you were unlucky, a deadlock would have happened here". There are a couple of "well known" false positives, which we need to do a better job of documenting to prevent spurious reports. The non-well-known ones typically correspond to bugs in newly added locking, as lock order reversals usually get fixed pretty quickly because Witness is busy generating warnings :-). I believe the reversal you've reported is a false positive. Thanks, Robert N M Watson FreeBSD Core Team, TrustedBSD Projects [email protected] Senior Research Scientist, McAfee Research _______________________________________________ [email protected] mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-current To unsubscribe, send any mail to "[email protected]"