On Wed, Jun 08, 2016 at 04:56:35PM +0300, Konstantin Belousov wrote: > On Wed, Jun 08, 2016 at 06:35:08AM -0700, Mark Johnston wrote: > > On Wed, Jun 08, 2016 at 07:30:55AM +0300, Konstantin Belousov wrote: > > > On Tue, Jun 07, 2016 at 11:19:19PM +0200, Jilles Tjoelker wrote: > > > > I also wonder whether we may be overengineering things here. Perhaps > > > > the advlock sleep can simply turn off TDF_SBDRY. > > > Well, this was the very first patch suggested. I would be fine with that, > > > but again, out-of-tree code seems to be not quite fine with that local > > > solution.
> > In our particular case, we could possibly use a similar approach. In > > general, it seems incorrect to clear TDF_SBDRY if the thread calling > > sx_sleep() has any locks held. It is easy to verify that all callers of > > lf_advlock() are safe in this respect, but this kind of auditing is > > generally hard. In fact, I believe the sx_sleep that led to the problem > > described in D2612 is the same as the one in my case. That is, the > > sleeping thread may or may not hold a vnode lock depending on context. > I do not think that in-tree code sleeps with a vnode lock held in > the lf_advlock(). Otherwise, system would hang in lock cascade by > an attempt to obtain an advisory lock. I think we can even assert > this with witness. > There is another sleep, which Jilles mentioned, in lf_purgelocks(), > called from vgone(). This sleep indeed occurs under the vnode lock, and > as such must be non-suspendable. The sleep waits until other threads > leave the lf_advlock() for the reclaimed vnode, and they should leave in > deterministic time due to issued wakeups. So this sleep is exempt from > the considerations, and TDF_SBDRY there is correct. > I am fine with either the braces around sx_sleep() in lf_advlock() to > clear TDF_SBDRY (sigdeferstsop()), or with the latest patch I sent, > which adds temporal override for TDF_SBDRY with TDF_SRESTART. My > understanding is that you prefer the later. If I do not mis-represent > your position, I understand why you do prefer that. The TDF_SRESTART change does fix some more problems such as umount -f getting stuck in lf_purgelocks(). However, it introduces some subtle issues that may not necessarily be a sufficient objection. Firstly, adding this closes the door on fixing signal handling for fcntl(F_SETLKW). Per POSIX, any caught signal interrupts fcntl(F_SETLKW), even if SA_RESTART is set for the signal, and the Linux man page documents the same. Our man page has documented that SA_RESTART behaves normally with fcntl(F_SETLKW) since at least FreeBSD 2.0. This could normally be fixed via if (error == ERESTART) error = EINTR; but that is no longer possible if there are [ERESTART] errors that should still restart. Secondly, fcntl(F_SETLKW) restarting after a stop may actually be observable, contrary to what I wrote before. This is due to the fair queuing. Suppose thread A has locked byte 1 a while ago and thread B is trying to lock byte 1 and 2 right now. Then thread C will be able to lock byte 2 iff thread B has not blocked yet. If thread C will not be allowed to lock byte 2 and will block on it, the TDF_SRESTART change will cause it to be awakened if thread B is stopped. When thread B resumes, the region to be locked will be recomputed. This scenario unambiguously violates the POSIX requirement but I don't know how bad it is. Note that all these threads must be in separate processes because of fcntl locks' strange semantics. -- Jilles Tjoelker _______________________________________________ email@example.com mailing list https://lists.freebsd.org/mailman/listinfo/freebsd-current To unsubscribe, send any mail to "freebsd-current-unsubscr...@freebsd.org"