On Mon 11-07-16 07:03:31, Jeff Layton wrote: > On Mon, 2016-07-11 at 09:23 +0200, Michal Hocko wrote: > > On Fri 08-07-16 10:27:38, Jeff Layton wrote: > > > On Fri, 2016-07-08 at 16:23 +0200, Michal Hocko wrote: > > > > On Fri 08-07-16 08:51:54, Jeff Layton wrote: > > > > > > > > > > On Fri, 2016-07-08 at 14:22 +0200, Michal Hocko wrote: > > > > [...] > > > > > > > > > > > > > > > > > Apart from alternative Dave was mentioning in other email, what > > > > > > is the > > > > > > point to use freezable wait from this path in the first place? > > > > > > > > > > > > nfs4_handle_exception does nfs4_wait_clnt_recover from the same > > > > > > path and > > > > > > that does wait_on_bit_action with TASK_KILLABLE so we are waiting > > > > > > in two > > > > > > different modes from the same path AFAICS. There do not seem to > > > > > > be other > > > > > > callers of nfs4_delay outside of nfs4_handle_exception. Sounds > > > > > > like > > > > > > something is not quite right here to me. If the nfs4_delay did > > > > > > regular > > > > > > wait then the freezing would fail as well but at least it would > > > > > > be clear > > > > > > who is the culrprit rather than having an indirect dependency. > > > > > The codepaths involved there are a lot more complex than that > > > > > unfortunately. > > > > > > > > > > nfs4_delay is the function that we use to handle the case where the > > > > > server returns NFS4ERR_DELAY. Basically telling us that it's too > > > > > busy > > > > > right now or has some transient error and the client should retry > > > > > after > > > > > a small, sliding delay. > > > > > > > > > > That codepath could probably be made more freezer-safe. The typical > > > > > case however, is that we've sent a call and just haven't gotten a > > > > > reply. That's the trickier one to handle. > > > > Why using a regular non-freezable wait would be a problem? > > > > > > It has been a while since I looked at that code, but IIRC, that could > > > block the freezer for up to 15s, which is a significant portion of the > > > 20s that you get before the freezer gives up. > > > > But how does that differ from the situation when the freezer has to give > > up on the timeout because another task fails due to lock dependency. > > > > As Trond and Dave have written in other emails. It is really danngerous > > to freeze a task while it is holding locks and other resources. > > It's not really dangerous if you're freezing every task on the host. > Sure, you're freezing with locks held, but everything else is freezing > too, so nothing will be contending for those locks. But the very same path is used also for cgroup freezer so you can end up freezing a task while it holds locks which might block tasks from unrelated cgroups, right? > I'm not at all opposed to changing how all of that works. My only > stipulation is that we not break the ability to reliably suspend a host > that is actively using an NFS mount. If you can come up with a way to > do that that also works for freezing cgroups, then I'm all for it. My knowledge of NFS is too limited to help you out here but I guess it would be a good start to stop using unsafe freezer APIs. Or use it only when you are sure you cannot block any resources. -- Michal Hocko SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
