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Date: Mon, 20 Dec 2010 19:58:12 +0100 From: Borislav Petkov <bp@...en8.de> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Tony Luck <tony.luck@...el.com>, Tony Luck <tony.luck@...il.com>, "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org, tglx@...utronix.de, mingo@...e.hu, greg@...ah.com, akpm@...ux-foundation.org, ying.huang@...el.com, David Miller <davem@...emloft.net>, Alan Cox <alan@...rguk.ukuu.org.uk>, Jim Keniston <jkenisto@...ux.vnet.ibm.com>, Kyungmin Park <kmpark@...radead.org>, Geert Uytterhoeven <geert@...ux-m68k.org> Subject: Re: [concept & "good taste" review] persistent store On Mon, Dec 20, 2010 at 09:18:25AM -0800, Linus Torvalds wrote: > On Sun, Dec 19, 2010 at 11:26 PM, Borislav Petkov <bp@...en8.de> wrote: > > > > IOW, the simple (maybe too simple) algo of the pstore could be something > > like: > > Simple? > > > 1. Got a relevant message from kernel, log it. > > > > 2. Am I still alive? > > Umm. The "am I still alive" question is traditionally called "the > stopping problem", and is considered to be the traditional example of > _least_ simple problem there is. As in "fundamentally unsolvable". Yeah, I meant simple in the sense of only two steps required. > Did we kill X? Did we happen to hold some critical lock when oopsing? > Was it syslogd itself that died and caused nothing further to be > saved, even if the machine otherwise seems to be fine? Or did the > filesystem go into read-only mode due to the problem and the rest of > the system is fine, but the disk is never going to see the messages? > > In other words, the problem really is that "am I still alive" thing. > That's a seriously impossible question to answer. Maybe we should rephrase this as "am I still alive and well," for a specific definition of well. > What _can_ be answered is "did somebody write out the oops, then > fsync, and then notify us about it?" But without explicit notification > of "yeah, it really is saved off somewhere else", we really can't > tell. That could work, I should look deeper into that. > We could do heuristics, of course, and they might even work in > practice (like "flush after half an hour if there has been actual work > done and the machine is clearly making progress"). Yes, this was exactly what I was trying to say! Do something in a watchdog handler path that shows that we actually made progress. But you're right, we'd still need the notification. My look at "did we make a progress" was too simple and there _are_ nuances which need to be accounted for. Thanks. -- Regards/Gruss, Boris. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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