Date: Thu, 2 Aug 2012 22:04:08 +0100 From: Attilio Rao <attilio@freebsd.org> To: Konstantin Belousov <kostikbel@gmail.com> Cc: Davide Italiano <davide@freebsd.org>, svn-src-projects@freebsd.org, Alexander Motin <mav@freebsd.org>, src-committers@freebsd.org Subject: Re: svn commit: r238907 - projects/calloutng/sys/kern Message-ID: <CAJ-FndCYzfU7A4-fXaCYisgM=xU1P27VGXmro0M1aHF5cv3FyQ@mail.gmail.com> In-Reply-To: <20120731095922.GC2676@deviant.kiev.zoral.com.ua> References: <201207301350.q6UDobCI099069@svn.freebsd.org> <CAJ-FndBJNNBNDUEDsDBUvwoVExZpnXmoJmpY58gE3QQbw3hRGA@mail.gmail.com> <CACYV=-HmOwZ=E8Pw3-mUw0994SbvZaA3eMfcwM0fDTu_zykBJg@mail.gmail.com> <CAJ-FndBmXkyJJ=fCkEpVm84E56A2_EoM6kbch03e4RMEM6WCGQ@mail.gmail.com> <20120730143943.GY2676@deviant.kiev.zoral.com.ua> <CAJ-FndByYcZ%2BUhnkFT_n2=W=UheqUCi0%2BUAX%2BF07EqbVU=6iDQ@mail.gmail.com> <20120730145912.GZ2676@deviant.kiev.zoral.com.ua> <CAJ-FndAdyL5-29vjkS1deAhc4ewYTmA6tEhXUNh%2BqQzUCcTpGw@mail.gmail.com> <20120731093735.GB2676@deviant.kiev.zoral.com.ua> <5017A82B.3040704@FreeBSD.org> <20120731095922.GC2676@deviant.kiev.zoral.com.ua>
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On 7/31/12, Konstantin Belousov <kostikbel@gmail.com> wrote: > On Tue, Jul 31, 2012 at 12:40:59PM +0300, Alexander Motin wrote: >> On 31.07.2012 12:37, Konstantin Belousov wrote: >> >On Mon, Jul 30, 2012 at 09:48:08PM +0100, Attilio Rao wrote: >> >>On Mon, Jul 30, 2012 at 3:59 PM, Konstantin Belousov >> >><kostikbel@gmail.com> wrote: >> >>>On Mon, Jul 30, 2012 at 03:51:22PM +0100, Attilio Rao wrote: >> >>>>On 7/30/12, Konstantin Belousov <kostikbel@gmail.com> wrote: >> >>>>>On Mon, Jul 30, 2012 at 03:24:26PM +0100, Attilio Rao wrote: >> >>>>>>On 7/30/12, Davide Italiano <davide@freebsd.org> wrote: >> >>>>>>>On Mon, Jul 30, 2012 at 4:02 PM, Attilio Rao <attilio@freebsd.org> >> >>>>>>>wrote: >> >>>>>>>Thanks for the comment, Attilio. >> >>>>>>>Yes, it's exactly what you thought. If direct flag is equal to one >> >>>>>>>you're sure you're processing a callout which runs directly from >> >>>>>>>hardware interrupt context. In this case, the running thread >> >>>>>>> cannot >> >>>>>>>sleep and it's likely you have TDP_NOSLEEPING flags set, failing >> >>>>>>> the >> >>>>>>>KASSERT() in THREAD_NO_SLEEPING() and leading to panic if kernel >> >>>>>>> is >> >>>>>>>compiled with INVARIANTS. >> >>>>>>>In case you're running from SWI context (direct equals to zero) >> >>>>>>> code >> >>>>>>>remains the same as before. >> >>>>>>>I think what I'm doing works due the assumption thread running >> >>>>>>> never >> >>>>>>>sleeps. Do you suggest some other way to handle this? >> >>>>>> >> >>>>>>Possibly the quicker way to do this is to have a way to deal with >> >>>>>> the >> >>>>>>TDP_NOSLEEPING flag in recursed way, thus implement the same logic >> >>>>>> as >> >>>>>>VFS_LOCK_GIANT() does, for example. >> >>>>>>You will need to change the few callers of THREAD_NO_SLEEPING(), >> >>>>>> but >> >>>>>>the patch should be no longer than 10/15 lines. >> >>>>> >> >>>>>There are already curthread_pflags_set/restore KPI designed exactly >> >>>>> to >> >>>>>handle >> >>>>>nested private thread flags. >> >>>> >> >>>>Yes, however I would use curthread_pflags* KPI within >> >>>>THREAD_NO_SLEEPING() as this name is much more explicit. >> >>>> >> >>>Sure, hiding it in THREAD_NO_SLEEPING (THREAD_NO_SLEEP_ENTER/LEAVE ?) >> >>>is the way to use curthread_pflags_set there. >> >>> >> >>>As a second though, on the other hand, is it safe to modify td_flags >> >>>from the interrupt context at all ? Probably yes if interrupt handler >> >>>always leave td_pflags in the same state on leave as it was on entry, >> >>>but couldn't too smart compiler cause inconsistent view of td_pflags >> >>>inside the handler ? >> >> >> >>Can you think of any? Because I cannot think of a case where a nested >> >>interrupt can messup with already compiled code, unless it leaks a >> >>cleanup. >> >In principle, compiler might compile the >> > x |= a; >> >into whatever it finds suitable, e.g. it could write 0 temporary into >> >x if the corresponding instruction sequence is considered faster. >> > >> >No sane compiler for x86 does this. >> >> >> >>I was more worried about the compiler reordering operations before >> >>locking could really see it, but I think in this case the functions >> >>call to sleepqueue (at least) works as a sequence point so we are >> >>safe. >> >> >> >>> >> >>>>>Also, I wonder, should you assert somehow that direct dispatch cannot >> >>>>> >> >>>>>block >> >>>>>as well ? >> >>>> >> >>>>Yes, it would be optimal, but I don't think we have a flag for that >> >>>>right now, do we? >> >>> >> >>>I am not aware of such flag, this might be a good reason to introduce >> >>> it, >> >>>if issue about td_pflags is just a product of my imagination. >> >> >> >>I think you should be good to go. Do you plan to work on such a patch? >> > >> >Ok, I looked closely at the direct dispatch and TD_NOBLOCKING. I now >> >think that such flag is not needed. >> > >> >Am I right that direct dispatch executes callback while owning cc_lock >> >spinlock ? >> >> No, does not now. It was so originally, but was fixed recently, as it >> caused LOR deadlocks. > Hm, ok. Probably I misread the diff. > > Anyway, I believe that both direct interrupt dispatch and IPIs take > critical sections around handlers. This should have the same effect > for assertion in the mi_switch(). I agree, this is certainly true. Attilio -- Peace can only be achieved by understanding - A. Einstein
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