Date: Tue, 31 Jul 2012 12:40:59 +0300 From: Alexander Motin <mav@FreeBSD.org> To: Konstantin Belousov <kostikbel@gmail.com> Cc: Attilio Rao <attilio@freebsd.org>, Davide Italiano <davide@freebsd.org>, src-committers@freebsd.org, svn-src-projects@freebsd.org Subject: Re: svn commit: r238907 - projects/calloutng/sys/kern Message-ID: <5017A82B.3040704@FreeBSD.org> In-Reply-To: <20120731093735.GB2676@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>
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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. > If true, then TD_NOBLOCKING is definitely not needed for > direct dispatch. For thread to be blocked, it shall be scheduled off the > CPU, going through mi_switch(). And mi_switch() asserts that critical > section level is exactly 1, which assertion fails due to direct dispatch > context owning spinlock. -- Alexander Motin
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