Date: Mon, 30 Jul 2012 22:58:37 +0200 From: Davide Italiano <davide@freebsd.org> To: attilio@freebsd.org Cc: Konstantin Belousov <kostikbel@gmail.com>, svn-src-projects@freebsd.org, src-committers@freebsd.org Subject: Re: svn commit: r238907 - projects/calloutng/sys/kern Message-ID: <CACYV=-H4Z9XXYYvWRfyFCg3O0s7yHGgOcyH4OBC7HAEx0083iA@mail.gmail.com> In-Reply-To: <CAJ-FndAdyL5-29vjkS1deAhc4ewYTmA6tEhXUNh%2BqQzUCcTpGw@mail.gmail.com> 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>
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On Mon, Jul 30, 2012 at 10:48 PM, Attilio Rao <attilio@freebsd.org> 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. > > 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? I may work on that as final part of my GSoC work considering I've an interest in this. Though, I could need some guidance and help in review. Can you provide these? > > Thanks, > Attilio > > > -- > Peace can only be achieved by understanding - A. Einstein Thanks, Davide
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