Date: Mon, 30 Jul 2012 21:48:08 +0100 From: Attilio Rao <attilio@freebsd.org> To: Konstantin Belousov <kostikbel@gmail.com> Cc: Davide Italiano <davide@freebsd.org>, svn-src-projects@freebsd.org, src-committers@freebsd.org Subject: Re: svn commit: r238907 - projects/calloutng/sys/kern Message-ID: <CAJ-FndAdyL5-29vjkS1deAhc4ewYTmA6tEhXUNh%2BqQzUCcTpGw@mail.gmail.com> In-Reply-To: <20120730145912.GZ2676@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>
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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? Thanks, Attilio -- Peace can only be achieved by understanding - A. Einstein
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