Date: Tue, 31 Jul 2012 12:37:35 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Attilio Rao <attilio@freebsd.org> 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: <20120731093735.GB2676@deviant.kiev.zoral.com.ua> 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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--K2wHSdELFynQv3TW Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable 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 cann= ot > >> >> > 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) c= ode > >> >> > remains the same as before. > >> >> > I think what I'm doing works due the assumption thread running ne= ver > >> >> > 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 t= he > >> >> 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 ? >=20 > 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 |=3D 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. >=20 > 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. >=20 > > > >> > Also, I wonder, should you assert somehow that direct dispatch canno= t 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 i= t, > > if issue about td_pflags is just a product of my imagination. >=20 > 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 ? 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. --K2wHSdELFynQv3TW Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (FreeBSD) iEYEARECAAYFAlAXp18ACgkQC3+MBN1Mb4h7rwCg8GCxQ7mEcSbaaAi9dZ6CGdXd BQoAoN31pfwlL0waULSLfAFEHDZBH2QB =ClUt -----END PGP SIGNATURE----- --K2wHSdELFynQv3TW--
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