Date: Thu, 3 May 2012 22:06:53 +0100 From: Attilio Rao <attilio@freebsd.org> To: Konstantin Belousov <kostikbel@gmail.com> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org, christian.esken@trivago.com Subject: Re: svn commit: r234952 - in head/sys: kern sys Message-ID: <CAJ-FndAhRPyMsFwCEh0w6=VmRy4faL4L0rC4kG4de8rNkX-DQw@mail.gmail.com> In-Reply-To: <20120503132715.GN2358@deviant.kiev.zoral.com.ua> References: <201205031038.q43Ac2eZ032779@svn.freebsd.org> <CAJ-FndBF7%2BfLwivrMdH=rMHsEVQ5jojk3noceN8wxmaj2ZaTvg@mail.gmail.com> <20120503114913.GJ2358@deviant.kiev.zoral.com.ua> <CAJ-FndCa8xBmKJNnm6kfVpAyCtWtJmoti75XvCBiOR7ppREqng@mail.gmail.com> <20120503132715.GN2358@deviant.kiev.zoral.com.ua>
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2012/5/3 Konstantin Belousov <kostikbel@gmail.com>: > On Thu, May 03, 2012 at 02:14:20PM +0100, Attilio Rao wrote: >> 2012/5/3, Konstantin Belousov <kostikbel@gmail.com>: >> > On Thu, May 03, 2012 at 12:02:08PM +0100, Attilio Rao wrote: >> >> 2012/5/3, Konstantin Belousov <kib@freebsd.org>: >> >> > Author: kib >> >> > Date: Thu May =C2=A03 10:38:02 2012 >> >> > New Revision: 234952 >> >> > URL: http://svn.freebsd.org/changeset/base/234952 >> >> > >> >> > Log: >> >> > =C2=A0 When callout_reset_on() cannot immediately migrate a callout= since it >> >> > =C2=A0 is running on other cpu, the CALLOUT_PENDING flag is tempora= rily >> >> > =C2=A0 cleared. Then, callout_stop() on this, in fact active, callo= ut fails >> >> > =C2=A0 because CALLOUT_PENDING is not set, and callout_stop() retur= ns 0. >> >> > >> >> > =C2=A0 Now, in sleepq_check_timeout(), the failed callout_stop() ca= uses the >> >> > =C2=A0 sleepq code to execute mi_switch() without even setting the = wmesg, >> >> > =C2=A0 since the switch-out is supposed to be transient. In fact, t= he thread >> >> > =C2=A0 is put off the CPU for full timeout interval, instead of bei= ng put on >> >> > =C2=A0 runq immediately. =C2=A0Until timeout fires, the process is = unkillable for >> >> > =C2=A0 obvious reasons. >> >> > >> >> > =C2=A0 Fix this by marking the migrating callouts with CALLOUT_DFRM= IGRATION >> >> > =C2=A0 flag. The flag is cleared by callout_stop_safe() when the fu= nction >> >> > =C2=A0 detects a migration, besides returning the success. The soft= clock() >> >> > =C2=A0 rechecks the flag for migrating callout and cancels its exec= ution if >> >> > =C2=A0 the flag was cleared meantime. >> >> >> >> Can you please clarify why you cannot simply drop the deferred >> >> migration in the case !CALLOUT_PENDING in callout_stop_safe()? >> > >> > I probably can, I think I went with the route of committed patch >> > because it is slightly less work. Also, the comment in the while() >> > loop suggested me to rely on softclock. >> >> I don't think this is more work at all, the attached patch >> (pre-r234952, untested) should address it properly in few than 10 >> lines: >> http://www.freebsd.org/~attilio/callout_cancel_mig_stop.patch >> >> without the need to add further flags and re-using existing mechanisms. > > (cc->cc_curr !=3D c) is not the case which caused the issue. It might be > needed to treatened this way, but the reported case is opposite. Yes, of course, because the migration handover happens in the same critical context of cc->cc_curr =3D=3D c, but now I wonder if this fix is really right. It seems to me that in the case you describe callout_stop() must return 0 and the migration must not be cancelled because the callout is not stopped. It is not stopped not because of the deferred migration but because cc->cc_curr =3D=3D c. It seems a perfectly valid situation to me. Probabilly the bug is in the sleepq use of this mechanism? Attilio --=20 Peace can only be achieved by understanding - A. Einstein
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