Date: Fri, 4 May 2012 00:28:34 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Attilio Rao <attilio@freebsd.org> 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: <20120503212834.GP2358@deviant.kiev.zoral.com.ua> In-Reply-To: <CAJ-FndAhRPyMsFwCEh0w6=VmRy4faL4L0rC4kG4de8rNkX-DQw@mail.gmail.com> 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> <CAJ-FndAhRPyMsFwCEh0w6=VmRy4faL4L0rC4kG4de8rNkX-DQw@mail.gmail.com>
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[-- Attachment #1 --] On Thu, May 03, 2012 at 10:06:53PM +0100, Attilio Rao wrote: > 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 3 10:38:02 2012 > >> >> > New Revision: 234952 > >> >> > URL: http://svn.freebsd.org/changeset/base/234952 > >> >> > > >> >> > Log: > >> >> > When callout_reset_on() cannot immediately migrate a callout since it > >> >> > is running on other cpu, the CALLOUT_PENDING flag is temporarily > >> >> > cleared. Then, callout_stop() on this, in fact active, callout fails > >> >> > because CALLOUT_PENDING is not set, and callout_stop() returns 0. > >> >> > > >> >> > Now, in sleepq_check_timeout(), the failed callout_stop() causes the > >> >> > sleepq code to execute mi_switch() without even setting the wmesg, > >> >> > since the switch-out is supposed to be transient. In fact, the thread > >> >> > is put off the CPU for full timeout interval, instead of being put on > >> >> > runq immediately. Until timeout fires, the process is unkillable for > >> >> > obvious reasons. > >> >> > > >> >> > Fix this by marking the migrating callouts with CALLOUT_DFRMIGRATION > >> >> > flag. The flag is cleared by callout_stop_safe() when the function > >> >> > detects a migration, besides returning the success. The softclock() > >> >> > rechecks the flag for migrating callout and cancels its execution if > >> >> > 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 != 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 == 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 == c. It seems a perfectly valid > situation to me. Yes, and my patch makes the callout to be indeed stopped right after migration is finished. Did you looked at the patch itself ? What is the valid situation ? callout_stop returning 0 but not stopping a pending callout ? I have to disagree. > > Probabilly the bug is in the sleepq use of this mechanism? And, what the bug is, then ? [-- Attachment #2 --] -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (FreeBSD) iEYEARECAAYFAk+i+IIACgkQC3+MBN1Mb4g8gACdFRb4hFMRIYccDzPtofqjJk2F oFIAoKunxB+SHTkWgKRHleB37GPxYgE0 =iAP5 -----END PGP SIGNATURE-----
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