Date: Thu, 3 May 2012 14:14:20 +0100 From: Attilio Rao <attilio@freebsd.org> To: Konstantin Belousov <kostikbel@gmail.com>, christian.esken@trivago.com Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org Subject: Re: svn commit: r234952 - in head/sys: kern sys Message-ID: <CAJ-FndCa8xBmKJNnm6kfVpAyCtWtJmoti75XvCBiOR7ppREqng@mail.gmail.com> In-Reply-To: <20120503114913.GJ2358@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>
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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. Attilio -- Peace can only be achieved by understanding - A. Einstein
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