Date: Thu, 19 May 2011 19:32:36 +0300 From: Andriy Gapon <avg@FreeBSD.org> To: Attilio Rao <attilio@FreeBSD.org> Cc: mdf@FreeBSD.org, src-committers@FreeBSD.org, Pawel Jakub Dawidek <pjd@FreeBSD.org>, svn-src-user@FreeBSD.org Subject: Re: svn commit: r222060 - in user/avg/xcpu/sys: kern sys Message-ID: <4DD54624.5050202@FreeBSD.org> In-Reply-To: <BANLkTinn7ttBxKDDsj35Tmib=1f=O8Fy-A@mail.gmail.com> References: <201105181508.p4IF8UoS096841@svn.freebsd.org> <20110518182441.GB2273@garage.freebsd.pl> <4DD4243C.4070301@FreeBSD.org> <BANLkTikAnB-3XbvDwGHgyqyJquH9BhqzOQ@mail.gmail.com> <BANLkTikmxbsCV_A-zD7XdkWyOEBzy67iZQ@mail.gmail.com> <BANLkTinn7ttBxKDDsj35Tmib=1f=O8Fy-A@mail.gmail.com>
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on 18/05/2011 23:50 Attilio Rao said the following: > 2011/5/18 <mdf@freebsd.org>: >> On Wed, May 18, 2011 at 1:06 PM, Attilio Rao <attilio@freebsd.org> wrote: >>> 2011/5/18 Andriy Gapon <avg@freebsd.org>: >>>> on 18/05/2011 21:24 Pawel Jakub Dawidek said the following: >>>>> On Wed, May 18, 2011 at 03:08:30PM +0000, Andriy Gapon wrote: >>>>> [...] >>>>>> --- user/avg/xcpu/sys/sys/proc.h Wed May 18 15:07:36 2011 (r222059) >>>>>> +++ user/avg/xcpu/sys/sys/proc.h Wed May 18 15:08:30 2011 (r222060) >>>>>> @@ -781,7 +781,7 @@ MALLOC_DECLARE(M_SUBPROC); >>>>>> * Otherwise, the kernel will deadlock since the scheduler isn't >>>>>> * going to run the thread that holds the lock we need. >>>>>> */ >>>>>> -#define THREAD_PANICED() \ >>>>>> +#define TD_IS_INPANIC() \ >>>>>> (panicstr != NULL && (curthread->td_flags & TDF_INPANIC) != 0) >>>>> >>>>> Does TDF_INPANIC make sense without panicstr being set? >>>> >>>> Very good observation. It seems that TDF_INPANIC can never be set unless >>>> panicstr is set. So, I guess it should be OK to simplify the macro further. >>>> Thank you. >>> >>> However I think that TDF_INPANIC handling is not optimal. >>> You should really acquire thread_lock otherwise you are going to break >>> choosethread() concurrency. >>> >>> I would prefer to make TDF_INPANIC a private flag and just use it with >>> curthread, if possible, but I still don't have a good way to resolve >>> choosethread() (I would dig the runqueue adding path and resolve the >>> problem later in the codeflow, I think). >> >> I know it's almost required now (sync on reboot?!?!), but I would >> strongly question, from an architectural standpoint, why the scheduler >> should be running at all in panic. Once a thread pulls the panic >> trigger, nothing else should run except ddb in that thread's context. This is true. The code that I propose makes sure that there is only one CPU and only one thread running when TDF_INPANIC is set. If someone decides to enable sync-on-panic and expect it to work, I can only pray for them. Yes, there most likely will be a knob to turn off CPU stopping for panic, but I plan it mostly as a quick way to switch back to legacy (present) behavior if a need arises (bugs, tec). It may work to the same degree as panic on SMP works now, but without any guarantees. > Yes, I mostly agree, so for avg's purpose he should maybe assert > choosethread() doesn't pickup the TDF_INPANIC threads (or just skip > the check at all, as it should be made private). Unfortunately I can not understand your concern and proposal. Not sure why choosethread() would be called at all, why it shouldn't pick up TDF_INPANIC thread(s), and which check could/should be skipped and what should be made private. Sorry :( -- Andriy Gapon
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