Messages in this thread | | | Date | Tue, 26 Jan 2010 11:37:18 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: [PATCH 3/4] kgdb,clocksource: Prevent kernel hang in kernel debugger |
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On Tue, 26 Jan 2010, Dongdong Deng wrote: > On Tue, Jan 26, 2010 at 4:50 PM, Thomas Gleixner <tglx@linutronix.de> wrote: > > On Tue, 26 Jan 2010, Martin Schwidefsky wrote: > >> On Mon, 25 Jan 2010 22:26:39 -0600 > >> Jason Wessel <jason.wessel@windriver.com> wrote: > >> > >> > This is a regression fix against: 0f8e8ef7c204988246da5a42d576b7fa5277a8e4 > >> > > >> > Spin locks were added to the clocksource_resume_watchdog() which cause > >> > the kernel debugger to deadlock on an SMP system frequently. > >> > > >> > The kernel debugger can try for the lock, but if it fails it should > >> > continue to touch the clocksource watchdog anyway, else it will trip > >> > if the general kernel execution has been paused for too long. > >> > > >> > This introduces an possible race condition where the kernel debugger > >> > might not process the list correctly if a clocksource is being added > >> > or removed at the time of this call. This race is sufficiently rare vs > >> > having the kernel debugger hang the kernel > >> > > >> > CC: Thomas Gleixner <tglx@linutronix.de> > >> > CC: Martin Schwidefsky <schwidefsky@de.ibm.com> > >> > CC: John Stultz <johnstul@us.ibm.com> > >> > CC: Andrew Morton <akpm@linux-foundation.org> > >> > CC: Magnus Damm <damm@igel.co.jp> > >> > Signed-off-by: Jason Wessel <jason.wessel@windriver.com> > >> > >> The first question I would ask is why does the kernel deadlock? Can we > >> have a backchain of a deadlock please? > > > > The problem arises when the kernel is stopped inside the watchdog code > > with watchdog_lock held. When kgdb restarts execution then it touches > > the watchdog to avoid that TSC gets marked unstable. > > > >> Hmm, there are all kinds of races if the watchdog code gets interrupted > >> by the kernel debugger. Wouldn't it be better to just disable the > >> watchdog while the kernel debugger is active? > > > > No, we can keep it and in most cases it clocksource_touch_watchdog() > > helps to keep TSC alive. A simple "if (!trylock) return;" should solve > > the deadlock problem for kgdb without opening a can of worms. > > Is it possible that we reset the clocksource watchdog during in > clocksource_watchdog() ? > > >From the code view, The action of reset clocksource watchdog is just > set the CLOCK_SOURCE_WATCHDOG flag. > thus if we reset it before using, I think the logic will be right.
No, it's not. It just brings back the old flag based logic which we removed.
The correct way to solve this is a documented
if (!trylock()) return;
in clocksource_touch_watchdog(). And that's what I'm going to push linuswards.
There is no sane way to reliably prevent TSC from becoming unstable when kgdb stops the kernel inside the watchdog code. And I do not care about that at all.
I'm not going to clutter code with crazy workarounds just because some people believe that using a kernel debugger is a good idea. If people insist on using kgdb then the possible "TSC becomes unstable" side effect is the least of their problems.
Thanks,
tglx | |