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SubjectRe: printk deadlock due to double lock attempt on current CPU's runqueue
On Wed, Nov 10, 2021 at 12:20:05PM +0100, Peter Zijlstra wrote:
> On Wed, Nov 10, 2021 at 11:50:38AM +0100, Petr Mladek wrote:
> > On Tue 2021-11-09 12:06:48, Sultan Alsawaf wrote:
> > > Hi,
> > >
> > > I encountered a printk deadlock on 5.13 which appears to still affect the latest
> > > kernel. The deadlock occurs due to printk being used while having the current
> > > CPU's runqueue locked, and the underlying framebuffer console attempting to lock
> > > the same runqueue when printk tries to flush the log buffer.
> > >
> > > I'm not sure what the *correct* solution is here (don't use printk while having
> > > a runqueue locked? don't use schedule_work() from the fbcon path? tell printk
> > > to use one of its lock-less backends?), so I've cc'd all the relevant folks.
> >
> > At the moment, printk_deferred() could be used here. It defers the
> > console handling via irq_work().
>
> I think I've rejected that patch at least twice now :-) John's printk
> stuff will really land real soon now, right.

Yeah whacking all affected prinkt callers just because of fbcon does not
sound like a good idea to me either.
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

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