On Fri, Sep 06, 2019 at 04:01:26PM +0200, Peter Zijlstra wrote: > On Fri, Sep 06, 2019 at 02:42:11PM +0200, Petr Mladek wrote:
> > 7. People would complain when continuous lines become less > > reliable. It might be most visible when mixing backtraces > > from all CPUs. Simple sorting by prefix will not make > > it readable. The historic way was to synchronize CPUs > > by a spin lock. But then the cpu_lock() could cause > > deadlock. > > Why? I'm running with that thing on, I've never seen a deadlock ever > because of it. In fact, i've gotten output that is plain impossible with > the current junk. > > The cpu-lock is inside the all-backtrace spinlock, not outside. And as I > said yesterday, only the lockless console has any wait-loops while > holding the cpu-lock. It _will_ make progress. So I've been a huge flaming idiot.. so while I'm not particularly sympathetic to NMIs that block, there are a number of really trivial deadlocks possible -- and it is a minor miracle I've not actually hit them (I suppose because printk() isn't really all that common). The whole cpu-lock thing I had needs to go. But not having it makes lockless console output unreadable and unsable garbage. I've got some ideas on a replacement, but I need to further consider it. :-/
