Re: [Qemu-devel] [s390] possible deadlock in handle_sigp?

[David Hildenbrand]

> On 09/12/2016 08:03 PM, Paolo Bonzini wrote:
> > 
> > 
> > On 12/09/2016 19:37, Christian Borntraeger wrote:  
> >> On 09/12/2016 06:44 PM, Paolo Bonzini wrote:  
> >>> I think that two CPUs doing reciprocal SIGPs could in principle end up
> >>> waiting on each other to complete their run_on_cpu.  If the SIGP has to
> >>> be synchronous the fix is not trivial (you'd have to put the CPU in a
> >>> state similar to cpu->halted = 1), otherwise it's enough to replace
> >>> run_on_cpu with async_run_on_cpu.  
> >>
> >> IIRC the sigps are supossed to be serialized by the big QEMU lock. WIll 
> >> have a look.  
> > 
> > Yes, but run_on_cpu drops it when it waits on the qemu_work_cond
> > condition variable.  (Related: I stumbled upon it because I wanted to
> > remove the BQL from run_on_cpu work items).  
> 
> Yes, seems you are right. If both CPUs have just exited from KVM doing a
> crossover sigp, they will do the arch_exit handling before the run_on_cpu
> stuff which might result in this hang. (luckily it seems quite unlikely 
> but still we need to fix it).
> We cannot simply use async as the callbacks also provide the condition
> code for the initiater, so this requires some rework.
> 
> 

Smells like having to provide a lock per CPU. Trylock that lock, if that's not
possible, cc=busy. SIGP SET ARCHITECTURE has to lock all CPUs.

That was the initital design, until I realized that this was all protected by
the BQL.

David