Re: [PATCH v2 2/2] kgdb: Fix kgdb_roundup_cpus() for arches who used smp_call_function()

[Daniel Thompson]

On Wed, Oct 31, 2018 at 02:49:26PM +0100, Peter Zijlstra wrote:
> On Tue, Oct 30, 2018 at 03:18:43PM -0700, Douglas Anderson wrote:
> > Looking closely at it, it seems like a really bad idea to be calling
> > local_irq_enable() in kgdb_roundup_cpus().  If nothing else that seems
> > like it could violate spinlock semantics and cause a deadlock.
> > 
> > Instead, let's use a private csd alongside
> > smp_call_function_single_async() to round up the other CPUs.  Using
> > smp_call_function_single_async() doesn't require interrupts to be
> > enabled so we can remove the offending bit of code.
> 
> You might want to mention that the only reason this isn't a deadlock
> itself is because there is a timeout on waiting for the slaves to
> check-in.

dbg_master_lock must be owned to call kgdb_roundup_cpus() so
the calls to smp_call_function_single_async() should never deadlock the
calling CPU unless there has been a previous failure to round up (e.g.
cores that cannot react to the round up signal).

When there is a failure to round up when we resume, there is a window (before
whatever locks that prevented the IPI being serviced are released) during which
the system will deadlock if the debugger is re entered.

I don't think there is any point trying to round up a CPU that did not
previously respond... it should still have an IPI pending. The deadlock
can be eliminated by getting the round up code to avoid CPUs whose csd->flags
are non-zero either by checking the flag in the kgdb code or adding something
like smp_trycall_function_single_async().


Daniel.