[V3 PATCH 3/4] kexec: Fix race between panic() and crash_kexec() called directly

[Peter Zijlstra]

On Sat, Aug 22, 2015 at 02:35:24AM +0000, 河合英宏 / KAWAI，HIDEHIRO wrote:
> > From: Peter Zijlstra [mailto:peterz at infradead.org]
> > 
> > On Thu, Aug 06, 2015 at 02:45:43PM +0900, Hidehiro Kawai wrote:
> > >  void crash_kexec(struct pt_regs *regs)
> > >  {
> > > +	int old_cpu, this_cpu;
> > > +
> > > +	/*
> > > +	 * `old_cpu == -1' means we are the first comer and crash_kexec()
> > > +	 * was called without entering panic().
> > > +	 * `old_cpu == this_cpu' means crash_kexec() was called from panic().
> > > +	 */
> > > +	this_cpu = raw_smp_processor_id();
> > > +	old_cpu = atomic_cmpxchg(&panic_cpu, -1, this_cpu);
> > > +	if (old_cpu != -1 && old_cpu != this_cpu)
> > > +		return;
> > 
> > This allows recursive calling of crash_kexec(), the Changelog did not
> > mention that. Is this really required?
> 
> What part are you arguing?  Recursive call of crash_kexec() doesn't
> happen.  In the first place, one of the purpose of this patch is
> to prevent a recursive call of crash_kexec() in the following case
> as I stated in the description:
> 
> CPU 0:
>   oops_end()
>     crash_kexec()
>       mutex_trylock() // acquired
>         <NMI>
>         io_check_error()
>           panic()
>             crash_kexec()
>               mutex_trylock() // failed to acquire
>             infinite loop
> 

Yes, but what to we want to do there? It seems to me that is wrong, we
do not want to let a recursive crash_kexec() proceed.

Whereas the condition you created explicitly allows this recursion by
virtue of the 'old_cpu != this_cpu' check.

You changelog does not explain why you want a recursive crash_kexec().

> Also, the logic doesn't change form V1 (although the implementation
> changed), so I didn't add changelogs any more.

I cannot remember V1, nor can any prior patch be relevant.