Re: [PATCH 1/3] kvm/ppc/booke: Hold srcu lock when calling gfn functions

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, May 01, 2013 at 07:27:23PM -0500, Scott Wood wrote:
> On 05/01/2013 07:15:53 PM, Marcelo Tosatti wrote:
> >On Fri, Apr 26, 2013 at 07:53:38PM -0500, Scott Wood wrote:
> >> diff --git a/arch/powerpc/kvm/booke.c b/arch/powerpc/kvm/booke.c
> >> index 1020119..506c87d 100644
> >> --- a/arch/powerpc/kvm/booke.c
> >> +++ b/arch/powerpc/kvm/booke.c
> >> @@ -832,6 +832,8 @@ int kvmppc_handle_exit(struct kvm_run *run,
> >struct kvm_vcpu *vcpu,
> >>  {
> >>  	int r = RESUME_HOST;
> >>  	int s;
> >> +	int idx = 0; /* silence bogus uninitialized warning */
> >> +	bool need_srcu = false;
> >>
> >>  	/* update before a new last_exit_type is rewritten */
> >>  	kvmppc_update_timing_stats(vcpu);
> >> @@ -847,6 +849,20 @@ int kvmppc_handle_exit(struct kvm_run *run,
> >struct kvm_vcpu *vcpu,
> >>  	run->exit_reason = KVM_EXIT_UNKNOWN;
> >>  	run->ready_for_interrupt_injection = 1;
> >>
> >> +	/*
> >> +	 * Don't get the srcu lock unconditionally, because kvm_ppc_pv()
> >> +	 * can call kvm_vcpu_block(), and kvm_ppc_pv() is shared with
> >> +	 * book3s, so dropping the srcu lock there would be awkward.
> >> +	 */
> >> +	switch (exit_nr) {
> >> +	case BOOKE_INTERRUPT_ITLB_MISS:
> >> +	case BOOKE_INTERRUPT_DTLB_MISS:
> >> +		need_srcu = true;
> >> +	}
> >
> >This is not good practice (codepaths should either hold srcu or
> >not hold
> >it, unconditionally).
> 
> How is it different from moving the srcu lock into individual cases
> of the switch?  I just did it this way to make it easier to add new
> exception types if necessary (e.g. at the time I thought I'd end up
> adding exceptions which lead to instruction emulation, but I ended
> up acquiring the lock further down the path in that case).

Question: is this piece of code accessing this data structure?
Answer: it depends on a given runtime configuration.

Its confusing.

> >Can you give more details of the issue? (not obvious)
> 
> ITLB/DTLB miss call things like gfn_to_memslot() which need the lock
> (but don't grab it themselves -- that seems like the real bad
> practice here...).  The syscall exceptions can't have the SRCU lock
> held, because they call kvmppc_kvm_pv which can call
> kvm_vcpu_block() (yes, you can sleep with SRCU, but not
> indefinitely...).  kvmppc_kvm_pv is shared with book3s code, so
> adding code to drop the srcu lock there would be a problem since
> book3s doesn't hold the SRCU lock then...
> 
> -Scott

Its OK to nest srcu calls as long as there are properly ordered releases:

idx1 = srcu_read_lock()
idx2 = srcu_read_lock()

srcu_read_unlock(idx2)
srcu_read_unlock(idx1)

Is that helpful?