----- On Aug 19, 2021, at 7:33 PM, Sean Christopherson seanjc@google.com wrote:
On Thu, Aug 19, 2021, Mathieu Desnoyers wrote:
----- On Aug 17, 2021, at 8:12 PM, Sean Christopherson seanjc@google.com wrote:
Add a test to verify an rseq's CPU ID is updated correctly if the task is migrated while the kernel is handling KVM_RUN. This is a regression test for a bug introduced by commit 72c3c0fe54a3 ("x86/kvm: Use generic xfer to guest work function"), where TIF_NOTIFY_RESUME would be cleared by KVM without updating rseq, leading to a stale CPU ID and other badness.
Signed-off-by: Sean Christopherson seanjc@google.com
[...]
- while (!done) {
vcpu_run(vm, VCPU_ID);TEST_ASSERT(get_ucall(vm, VCPU_ID, NULL) == UCALL_SYNC,"Guest failed?");cpu = sched_getcpu();rseq_cpu = READ_ONCE(__rseq.cpu_id);/** Verify rseq's CPU matches sched's CPU, and that sched's CPU* is stable. This doesn't handle the case where the task is* migrated between sched_getcpu() and reading rseq, and again* between reading rseq and sched_getcpu(), but in practice no* false positives have been observed, while on the other hand* blocking migration while this thread reads CPUs messes with* the timing and prevents hitting failures on a buggy kernel.*/I think you could get a stable cpu id between sched_getcpu and __rseq_abi.cpu_id if you add a pthread mutex to protect:
sched_getcpu and __rseq_abi.cpu_id reads
vs
sched_setaffinity calls within the migration thread.
Thoughts ?
I tried that and couldn't reproduce the bug. That's what I attempted to call out in the blurb "blocking migration while this thread reads CPUs ... prevents hitting failures on a buggy kernel".
I considered adding arbitrary delays around the mutex to try and hit the bug, but I was worried that even if I got it "working" for this bug, the test would be too tailored to this bug and potentially miss future regression. Letting the two threads run wild seemed like it would provide the best coverage, at the cost of potentially causing to false failures.
OK, so your point is that using mutual exclusion to ensure stability of the cpu id changes the timings too much, to a point where the issues don't reproduce. I understand that this mutex ties the migration thread timings to the vcpu thread's use of the mutex, which will reduce timings randomness, which is unwanted here.
I still really hate flakiness in tests, because then people stop caring when they fail once in a while. And with the nature of rseq, a once-in-a-while failure is a big deal. Let's see if we can use other tricks to ensure stability of the cpu id without changing timings too much.
One idea would be to use a seqcount lock. But even if we use that, I'm concerned that the very long writer critical section calling sched_setaffinity would need to be alternated with a sleep to ensure the read-side progresses. The sleep delay could be relatively small compared to the duration of the sched_setaffinity call, e.g. ratio 1:10.
static volatile uint64_t seqcnt;
The thread responsible for setting the affinity would do something like:
for (;;) { atomic_inc_seq_cst(&seqcnt); sched_setaffinity(..., n++ % nr_cpus); atomic_inc_seq_cst(&seqcnt); usleep(1); /* this is where read-side is allowed to progress. */ }
And the thread reading the rseq cpu id and calling sched_getcpu():
uint64_t snapshot;
do { snapshot = atomic_load(&seqcnt) & ~1; /* force retry if odd */ smp_rmb(); cpu = sched_getcpu(); rseq_cpu = READ_ONCE(__rseq.cpu_id); smp_rmb(); } while (snapshot != atomic_load(&seqcnt));
So the reader retry the cpu id reads whenever sched_setaffinity is being called by the migration thread, and whenever it is preempted for more than one migration thread loop.
That should achieve our goal of providing cpu id stability without significantly changing the timings of the migration thread, given that it never blocks waiting for the reader.
Thoughts ?
Thanks,
Mathieu