Re: [PATCH] KVM: selftests: Double check on the current CPU in rseq_test

Paolo Bonzini
2022-07-14T13:55:51Z

On 7/14/22 10:06, Gavin Shan wrote:
> In rseq_test, there are two threads created. Those two threads are
> 'main' and 'migration_thread' separately. We also have the assumption
> that non-migration status on 'migration-worker' thread guarantees the
> same non-migration status on 'main' thread. Unfortunately, the assumption
> isn't true. The 'main' thread can be migrated from one CPU to another
> one between the calls to sched_getcpu() and READ_ONCE(__rseq.cpu_id).
> The following assert is raised eventually because of the mismatched
> CPU numbers.
>
> The issue can be reproduced on arm64 system occasionally.
>
> host# uname -r
> 5.19.0-rc6-gavin+
> host# # cat /proc/cpuinfo | grep processor | tail -n 1
> processor : 223
> host# pwd
> /home/gavin/sandbox/linux.main/tools/testing/selftests/kvm
> host# for i in `seq 1 100`; \
> do echo "--------> $i"; \
> ./rseq_test; sleep 3; \
> done
> --------> 1
> --------> 2
> --------> 3
> --------> 4
> --------> 5
> --------> 6
> ==== Test Assertion Failure ====
> rseq_test.c:265: rseq_cpu == cpu
> pid=3925 tid=3925 errno=4 - Interrupted system call
> 1 0x0000000000401963: main at rseq_test.c:265 (discriminator 2)
> 2 0x0000ffffb044affb: ?? ??:0
> 3 0x0000ffffb044b0c7: ?? ??:0
> 4 0x0000000000401a6f: _start at ??:?
> rseq CPU = 4, sched CPU = 27
>
> This fixes the issue by double-checking on the current CPU after
> call to READ_ONCE(__rseq.cpu_id) and restarting the test if the
> two consecutive CPU numbers aren't euqal.
>
> Fixes: 61e52f1630f5 ("KVM: selftests: Add a test for KVM_RUN+rseq to detect task migration bugs")
> Signed-off-by: Gavin Shan <gshan@redhat.com>
> ---
> tools/testing/selftests/kvm/rseq_test.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/tools/testing/selftests/kvm/rseq_test.c b/tools/testing/selftests/kvm/rseq_test.c
> index 4158da0da2bb..74709dd9f5b2 100644
> --- a/tools/testing/selftests/kvm/rseq_test.c
> +++ b/tools/testing/selftests/kvm/rseq_test.c
> @@ -207,7 +207,7 @@ int main(int argc, char *argv[])
> {
> int r, i, snapshot;
> struct kvm_vm *vm;
> - u32 cpu, rseq_cpu;
> + u32 cpu, rseq_cpu, last_cpu;
>
> /* Tell stdout not to buffer its content */
> setbuf(stdout, NULL);
> @@ -259,8 +259,9 @@ int main(int argc, char *argv[])
> smp_rmb();
> cpu = sched_getcpu();
> rseq_cpu = READ_ONCE(__rseq.cpu_id);
> + last_cpu = sched_getcpu();
> smp_rmb();
> - } while (snapshot != atomic_read(&seq_cnt));
> + } while (snapshot != atomic_read(&seq_cnt) || cpu != last_cpu);
>
> TEST_ASSERT(rseq_cpu == cpu,
> "rseq CPU = %d, sched CPU = %d\n", rseq_cpu, cpu);

Queued for -rc, thanks.

Paolo