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

Gavin Shan
2022-07-14T06:07:20Z

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);
--
2.23.0