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

[Gavin Shan <gshan@redhat.com>]

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