Messages in this thread Patch in this message | | | Date | Thu, 14 Jul 2022 16:03:00 +0200 | Subject | Re: [PATCH] KVM: selftests: Double check on the current CPU in rseq_test | From | Paolo Bonzini <> |
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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.
Hmm, this does not seem a correct patch - the threads are already synchronizing using seq_cnt, like this:
migration main ---------------------- -------------------------------- seq_cnt = 1 smp_wmb() snapshot = 0 smp_rmb() cpu = sched_getcpu() reads 23 sched_setaffinity() rseq_cpu = __rseq.cpuid reads 35 smp_rmb() snapshot != seq_cnt -> retry smp_wmb() seq_cnt = 2
sched_setaffinity() is guaranteed to block until the task is enqueued on an allowed CPU.
Can you check that smp_rmb() and smp_wmb() generate correct instructions on arm64?
Paolo
> 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);
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