On 02.04.20 13:02, Christian Borntraeger wrote:
> make sure that sigp sense running status returns a sane value for
> stopped CPUs. To avoid potential races with the stop being processed we
> wait until sense running status is first 0.
>
> Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> ---
> lib/s390x/smp.c | 2 +-
> lib/s390x/smp.h | 2 +-
> s390x/smp.c | 13 +++++++++++++
> 3 files changed, 15 insertions(+), 2 deletions(-)
>
> diff --git a/lib/s390x/smp.c b/lib/s390x/smp.c
> index 5ed8b7b..492cb05 100644
> --- a/lib/s390x/smp.c
> +++ b/lib/s390x/smp.c
> @@ -58,7 +58,7 @@ bool smp_cpu_stopped(uint16_t addr)
> return !!(status & (SIGP_STATUS_CHECK_STOP|SIGP_STATUS_STOPPED));
> }
>
> -bool smp_cpu_running(uint16_t addr)
> +bool smp_sense_running_status(uint16_t addr)
> {
> if (sigp(addr, SIGP_SENSE_RUNNING, 0, NULL) != SIGP_CC_STATUS_STORED)
> return true;
> diff --git a/lib/s390x/smp.h b/lib/s390x/smp.h
> index a8b98c0..639ec92 100644
> --- a/lib/s390x/smp.h
> +++ b/lib/s390x/smp.h
> @@ -40,7 +40,7 @@ struct cpu_status {
> int smp_query_num_cpus(void);
> struct cpu *smp_cpu_from_addr(uint16_t addr);
> bool smp_cpu_stopped(uint16_t addr);
> -bool smp_cpu_running(uint16_t addr);
> +bool smp_sense_running_status(uint16_t addr);
> int smp_cpu_restart(uint16_t addr);
> int smp_cpu_start(uint16_t addr, struct psw psw);
> int smp_cpu_stop(uint16_t addr);
> diff --git a/s390x/smp.c b/s390x/smp.c
> index 79cdc1f..b4b1ff2 100644
> --- a/s390x/smp.c
> +++ b/s390x/smp.c
> @@ -210,6 +210,18 @@ static void test_emcall(void)
> report_prefix_pop();
> }
>
> +static void test_sense_running(void)
> +{
> + report_prefix_push("sense_running");
> + /* make sure CPU is stopped */
> + smp_cpu_stop(1);
> + /* wait for stop to succeed. */
> + while(smp_sense_running_status(1));
> + report(!smp_sense_running_status(1), "CPU1 sense claims not running");
> + report_prefix_pop();
> +}
> +
> +
> /* Used to dirty registers of cpu #1 before it is reset */
> static void test_func_initial(void)
> {
> @@ -319,6 +331,7 @@ int main(void)
> test_store_status();
> test_ecall();
> test_emcall();
> + test_sense_running();
> test_reset();
> test_reset_initial();
> smp_cpu_destroy(1);
>
TBH, I am still not sure if this is completely free of races.
Assume CPU 1 is in handle_stop()
if (!kvm_s390_user_cpu_state_ctrl(vcpu->kvm))
kvm_s390_vcpu_stop(vcpu);
// CPU 1: gets scheduled out.
// CPU 0: while(smp_sense_running_status(1)); finishes
// CPU 1: gets scheduled in to return to user space
return -EOPNOTSUPP;
// CPU 0: report(!smp_sense_running_status(1), "CPU1 sense claims not
running"); fails
SIGP SENSE RUNNING is simply racy as hell and doesn't give you any
guarantees. Which is good enough for some performance improvements
(e.g., spinlocks).
Now, I can queue this, but I wouldn't be surprised if we see random
failures at one point.
--
Thanks,
David / dhildenb
