Re: [Xen-devel] [PATCH v2 4/4] xen/rcu: add assertions to debug build

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Feb 18, 2020 at 01:21:14PM +0100, Juergen Gross wrote:
> Xen's RCU implementation relies on no softirq handling taking place
> while being in a RCU critical section. Add ASSERT()s in debug builds
> in order to catch any violations.
> 
> For that purpose modify rcu_read_[un]lock() to use a dedicated percpu
> counter instead of preempt_[en|dis]able() as this enables to test
> that condition in __do_softirq() (ASSERT_NOT_IN_ATOMIC() is not
> usable there due to __cpu_up() calling process_pending_softirqs()
> while holding the cpu hotplug lock).
> 
> Dropping the now no longer needed #include of preempt.h in rcupdate.h
> requires adding it in some sources.
> 
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> ---
>  xen/common/multicall.c     |  1 +
>  xen/common/rcupdate.c      |  4 ++++
>  xen/common/softirq.c       |  2 ++
>  xen/common/wait.c          |  1 +
>  xen/include/xen/rcupdate.h | 21 +++++++++++++++++----
>  5 files changed, 25 insertions(+), 4 deletions(-)
> 
> diff --git a/xen/common/multicall.c b/xen/common/multicall.c
> index 5a199ebf8f..67f1a23485 100644
> --- a/xen/common/multicall.c
> +++ b/xen/common/multicall.c
> @@ -10,6 +10,7 @@
>  #include <xen/multicall.h>
>  #include <xen/guest_access.h>
>  #include <xen/perfc.h>
> +#include <xen/preempt.h>
>  #include <xen/trace.h>
>  #include <asm/current.h>
>  #include <asm/hardirq.h>
> diff --git a/xen/common/rcupdate.c b/xen/common/rcupdate.c
> index e6add0b120..b03f4b44d9 100644
> --- a/xen/common/rcupdate.c
> +++ b/xen/common/rcupdate.c
> @@ -46,6 +46,10 @@
>  #include <xen/cpu.h>
>  #include <xen/stop_machine.h>
>  
> +#ifndef NDEBUG
> +DEFINE_PER_CPU(unsigned int, rcu_lock_cnt);
> +#endif
> +
>  /* Global control variables for rcupdate callback mechanism. */
>  static struct rcu_ctrlblk {
>      long cur;           /* Current batch number.                      */
> diff --git a/xen/common/softirq.c b/xen/common/softirq.c
> index 3fe75ca3e8..18be8db0c6 100644
> --- a/xen/common/softirq.c
> +++ b/xen/common/softirq.c
> @@ -30,6 +30,8 @@ static void __do_softirq(unsigned long ignore_mask, bool 
> rcu_allowed)
>      unsigned int i, cpu;
>      unsigned long pending;
>  
> +    ASSERT(!rcu_allowed || rcu_quiesce_allowed());
> +
>      for ( ; ; )
>      {
>          /*
> diff --git a/xen/common/wait.c b/xen/common/wait.c
> index 24716e7676..9cdb174036 100644
> --- a/xen/common/wait.c
> +++ b/xen/common/wait.c
> @@ -19,6 +19,7 @@
>   * along with this program; If not, see <http://www.gnu.org/licenses/>.
>   */
>  
> +#include <xen/preempt.h>
>  #include <xen/sched.h>
>  #include <xen/softirq.h>
>  #include <xen/wait.h>
> diff --git a/xen/include/xen/rcupdate.h b/xen/include/xen/rcupdate.h
> index 87f35b7704..a5ee7fec2b 100644
> --- a/xen/include/xen/rcupdate.h
> +++ b/xen/include/xen/rcupdate.h
> @@ -34,10 +34,23 @@
>  #include <xen/cache.h>
>  #include <xen/spinlock.h>
>  #include <xen/cpumask.h>
> -#include <xen/preempt.h>
> +#include <xen/percpu.h>
>  
>  #define __rcu
>  
> +#ifndef NDEBUG
> +DECLARE_PER_CPU(unsigned int, rcu_lock_cnt);
> +
> +#define rcu_quiesce_disable() (this_cpu(rcu_lock_cnt))++
> +#define rcu_quiesce_enable()  (this_cpu(rcu_lock_cnt))--

I think you need a barrier here like it's currently used in
preempt_{enabled/disable}, or use arch_lock_{acquire/release}_barrier
which would be better IMO.

> +#define rcu_quiesce_allowed() (!this_cpu(rcu_lock_cnt))

ASSERT_NOT_IN_ATOMIC should be expanded to also assert
!this_cpu(rcu_lock_cnt), or else missing pairs of
rcu_read_{lock/unlock} would be undetected.

Thanks, Roger.

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