Re: [PATCH] srcu: Eliminate the case that snp_seq bigger than snap in srcu_funnel_gp_start()

[Pingfan Liu <kernelfans@gmail.com>]

Sorry for the fragment, but I am just aware of this and think it is
better to fold into the same series.

Thanks,

    Pingfan

On Sun, Nov 20, 2022 at 11:20 PM Pingfan Liu <kernelfans@gmail.com> wrote:
>
> Since the srcu read lock is still held during srcu_funnel_gp_start(),
> the seq snap should be the largest number for the slot
> srcu_have_cbs[idx].
>
> Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
> Cc: Lai Jiangshan <jiangshanlai@gmail.com>
> Cc: "Paul E. McKenney" <paulmck@kernel.org>
> Cc: Frederic Weisbecker <frederic@kernel.org>
> Cc: Josh Triplett <josh@joshtriplett.org>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> To: rcu@vger.kernel.org
> ---
>  include/linux/rcupdate.h |  1 +
>  kernel/rcu/srcutree.c    | 10 +++++-----
>  2 files changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 08605ce7379d..a09007236660 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -32,6 +32,7 @@
>  #include <linux/context_tracking_irq.h>
>
>  #define ULONG_CMP_GE(a, b)     (ULONG_MAX / 2 >= (a) - (b))
> +#define ULONG_CMP_GT(a, b)     (ULONG_MAX / 2 > (a) - (b))
>  #define ULONG_CMP_LT(a, b)     (ULONG_MAX / 2 < (a) - (b))
>  #define ulong2long(a)          (*(long *)(&(a)))
>  #define USHORT_CMP_GE(a, b)    (USHRT_MAX / 2 >= (unsigned short)((a) - (b)))
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index c54d6c04751f..057752db1125 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -900,14 +900,14 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
>                 for (snp = snp_leaf; snp != NULL; snp = snp->srcu_parent) {
>                         spin_lock_irqsave_rcu_node(snp, flags);
>                         snp_seq = snp->srcu_have_cbs[idx];
> -                       if (!srcu_invl_snp_seq(snp_seq) && ULONG_CMP_GE(snp_seq, s)) {
> +                       /* s should be the biggest in the current slot. Hence only LE is
> +                        * valid
> +                        */
> +                       BUG_ON(ULONG_CMP_GT(snp_seq, s));
> +                       if (!srcu_invl_snp_seq(snp_seq) && (snp_seq == s)) {
>                                 if (snp == snp_leaf && snp_seq == s)
>                                         snp->srcu_data_have_cbs[idx] |= sdp->grpmask;
>                                 spin_unlock_irqrestore_rcu_node(snp, flags);
> -                               if (snp == snp_leaf && snp_seq != s) {
> -                                       srcu_schedule_cbs_sdp(sdp, do_norm ? SRCU_INTERVAL : 0);
> -                                       return;
> -                               }
>                                 if (!do_norm)
>                                         srcu_funnel_exp_start(ssp, snp, s);
>                                 return;
> --
> 2.31.1
>