Re: [PATCH AUTOSEL 5.19 05/29] powerpc/hw_breakpoint: Avoid relying on caller synchronization

[Marco Elver <elver@google.com>]

On Mon, 17 Oct 2022 at 17:08, Sasha Levin <sashal@kernel.org> wrote:
>
> From: Marco Elver <elver@google.com>
>
> [ Upstream commit f95e5a3d59011eec1257d0e76de1e1f8969d426f ]
>
> Internal data structures (cpu_bps, task_bps) of powerpc's hw_breakpoint
> implementation have relied on nr_bp_mutex serializing access to them.
>
> Before overhauling synchronization of kernel/events/hw_breakpoint.c,
> introduce 2 spinlocks to synchronize cpu_bps and task_bps respectively,
> thus avoiding reliance on callers synchronizing powerpc's hw_breakpoint.
>
> Reported-by: Dmitry Vyukov <dvyukov@google.com>
> Signed-off-by: Marco Elver <elver@google.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Acked-by: Dmitry Vyukov <dvyukov@google.com>
> Acked-by: Ian Rogers <irogers@google.com>
> Link: https://lore.kernel.org/r/20220829124719.675715-10-elver@google.com
> Signed-off-by: Sasha Levin <sashal@kernel.org>

Backporting this patch seems unnecessary if we're not backporting the
hw_breakpoint overhaul.

Without the overhaul, nothing will break without this patch.

Thanks,
-- Marco

> ---
>  arch/powerpc/kernel/hw_breakpoint.c | 53 ++++++++++++++++++++++-------
>  1 file changed, 40 insertions(+), 13 deletions(-)
>
> diff --git a/arch/powerpc/kernel/hw_breakpoint.c b/arch/powerpc/kernel/hw_breakpoint.c
> index 2669f80b3a49..8db1a15d7acb 100644
> --- a/arch/powerpc/kernel/hw_breakpoint.c
> +++ b/arch/powerpc/kernel/hw_breakpoint.c
> @@ -15,6 +15,7 @@
>  #include <linux/kernel.h>
>  #include <linux/sched.h>
>  #include <linux/smp.h>
> +#include <linux/spinlock.h>
>  #include <linux/debugfs.h>
>  #include <linux/init.h>
>
> @@ -129,7 +130,14 @@ struct breakpoint {
>         bool ptrace_bp;
>  };
>
> +/*
> + * While kernel/events/hw_breakpoint.c does its own synchronization, we cannot
> + * rely on it safely synchronizing internals here; however, we can rely on it
> + * not requesting more breakpoints than available.
> + */
> +static DEFINE_SPINLOCK(cpu_bps_lock);
>  static DEFINE_PER_CPU(struct breakpoint *, cpu_bps[HBP_NUM_MAX]);
> +static DEFINE_SPINLOCK(task_bps_lock);
>  static LIST_HEAD(task_bps);
>
>  static struct breakpoint *alloc_breakpoint(struct perf_event *bp)
> @@ -174,7 +182,9 @@ static int task_bps_add(struct perf_event *bp)
>         if (IS_ERR(tmp))
>                 return PTR_ERR(tmp);
>
> +       spin_lock(&task_bps_lock);
>         list_add(&tmp->list, &task_bps);
> +       spin_unlock(&task_bps_lock);
>         return 0;
>  }
>
> @@ -182,6 +192,7 @@ static void task_bps_remove(struct perf_event *bp)
>  {
>         struct list_head *pos, *q;
>
> +       spin_lock(&task_bps_lock);
>         list_for_each_safe(pos, q, &task_bps) {
>                 struct breakpoint *tmp = list_entry(pos, struct breakpoint, list);
>
> @@ -191,6 +202,7 @@ static void task_bps_remove(struct perf_event *bp)
>                         break;
>                 }
>         }
> +       spin_unlock(&task_bps_lock);
>  }
>
>  /*
> @@ -200,12 +212,17 @@ static void task_bps_remove(struct perf_event *bp)
>  static bool all_task_bps_check(struct perf_event *bp)
>  {
>         struct breakpoint *tmp;
> +       bool ret = false;
>
> +       spin_lock(&task_bps_lock);
>         list_for_each_entry(tmp, &task_bps, list) {
> -               if (!can_co_exist(tmp, bp))
> -                       return true;
> +               if (!can_co_exist(tmp, bp)) {
> +                       ret = true;
> +                       break;
> +               }
>         }
> -       return false;
> +       spin_unlock(&task_bps_lock);
> +       return ret;
>  }
>
>  /*
> @@ -215,13 +232,18 @@ static bool all_task_bps_check(struct perf_event *bp)
>  static bool same_task_bps_check(struct perf_event *bp)
>  {
>         struct breakpoint *tmp;
> +       bool ret = false;
>
> +       spin_lock(&task_bps_lock);
>         list_for_each_entry(tmp, &task_bps, list) {
>                 if (tmp->bp->hw.target == bp->hw.target &&
> -                   !can_co_exist(tmp, bp))
> -                       return true;
> +                   !can_co_exist(tmp, bp)) {
> +                       ret = true;
> +                       break;
> +               }
>         }
> -       return false;
> +       spin_unlock(&task_bps_lock);
> +       return ret;
>  }
>
>  static int cpu_bps_add(struct perf_event *bp)
> @@ -234,6 +256,7 @@ static int cpu_bps_add(struct perf_event *bp)
>         if (IS_ERR(tmp))
>                 return PTR_ERR(tmp);
>
> +       spin_lock(&cpu_bps_lock);
>         cpu_bp = per_cpu_ptr(cpu_bps, bp->cpu);
>         for (i = 0; i < nr_wp_slots(); i++) {
>                 if (!cpu_bp[i]) {
> @@ -241,6 +264,7 @@ static int cpu_bps_add(struct perf_event *bp)
>                         break;
>                 }
>         }
> +       spin_unlock(&cpu_bps_lock);
>         return 0;
>  }
>
> @@ -249,6 +273,7 @@ static void cpu_bps_remove(struct perf_event *bp)
>         struct breakpoint **cpu_bp;
>         int i = 0;
>
> +       spin_lock(&cpu_bps_lock);
>         cpu_bp = per_cpu_ptr(cpu_bps, bp->cpu);
>         for (i = 0; i < nr_wp_slots(); i++) {
>                 if (!cpu_bp[i])
> @@ -260,19 +285,25 @@ static void cpu_bps_remove(struct perf_event *bp)
>                         break;
>                 }
>         }
> +       spin_unlock(&cpu_bps_lock);
>  }
>
>  static bool cpu_bps_check(int cpu, struct perf_event *bp)
>  {
>         struct breakpoint **cpu_bp;
> +       bool ret = false;
>         int i;
>
> +       spin_lock(&cpu_bps_lock);
>         cpu_bp = per_cpu_ptr(cpu_bps, cpu);
>         for (i = 0; i < nr_wp_slots(); i++) {
> -               if (cpu_bp[i] && !can_co_exist(cpu_bp[i], bp))
> -                       return true;
> +               if (cpu_bp[i] && !can_co_exist(cpu_bp[i], bp)) {
> +                       ret = true;
> +                       break;
> +               }
>         }
> -       return false;
> +       spin_unlock(&cpu_bps_lock);
> +       return ret;
>  }
>
>  static bool all_cpu_bps_check(struct perf_event *bp)
> @@ -286,10 +317,6 @@ static bool all_cpu_bps_check(struct perf_event *bp)
>         return false;
>  }
>
> -/*
> - * We don't use any locks to serialize accesses to cpu_bps or task_bps
> - * because are already inside nr_bp_mutex.
> - */
>  int arch_reserve_bp_slot(struct perf_event *bp)
>  {
>         int ret;
> --
> 2.35.1
>