Re: [PATCH v2 08/13] powerpc/hw_breakpoint: Avoid relying on caller synchronization

Dmitry Vyukov
2022-06-28T13:25:21Z

On Tue, 28 Jun 2022 at 11:59, Marco Elver <elver@google.com> wrote:
>
> 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>

Acked-by: Dmitry Vyukov <dvyukov@google.com>

> ---
> v2:
> * New patch.
> ---
> 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;
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>