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

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Hi Marco,

Le 28/06/2022 à 11:58, Marco Elver a écrit :
> 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.

We have an still opened old issue in our database related to 
hw_breakpoint, I was wondering if it could have any link with the 
changes you are doing and whether you could handle it at the same time.

https://github.com/linuxppc/issues/issues/38

Maybe it is completely unrelated, but as your series modifies only 
powerpc and as the issue says that powerpc is the only one to do that, I 
thought it might be worth a hand up.

Thanks
Christophe

> 
> Reported-by: Dmitry Vyukov <dvyukov@google.com>
> Signed-off-by: Marco Elver <elver@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;