Re: [PATCH 1/2] drm/panthor: Replace sleep locks with spinlocks in fdinfo path

[Boris Brezillon <boris.brezillon@collabora.com>]

On Mon, 10 Feb 2025 12:41:59 +0000
Adrián Larumbe <adrian.larumbe@collabora.com> wrote:

> Panthor's fdinfo handler is routed through the /proc file system, which
> executes in an atomic context. That means we cannot use mutexes because
> they might sleep.
> 
> This bug was uncovered by enabling some of the kernel's mutex-debugging
> features:
> 
> CONFIG_DEBUG_RT_MUTEXES=y
> CONFIG_DEBUG_MUTEXES=y
> 
> Replace Panthor's group fdinfo data mutex with a guarded spinlock.
> 
> Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
> Fixes: e16635d88fa0 ("drm/panthor: add DRM fdinfo support")

Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>

> ---
>  drivers/gpu/drm/panthor/panthor_sched.c | 26 ++++++++++++-------------
>  1 file changed, 12 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_sched.c b/drivers/gpu/drm/panthor/panthor_sched.c
> index 0b93bf83a9b2..7a267d1efeb6 100644
> --- a/drivers/gpu/drm/panthor/panthor_sched.c
> +++ b/drivers/gpu/drm/panthor/panthor_sched.c
> @@ -9,6 +9,7 @@
>  #include <drm/panthor_drm.h>
>  
>  #include <linux/build_bug.h>
> +#include <linux/cleanup.h>
>  #include <linux/clk.h>
>  #include <linux/delay.h>
>  #include <linux/dma-mapping.h>
> @@ -631,10 +632,10 @@ struct panthor_group {
>  		struct panthor_gpu_usage data;
>  
>  		/**
> -		 * @lock: Mutex to govern concurrent access from drm file's fdinfo callback
> -		 * and job post-completion processing function
> +		 * @fdinfo.lock: Spinlock to govern concurrent access from drm file's fdinfo
> +		 * callback and job post-completion processing function
>  		 */
> -		struct mutex lock;
> +		spinlock_t lock;
>  
>  		/** @fdinfo.kbo_sizes: Aggregate size of private kernel BO's held by the group. */
>  		size_t kbo_sizes;
> @@ -910,8 +911,6 @@ static void group_release_work(struct work_struct *work)
>  						   release_work);
>  	u32 i;
>  
> -	mutex_destroy(&group->fdinfo.lock);
> -
>  	for (i = 0; i < group->queue_count; i++)
>  		group_free_queue(group, group->queues[i]);
>  
> @@ -2861,12 +2860,12 @@ static void update_fdinfo_stats(struct panthor_job *job)
>  	struct panthor_job_profiling_data *slots = queue->profiling.slots->kmap;
>  	struct panthor_job_profiling_data *data = &slots[job->profiling.slot];
>  
> -	mutex_lock(&group->fdinfo.lock);
> -	if (job->profiling.mask & PANTHOR_DEVICE_PROFILING_CYCLES)
> -		fdinfo->cycles += data->cycles.after - data->cycles.before;
> -	if (job->profiling.mask & PANTHOR_DEVICE_PROFILING_TIMESTAMP)
> -		fdinfo->time += data->time.after - data->time.before;
> -	mutex_unlock(&group->fdinfo.lock);
> +	scoped_guard(spinlock, &group->fdinfo.lock) {
> +		if (job->profiling.mask & PANTHOR_DEVICE_PROFILING_CYCLES)
> +			fdinfo->cycles += data->cycles.after - data->cycles.before;
> +		if (job->profiling.mask & PANTHOR_DEVICE_PROFILING_TIMESTAMP)
> +			fdinfo->time += data->time.after - data->time.before;
> +	}
>  }
>  
>  void panthor_fdinfo_gather_group_samples(struct panthor_file *pfile)
> @@ -2880,12 +2879,11 @@ void panthor_fdinfo_gather_group_samples(struct panthor_file *pfile)
>  
>  	xa_lock(&gpool->xa);
>  	xa_for_each(&gpool->xa, i, group) {
> -		mutex_lock(&group->fdinfo.lock);
> +		guard(spinlock)(&group->fdinfo.lock);
>  		pfile->stats.cycles += group->fdinfo.data.cycles;
>  		pfile->stats.time += group->fdinfo.data.time;
>  		group->fdinfo.data.cycles = 0;
>  		group->fdinfo.data.time = 0;
> -		mutex_unlock(&group->fdinfo.lock);
>  	}
>  	xa_unlock(&gpool->xa);
>  }
> @@ -3531,7 +3529,7 @@ int panthor_group_create(struct panthor_file *pfile,
>  	mutex_unlock(&sched->reset.lock);
>  
>  	add_group_kbo_sizes(group->ptdev, group);
> -	mutex_init(&group->fdinfo.lock);
> +	spin_lock_init(&group->fdinfo.lock);
>  
>  	return gid;
>  
> 
> base-commit: 2eca617f12586abff62038db1c14cb3aa60a15aa
> prerequisite-patch-id: 7e787ce5973b5fc7e9f69f26aa4d7e5ec03d5caa
> prerequisite-patch-id: 03a619b8c741444b28435850e23d9ec463171c13
> prerequisite-patch-id: 290e1053f8bf4a8b80fb037a87cae7e096b5aa96
> prerequisite-patch-id: bc49bb8c29905650fb4788acc528bb44013c0240
> prerequisite-patch-id: 46cab4c980824c03e5164afc43085fec23e1cba5