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

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Message-ID: <2ec2a848-90f4-49bc-aaaf-8eb256f271db@ursulin.net>
Date: Mon, 10 Feb 2025 15:52:35 +0000
From: Tvrtko Ursulin <tursulin@...ulin.net>
To: Adrián Larumbe <adrian.larumbe@...labora.com>,
 Boris Brezillon <boris.brezillon@...labora.com>,
 Steven Price <steven.price@....com>, Liviu Dudau <liviu.dudau@....com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>
Cc: kernel@...labora.com, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] drm/panthor: Replace sleep locks with spinlocks in
 fdinfo path


On 10/02/2025 12:41, Adrián Larumbe 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.

Have the debug splat at hand? I am thinking it is not because of fdinfo 
reads, which are allowed to sleep, but has to be something else.

Regards,

Tvrtko

> 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@...labora.com>
> Fixes: e16635d88fa0 ("drm/panthor: add DRM fdinfo support")
> ---
>   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