Re: [PATCH v3 1/2] PM: wakeup: Add kfuncs to traverse over wakeup_sources

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Tue, Mar 31, 2026 at 08:34:10AM -0700, Samuel Wu wrote:
> Iterating through wakeup sources via sysfs or debugfs can be inefficient
> or restricted. Introduce BPF kfuncs to allow high-performance and safe
> in-kernel traversal of the wakeup_sources list.

What exactly is "inefficient"?  I think you might have some numbers in
your 0/2 patch, but putting it in here would be best.

And who is going to be calling these functions, just ebpf scripts?

> The new kfuncs include:
> - bpf_wakeup_sources_get_head() to obtain the list head.
> - bpf_wakeup_sources_read_lock/unlock() to manage the SRCU lock.

Does this mean we can stop exporting wakeup_sources_read_lock() now?

> For verifier safety, the underlying SRCU index is wrapped in an opaque
> 'struct bpf_ws_lock' pointer. This enables the use of KF_ACQUIRE and
> KF_RELEASE flags, allowing the BPF verifier to strictly enforce paired
> lock/unlock cycles and prevent resource leaks.

But it's an index, not a lock.  Is this just a verifier thing?

> 
> Signed-off-by: Samuel Wu <wusamuel@google.com>
> ---
>  drivers/base/power/power.h  |  7 ++++
>  drivers/base/power/wakeup.c | 72 +++++++++++++++++++++++++++++++++++--
>  2 files changed, 77 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/base/power/power.h b/drivers/base/power/power.h
> index 922ed457db19..8823aceeac8b 100644
> --- a/drivers/base/power/power.h
> +++ b/drivers/base/power/power.h
> @@ -168,3 +168,10 @@ static inline void device_pm_init(struct device *dev)
>  	device_pm_sleep_init(dev);
>  	pm_runtime_init(dev);
>  }
> +
> +#ifdef CONFIG_BPF_SYSCALL
> +struct bpf_ws_lock { };

An empty structure?  This is just an int, so you are casting an int to a
pointer?  Can we make wakeup_sources_read_lock() actually use a
structure instead to make this simpler?

> +struct bpf_ws_lock *bpf_wakeup_sources_read_lock(void);
> +void bpf_wakeup_sources_read_unlock(struct bpf_ws_lock *lock);
> +void *bpf_wakeup_sources_get_head(void);
> +#endif
> diff --git a/drivers/base/power/wakeup.c b/drivers/base/power/wakeup.c
> index b8e48a023bf0..8eda7d35d9cc 100644
> --- a/drivers/base/power/wakeup.c
> +++ b/drivers/base/power/wakeup.c
> @@ -1168,11 +1168,79 @@ static const struct file_operations wakeup_sources_stats_fops = {
>  	.release = seq_release_private,
>  };
>  
> -static int __init wakeup_sources_debugfs_init(void)
> +#ifdef CONFIG_BPF_SYSCALL
> +#include <linux/btf.h>
> +
> +__bpf_kfunc_start_defs();
> +
> +/**
> + * bpf_wakeup_sources_read_lock - Acquire the SRCU lock for wakeup sources
> + *
> + * The underlying SRCU lock returns an integer index. However, the BPF verifier
> + * requires a pointer (PTR_TO_BTF_ID) to strictly track the state of acquired
> + * resources using KF_ACQUIRE and KF_RELEASE semantics. We use an opaque
> + * structure pointer (struct bpf_ws_lock *) to satisfy the verifier while
> + * safely encoding the integer index within the pointer address itself.
> + *
> + * Return: An opaque pointer encoding the SRCU lock index + 1 (to avoid NULL).
> + */
> +__bpf_kfunc struct bpf_ws_lock *bpf_wakeup_sources_read_lock(void)
> +{
> +	return (struct bpf_ws_lock *)(long)(wakeup_sources_read_lock() + 1);

Why are you incrementing this by 1?

> +}
> +
> +/**
> + * bpf_wakeup_sources_read_unlock - Release the SRCU lock for wakeup sources
> + * @lock: The opaque pointer returned by bpf_wakeup_sources_read_lock()
> + *
> + * The BPF verifier guarantees that @lock is a valid, unreleased pointer from
> + * the acquire function. We decode the pointer back into the integer SRCU index
> + * by subtracting 1 and release the lock.
> + */
> +__bpf_kfunc void bpf_wakeup_sources_read_unlock(struct bpf_ws_lock *lock)
> +{
> +	wakeup_sources_read_unlock((int)(long)lock - 1);

Why decrementing by one?

So it's really an int, but you are casting it to a pointer, incrementing
it by one to make it a "fake" pointer value (i.e. unaligned mess), and
then when unlocking casting the pointer back to an int, and then
decrementing the value?

This feels "odd" :(


> +}
> +
> +/**
> + * bpf_wakeup_sources_get_head - Get the head of the wakeup sources list
> + *
> + * Return: The head of the wakeup sources list.
> + */
> +__bpf_kfunc void *bpf_wakeup_sources_get_head(void)
> +{
> +	return &wakeup_sources;
> +}
> +
> +__bpf_kfunc_end_defs();
> +
> +BTF_KFUNCS_START(wakeup_source_kfunc_ids)
> +BTF_ID_FLAGS(func, bpf_wakeup_sources_read_lock, KF_ACQUIRE)
> +BTF_ID_FLAGS(func, bpf_wakeup_sources_read_unlock, KF_RELEASE)
> +BTF_ID_FLAGS(func, bpf_wakeup_sources_get_head)
> +BTF_KFUNCS_END(wakeup_source_kfunc_ids)
> +
> +static const struct btf_kfunc_id_set wakeup_source_kfunc_set = {
> +	.owner = THIS_MODULE,

This isn't a module.

thanks,

greg k-h