Re: [PATCH v4 07/10] mmap locking API: add mmap_read_trylock_non_owner()

[Daniel Jordan]

On Tue, Apr 14, 2020 at 05:43:50PM -0700, Michel Lespinasse wrote:
> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> index 11d41f0c7005..998968659892 100644
> --- a/kernel/bpf/stackmap.c
> +++ b/kernel/bpf/stackmap.c
> @@ -317,7 +316,7 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
>  	 * with build_id.
>  	 */
>  	if (!user || !current || !current->mm || irq_work_busy ||
> -	    mmap_read_trylock(current->mm) == 0) {
> +	    !mmap_read_trylock_non_owner(current->mm)) {
>  		/* cannot access current->mm, fall back to ips */
>  		for (i = 0; i < trace_nr; i++) {
>  			id_offs[i].status = BPF_STACK_BUILD_ID_IP;
> @@ -342,16 +341,10 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
>  	}
>  
>  	if (!work) {
> -		mmap_read_unlock(current->mm);
> +		mmap_read_unlock_non_owner(current->mm);

These 'non_owner' calls are not intuitive because current _is the owner, so the
v3 version seems better, even if it adds a special wrapper for rwsem_release.

Though it makes some sense if you think, "we're consistently using the
non_owner APIs because there's a legitimate use somewhere else," so I'm fine
either way.

Reviewed-by: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>