linux-kernel - Re: [PATCH v4 07/10] mmap locking API: add mmap_read_trylock_non

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Date:   Mon, 20 Apr 2020 14:22:11 -0400
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     Michel Lespinasse <walken@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        Liam Howlett <Liam.Howlett@...cle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        David Rientjes <rientjes@...gle.com>,
        Hugh Dickins <hughd@...gle.com>, Ying Han <yinghan@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Daniel Jordan <daniel.m.jordan@...cle.com>
Subject: Re: [PATCH v4 07/10] mmap locking API: add
 mmap_read_trylock_non_owner()

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@...cle.com>