Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Mon, 20 Apr 2020 14:22:11 -0400
From:   Daniel Jordan <daniel.m.jordan@oracle.com>
To:     Michel Lespinasse <walken@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Laurent Dufour <ldufour@linux.ibm.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Matthew Wilcox <willy@infradead.org>,
        Liam Howlett <Liam.Howlett@oracle.com>,
        Jerome Glisse <jglisse@redhat.com>,
        Davidlohr Bueso <dave@stgolabs.net>,
        David Rientjes <rientjes@google.com>,
        Hugh Dickins <hughd@google.com>, Ying Han <yinghan@google.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Daniel Jordan <daniel.m.jordan@oracle.com>
Subject: Re: [PATCH v4 07/10] mmap locking API: add
 mmap_read_trylock_non_owner()
Message-ID: <20200420182211.exrzkszefraeylv2@ca-dmjordan1.us.oracle.com>
References: <20200415004353.130248-1-walken@google.com>
 <20200415004353.130248-8-walken@google.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200415004353.130248-8-walken@google.com>
User-Agent: NeoMutt/20180716
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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@oracle.com>