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;
MIME-Version: 1.0
References: <20200716060553.24618-1-ebiggers@kernel.org>
In-Reply-To: <20200716060553.24618-1-ebiggers@kernel.org>
From:   Dan Williams <dan.j.williams@intel.com>
Date:   Thu, 16 Jul 2020 08:53:30 -0700
Message-ID: <CAPcyv4g94hK7j7hknSmtt11hgq+5N-Yf-fwLdS4VrARyqKwJLw@mail.gmail.com>
Subject: Re: [PATCH] /dev/mem: Add missing memory barriers for devmem_inode
To:     Eric Biggers <ebiggers@kernel.org>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>, Ingo Molnar <mingo@redhat.com>,
        Kees Cook <keescook@chromium.org>,
        Matthew Wilcox <willy@infradead.org>,
        Russell King <linux@arm.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Wed, Jul 15, 2020 at 11:07 PM Eric Biggers <ebiggers@kernel.org> wrote:
>
> From: Eric Biggers <ebiggers@google.com>
>
> WRITE_ONCE() isn't the correct way to publish a pointer to a data
> structure, since it doesn't include a write memory barrier.  Therefore
> other tasks may see that the pointer has been set but not see that the
> pointed-to memory has finished being initialized yet.  Instead a
> primitive with "release" semantics is needed.
>
> Use smp_store_release() for this.
>
> The use of READ_ONCE() on the read side is still potentially correct if
> there's no control dependency, i.e. if all memory being "published" is
> transitively reachable via the pointer itself.  But this pairing is
> somewhat confusing and error-prone.  So just upgrade the read side to
> smp_load_acquire() so that it clearly pairs with smp_store_release().
>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Russell King <linux@arm.linux.org.uk>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Fixes: 3234ac664a87 ("/dev/mem: Revoke mappings when a driver claims the region")
> Signed-off-by: Eric Biggers <ebiggers@google.com>

Makes sense:

Acked-by: Dan Williams <dan.j.williams@intel.com>