linux-kernel - Re: [PATCH 00/13] Preparatory work to kill off ACCESS

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Message-Id: <20171009200223.GA27372@linux.vnet.ibm.com>
Date:   Mon, 9 Oct 2017 13:02:23 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-kernel@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>, akpm@...ux-foundation.org,
        benh@...nel.crashing.org, bp@...en8.de, davem@...emloft.net,
        fw@...len.de, jiangshanlai@...il.com, johannes.berg@...el.com,
        jonathanh@...dia.com, kadlec@...ckhole.kfki.hu, mchehab@...nel.org,
        mpe@...erman.id.au, pablo@...filter.org, paulus@...ba.org,
        petr@...drovec.name, sakari.ailus@...ux.intel.com,
        shuah@...nel.org, snitzer@...hat.com, thierry.reding@...il.com,
        thor.thayer@...ux.intel.com, tj@...nel.org, viro@...iv.linux.org.uk
Subject: Re: [PATCH 00/13] Preparatory work to kill off ACCESS_ONCE()

On Mon, Oct 09, 2017 at 07:28:37PM +0100, Mark Rutland wrote:
> Hi all,
> 
> There's a general want to kill off ACCESS_ONCE(), which is required to kill off
> smp_read_barrier_depends(), and to support debug features which require
> instrumenting reads and writes separately.
> 
> Thanks to preparatory work by a number of people, it's largely possible to
> script this with the Coccinelle patch below. However, this breaks a handful of
> cases, and renders some comments stale.
> 
> This series fixes up said cases, and comments. Where fixups have been made,
> I've converted the entire file for consistency. The remaining code can be
> converted by Coccinelle script, allowing for the subsequent removal of
> ACCESS_ONCE().
> 
> I've pushed this series, complete with an example treewide conversion of
> v4.14-rc4 to my core/access-once branch [1,2].

And this all that is left, aside from definitions and comments associated
with those definitions.  Cool!

Feel free to pull this into your preparation series.

							Thanx, Paul

------------------------------------------------------------------------

commit ed940234966f4857e23dd5f16aa8f200fc85dac6
Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Date:   Mon Oct 9 12:59:18 2017 -0700

    treewide: Kill off remaining ACCESS_ONCE()
    
    This commit removes instances of ACCESS_ONCE() not located by Mark Rutland's
    coccinelle script, for example, those in complex macro definitions and
    those in comments.  Removing ACCESS_ONCE() in favor of READ_ONCE() and
    WRITE_ONCE() provides tools such as KTSAN the read/write distinction that
    they need to better detect concurrency bugs.  In addition, removing
    ACCESS_ONCE() is one necessary step towards removing special cases for
    DEC Alpha from the Linux-kernel memory model.
    
    Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

diff --git a/Documentation/filesystems/path-lookup.md b/Documentation/filesystems/path-lookup.md
index 1b39e084a2b2..50ea32a51626 100644
--- a/Documentation/filesystems/path-lookup.md
+++ b/Documentation/filesystems/path-lookup.md
@@ -826,9 +826,9 @@ If the filesystem may need to revalidate dcache entries, then
 *is* passed the dentry but does not have access to the `inode` or the
 `seq` number from the `nameidata`, so it needs to be extra careful
 when accessing fields in the dentry.  This "extra care" typically
-involves using `ACCESS_ONCE()` or the newer [`READ_ONCE()`] to access
-fields, and verifying the result is not NULL before using it.  This
-pattern can be see in `nfs_lookup_revalidate()`.
+involves using [`READ_ONCE()`] to access fields, and verifying the
+result is not NULL before using it.  This pattern can be see in
+`nfs_lookup_revalidate()`.
 
 A pair of patterns
 ------------------
diff --git a/mm/memory.c b/mm/memory.c
index a728bed16c20..cae514e7dcfc 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3891,9 +3891,9 @@ static int handle_pte_fault(struct vm_fault *vmf)
 		/*
 		 * some architectures can have larger ptes than wordsize,
 		 * e.g.ppc44x-defconfig has CONFIG_PTE_64BIT=y and
-		 * CONFIG_32BIT=y, so READ_ONCE or ACCESS_ONCE cannot guarantee
-		 * atomic accesses.  The code below just needs a consistent
-		 * view for the ifs and we later double check anyway with the
+		 * CONFIG_32BIT=y, so READ_ONCE cannot guarantee atomic
+		 * accesses.  The code below just needs a consistent view
+		 * for the ifs and we later double check anyway with the
 		 * ptl lock held. So here a barrier will do.
 		 */
 		barrier();