linux-kernel - [PATCH 10/19] samples: mic/mpssd/mpssd.c: Kill off ACCESS

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Date:   Mon, 23 Oct 2017 14:07:20 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     mingo@...nel.org
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        will.deacon@....com, mark.rutland@....com, snitzer@...hat.com,
        thor.thayer@...ux.intel.com, viro@...iv.linux.org.uk,
        davem@...emloft.net, shuah@...nel.org, mpe@...erman.id.au,
        tj@...nel.org, torvalds@...ux-foundation.org
Subject: [PATCH 10/19] samples: mic/mpssd/mpssd.c: Kill off ACCESS_ONCE()

From: Mark Rutland <mark.rutland@....com>

For several reasons, it is desirable to use {READ,WRITE}_ONCE() in
preference to ACCESS_ONCE(), and new code is expected to use one of the
former. So far, there's been no reason to change most existing uses of
ACCESS_ONCE(), as these aren't currently harmful.

However, for some features it is necessary to instrument reads and
writes separately, which is not possible with ACCESS_ONCE(). This
distinction is critical to correct operation.

The bulk of the kernel code can be transformed via Coccinelle to use
{READ,WRITE}_ONCE(), though this only modifies users of ACCESS_ONCE(),
and not the implementation itself. As such, it has the potential to
break homebrew ACCESS_ONCE() macros seen in some user code in the kernel
tree (e.g. the virtio code, as fixed in commit ea9156fb3b71d9f7).

To avoid fragility if/when that transformation occurs, and to align with
the preferred usage of {READ,WRITE}_ONCE(), this patch updates the MPSSD
sample code to use READ_ONCE() rather than ACCESS_ONCE(). There should
be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@....com>
Cc: Shuah Khan <shuah@...nel.org>
---
 samples/mic/mpssd/mpssd.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/samples/mic/mpssd/mpssd.c b/samples/mic/mpssd/mpssd.c
index 49db1def1721..f42ce551bb48 100644
--- a/samples/mic/mpssd/mpssd.c
+++ b/samples/mic/mpssd/mpssd.c
@@ -65,7 +65,7 @@ static struct mic_info mic_list;
 /* to align the pointer to the (next) page boundary */
 #define PAGE_ALIGN(addr)        _ALIGN(addr, PAGE_SIZE)

-#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
+#define READ_ONCE(x) (*(volatile typeof(x) *)&(x))

 #define GSO_ENABLED		1
 #define MAX_GSO_SIZE		(64 * 1024)
@@ -382,7 +382,7 @@ disp_iovec(struct mic_info *mic, struct mic_copy_desc *copy,

 static inline __u16 read_avail_idx(struct mic_vring *vr)
 {
-	return ACCESS_ONCE(vr->info->avail_idx);
+	return READ_ONCE(vr->info->avail_idx);
 }

 static inline void txrx_prepare(int type, bool tx, struct mic_vring *vr,
@@ -523,7 +523,7 @@ spin_for_descriptors(struct mic_info *mic, struct mic_vring *vr)
 {
 	__u16 avail_idx = read_avail_idx(vr);

-	while (avail_idx == le16toh(ACCESS_ONCE(vr->vr.avail->idx))) {
+	while (avail_idx == le16toh(READ_ONCE(vr->vr.avail->idx))) {
 #ifdef DEBUG
 		mpsslog("%s %s waiting for desc avail %d info_avail %d\n",
 			mic->name, __func__,
-- 
2.5.2