linux-kernel - [PATCH 07/23] xarray: Switch __xa_cmpxchg() to use xas_store

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Date:   Wed, 22 Apr 2020 17:02:40 +0200
From:   Jan Kara <jack@...e.cz>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, Jan Kara <jack@...e.cz>
Subject: [PATCH 07/23] xarray: Switch __xa_cmpxchg() to use xas_store_noinit()

Currently there are four places that end up calling into __xa_cmpxchg().
Two in drivers/infiniband/hw/mlx5/odp.c, one in fs/erofs/utils.c and one
in mm/shmem.c. The xarray in the first three places do not contain any
marks ever, the fourth place originally used radix_tree_delete_item()
which didn't touch marks either. So we are safe to switch __xa_cmpxchg()
to use xas_store_noinit(). Also document that __xa_cmpxchg() now does
not touch marks at all - i.e., it has ordinary store semantics without
specialcasing NULL value. If someone ever needs xa_cmpxchg() equivalent
that would really cause mark clearing on storing NULL, we can create
xa_erase_item() function.

Signed-off-by: Jan Kara <jack@...e.cz>
---
 lib/xarray.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/lib/xarray.c b/lib/xarray.c
index a372c59e3914..d87045d120ad 100644
--- a/lib/xarray.c
+++ b/lib/xarray.c
@@ -1460,7 +1460,8 @@ EXPORT_SYMBOL(xa_store);
  *
  * You must already be holding the xa_lock when calling this function.
  * It will drop the lock if needed to allocate memory, and then reacquire
- * it afterwards.
+ * it afterwards. The call does not change any xarray marks except for
+ * XA_FREE_MARK if free tracking is enabled.
  *
  * Context: Any context.  Expects xa_lock to be held on entry.  May
  * release and reacquire xa_lock if @gfp flags permit.
@@ -1478,7 +1479,7 @@ void *__xa_cmpxchg(struct xarray *xa, unsigned long index,
 	do {
 		curr = xas_load(&xas);
 		if (curr == old) {
-			xas_store(&xas, entry);
+			xas_store_noinit(&xas, entry);
 			if (xa_track_free(xa)) {
 				if (entry && !curr)
 					xas_clear_mark(&xas, XA_FREE_MARK);
-- 
2.16.4