linux-kernel - [RFC 01/10] lib/idr.c: reused free bitmaps are already clear

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Message-Id: <1481160187-9652-2-git-send-email-linux@rasmusvillemoes.dk>
Date:   Thu,  8 Dec 2016 02:22:56 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Tejun Heo <tj@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Jens Axboe <axboe@...nel.dk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: [RFC 01/10] lib/idr.c: reused free bitmaps are already clear

If we're recycling a previously used struct ida_bitmap, it ended up in
->free_bitmap precisely because it was all zeroes. Thus, by using
kzalloc in ida_pre_get we can avoid doing the memset while holding
whatever locks protects the ida (which, judging by the number of
callers of ida_simple_get vs ida_get_new_above, is usually always the
global simple_ida_lock).

Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
---
 lib/idr.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/lib/idr.c b/lib/idr.c
index 6098336df267..9cbfae251d77 100644
--- a/lib/idr.c
+++ b/lib/idr.c
@@ -903,7 +903,7 @@ int ida_pre_get(struct ida *ida, gfp_t gfp_mask)
 	if (!ida->free_bitmap) {
 		struct ida_bitmap *bitmap;
 
-		bitmap = kmalloc(sizeof(struct ida_bitmap), gfp_mask);
+		bitmap = kzalloc(sizeof(struct ida_bitmap), gfp_mask);
 		if (!bitmap)
 			return 0;
 
@@ -962,7 +962,6 @@ int ida_get_new_above(struct ida *ida, int starting_id, int *p_id)
 		if (!bitmap)
 			return -EAGAIN;
 
-		memset(bitmap, 0, sizeof(struct ida_bitmap));
 		rcu_assign_pointer(pa[0]->ary[idr_id & IDR_MASK],
 				(void *)bitmap);
 		pa[0]->count++;
-- 
2.1.4