linux-kernel - [PATCH v2] linux/bitmap.h: comments on BITMAP_LAST_WORD

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Date:   Fri, 27 Jul 2018 16:03:31 +0800
From:   Wei Wang <wei.w.wang@...el.com>
To:     linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        andy.shevchenko@...il.com, ynorov@...iumnetworks.com,
        corbet@....net, linux@...musvillemoes.dk
Cc:     dgilbert@...hat.com, wei.w.wang@...el.com
Subject: [PATCH v2] linux/bitmap.h: comments on BITMAP_LAST_WORD_MASK

When "nbits = 0" which means no bits to mask, this macro is expected to
return 0, instead of 0xffffffff. Currently, the "nbits = 0" case is
avoided by the callers themselves (e.g.__bitmap_intersects,
__bitmap_weight). This corner case should be explicitly noted.

This patch simply adds a comment above the macro as a note to users about
the corner case.

Signed-off-by: Wei Wang <wei.w.wang@...el.com>
Cc: Andy Shevchenko <andy.shevchenko@...il.com>
Cc: Yury Norov <ynorov@...iumnetworks.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Rasmus Villemoes <linux@...musvillemoes.dk>
---
 include/linux/bitmap.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
index 1ee46f4..4e64d1f 100644
--- a/include/linux/bitmap.h
+++ b/include/linux/bitmap.h
@@ -194,6 +194,7 @@ extern int bitmap_print_to_pagebuf(bool list, char *buf,
 				   const unsigned long *maskp, int nmaskbits);
 
 #define BITMAP_FIRST_WORD_MASK(start) (~0UL << ((start) & (BITS_PER_LONG - 1)))
+/* "nbits = 0" is not applicable to this macro. Callers should avoid that. */
 #define BITMAP_LAST_WORD_MASK(nbits) (~0UL >> (-(nbits) & (BITS_PER_LONG - 1)))
 
 #define small_const_nbits(nbits) \
-- 
2.7.4