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Message-Id: <20100416145123.283f216c.akpm@linux-foundation.org>
Date: Fri, 16 Apr 2010 14:51:23 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Richard Kennedy <richard@....demon.co.uk>
Cc: Alexander Viro <viro@...iv.linux.org.uk>,
Jens Axboe <jens.axboe@...cle.com>,
lkml <linux-kernel@...r.kernel.org>,
Nick Piggin <npiggin@...e.de>, Jeff Mahoney <jeffm@...e.com>,
reiserfs-devel@...r.kernel.org
Subject: Re: [PATCH RFC] buffer_head: remove redundant test from
wait_on_buffer
On Fri, 16 Apr 2010 11:58:19 +0100
Richard Kennedy <richard@....demon.co.uk> wrote:
> The comment suggests that when b_count equals zero it is calling
> __wait_no_buffer to trigger some debug, but as there is no debug in
> __wait_on_buffer the whole thing is redundant.
>
> AFAICT from the git log this has been the case for at least 5 years, so
> it seems safe just to remove this.
>
> Signed-off-by: Richard Kennedy <richard@....demon.co.uk>
> ---
>
> This patch against 2.6.34-rc4
> compiled & tested on x86_64
>
> regards
> Richard
>
>
> diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
> index 16ed028..4c62dd4 100644
> --- a/include/linux/buffer_head.h
> +++ b/include/linux/buffer_head.h
> @@ -305,15 +305,10 @@ map_bh(struct buffer_head *bh, struct super_block *sb, sector_t block)
> bh->b_size = sb->s_blocksize;
> }
>
> -/*
> - * Calling wait_on_buffer() for a zero-ref buffer is illegal, so we call into
> - * __wait_on_buffer() just to trip a debug check. Because debug code in inline
> - * functions is bloaty.
> - */
> static inline void wait_on_buffer(struct buffer_head *bh)
> {
> might_sleep();
> - if (buffer_locked(bh) || atomic_read(&bh->b_count) == 0)
> + if (buffer_locked(bh))
> __wait_on_buffer(bh);
> }
That debug check got inadvertently crippled during some wait_on_bit()
conversion.
It's still a nasty bug to call wait_on_buffer() against a zero-ref
buffer so perhaps we should fix it up rather than removing its remains.
diff -puN include/linux/buffer_head.h~buffer_head-remove-redundant-test-from-wait_on_buffer-fix include/linux/buffer_head.h
--- a/include/linux/buffer_head.h~buffer_head-remove-redundant-test-from-wait_on_buffer-fix
+++ a/include/linux/buffer_head.h
@@ -305,10 +305,15 @@ map_bh(struct buffer_head *bh, struct su
bh->b_size = sb->s_blocksize;
}
+/*
+ * Calling wait_on_buffer() for a zero-ref buffer is illegal, so we call into
+ * __wait_on_buffer() just to trip a debug check. Because debug code in inline
+ * functions is bloaty.
+ */
static inline void wait_on_buffer(struct buffer_head *bh)
{
might_sleep();
- if (buffer_locked(bh))
+ if (buffer_locked(bh) || atomic_read(&bh->b_count) == 0)
__wait_on_buffer(bh);
}
diff -puN fs/buffer.c~buffer_head-remove-redundant-test-from-wait_on_buffer-fix fs/buffer.c
--- a/fs/buffer.c~buffer_head-remove-redundant-test-from-wait_on_buffer-fix
+++ a/fs/buffer.c
@@ -90,6 +90,12 @@ EXPORT_SYMBOL(unlock_buffer);
*/
void __wait_on_buffer(struct buffer_head * bh)
{
+ /*
+ * Calling wait_on_buffer() against a zero-ref buffer is a nasty bug
+ * because it will almost always "work". However this buffer can be
+ * reclaimed at any time. So check for it.
+ */
+ VM_BUG_ON(atomic_read(&bh->b_count) == 0);
wait_on_bit(&bh->b_state, BH_Lock, sync_buffer, TASK_UNINTERRUPTIBLE);
}
EXPORT_SYMBOL(__wait_on_buffer);
_
And while we're there...
This might make reiserfs explode.
From: Andrew Morton <akpm@...ux-foundation.org>
The first thing __wait_on_buffer()->wait_on_bit() does is to test that the
bit was set, so the buffer_locked() test is now redundant. And once we
remove that, we can remove the check for zero ->b_count also.
And now that wait_on_buffer() unconditionally calls __wait_on_buffer(), we
can move the might_sleep() check into __wait_on_buffer() to save some text.
The downside of all of this is that wait_on_buffer() against an unlocked
buffer will now always perform a function call. Is it a common case?
We can remove __wait_on_buffer() altogether now. For some strange reason
reiserfs calls __wait_on_buffer() directly. Maybe it's passing in
zero-ref buffers. If so, we'll get warnings now and shall need to look at
that.
Cc: Jens Axboe <jens.axboe@...cle.com>
Cc: Nick Piggin <nickpiggin@...oo.com.au>
Cc: Richard Kennedy <richard@....demon.co.uk>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
---
fs/buffer.c | 2 ++
include/linux/buffer_head.h | 4 +---
2 files changed, 3 insertions(+), 3 deletions(-)
diff -puN include/linux/buffer_head.h~wait_on_buffer-remove-the-buffer_locked-test include/linux/buffer_head.h
--- a/include/linux/buffer_head.h~wait_on_buffer-remove-the-buffer_locked-test
+++ a/include/linux/buffer_head.h
@@ -312,9 +312,7 @@ map_bh(struct buffer_head *bh, struct su
*/
static inline void wait_on_buffer(struct buffer_head *bh)
{
- might_sleep();
- if (buffer_locked(bh) || atomic_read(&bh->b_count) == 0)
- __wait_on_buffer(bh);
+ __wait_on_buffer(bh);
}
static inline int trylock_buffer(struct buffer_head *bh)
diff -puN fs/buffer.c~wait_on_buffer-remove-the-buffer_locked-test fs/buffer.c
--- a/fs/buffer.c~wait_on_buffer-remove-the-buffer_locked-test
+++ a/fs/buffer.c
@@ -90,6 +90,8 @@ EXPORT_SYMBOL(unlock_buffer);
*/
void __wait_on_buffer(struct buffer_head * bh)
{
+ might_sleep();
+
/*
* Calling wait_on_buffer() against a zero-ref buffer is a nasty bug
* because it will almost always "work". However this buffer can be
_
--
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