buffer_head: remove redundant test from wait_on_buffer

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 <[email protected]>

---

This patch against 2.6.34-rc4
compiled & tested on x86_64

regards
Richard

include/linux/buffer_head.h +0 -2

index 16ed028..4c62dd4 100644
@@ -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

And while we're there...

This might make reiserfs explode.
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.

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

include/linux/buffer_head.h renamed from include/linux/buffer_head.h~buffer_head-remove-redundant-test-from-wait_on_buffer-fix +6 -1

@@ -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

fs/buffer.c renamed from fs/buffer.c~buffer_head-remove-redundant-test-from-wait_on_buffer-fix +6 -0

@@ -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 <[email protected]>

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 <[email protected]>

Cc Nick Piggin <[email protected]>

Cc Richard Kennedy <[email protected]>

Signed-off-by Andrew Morton <[email protected]>

---

 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

include/linux/buffer_head.h renamed from include/linux/buffer_head.h~wait_on_buffer-remove-the-buffer_locked-test +1 -3

@@ -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

fs/buffer.c renamed from fs/buffer.c~wait_on_buffer-remove-the-buffer_locked-test +2 -0

@@ -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

_

Quoted 27 lines · On Fri, 16 Apr 2010 11:58:19 +0100 Richard Kennedy <[email protected]>…

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 <[email protected]>

---

This patch against 2.6.34-rc4
compiled & tested on x86_64

regards
Richard

include/linux/buffer_head.h +0 -2

index 16ed028..4c62dd4 100644
@@ -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

include/linux/buffer_head.h map_bh(struct buffer_head *bh, struct super_block *sb, sector_t block) · line 313

Quoted 10 lines

include/linux/buffer_head.h +1 -4

- * __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

include/linux/buffer_head.h renamed from include/linux/buffer_head.h~buffer_head-remove-redundant-test-from-wait_on_buffer-fix +6 -1

@@ -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

fs/buffer.c renamed from fs/buffer.c~buffer_head-remove-redundant-test-from-wait_on_buffer-fix +0 -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 <[email protected]>

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 <[email protected]>

Cc Nick Piggin <[email protected]>

Cc Richard Kennedy <[email protected]>

Signed-off-by Andrew Morton <[email protected]>

---

 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

include/linux/buffer_head.h renamed from include/linux/buffer_head.h~wait_on_buffer-remove-the-buffer_locked-test +1 -3

@@ -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

fs/buffer.c renamed from fs/buffer.c~wait_on_buffer-remove-the-buffer_locked-test +2 -0

@@ -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

_

Quoted 7 lines

fs/buffer.c renamed from fs/buffer.c~buffer_head-remove-redundant-test-from-wait_on_buffer-fix +1 -0

@@ -90,6 +90,12 @@ EXPORT_SYMBOL(unlock_buffer);
  */
 void __wait_on_buffer(struct buffer_head * bh)
 {
+	/*

fs/buffer.c EXPORT_SYMBOL(unlock_buffer); · line 98

Quoted 5 lines

fs/buffer.c renamed from fs/buffer.c~buffer_head-remove-redundant-test-from-wait_on_buffer-fix +5 -0

+	 * 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);

My system is failing this VM_BUG_ON() occasionally. I think this is due to wait_on_buffer() calls with b_count=0 from locations within fs/buffer.c.

These
occasional b_count=0 callers are caused by buf reads that complete quickly - after the I/O is issued but before it is waited upon. Such fs/buffer.c callers need to either bypass this assertion or increment b_count. I don't think they need to grab an b_count reference. I suggest a bypass routine in the patch below. Does this look good?