On Fri, 2010-04-16 at 14:51 -0700, Andrew Morton wrote:
> 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@xxxxxxxxxxxxxxxxxxxx>
>
> 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@xxxxxxxxxx>
> Cc: Nick Piggin <nickpiggin@xxxxxxxxxxxx>
> Cc: Richard Kennedy <richard@xxxxxxxxxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> ---
>
> 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
> _
>
Hi Andrew,
I've tested your patches against 2.6.34-rc4 on lvm/ext4. I'm not seeing
any vm bugs, so it all looks good to me.
thanks
Richard
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