Re: [PATCH 04/27] ext2: Add ext2_sb_info mutex

Jan Kara
2009-11-05T13:55:21Z

On Mon 02-11-09 17:57:52, Jan Blunck wrote:
> On Mon, Nov 02, Andi Kleen wrote:
>
> > > @@ -762,6 +767,12 @@ static int ext2_fill_super(struct super_block *sb, void *data, int silent)
> > > sbi->s_sb_block = sb_block;
> > >
> > > /*
> > > + * mutex for protection of modifications of the superblock while being
> > > + * write out by ext2_write_super() or ext2_sync_fs().
> > > + */
> > > + mutex_init(&sbi->s_mutex);
> >
> > I didn't go over all the code paths in detail, but if you replace
> > the BKL with a mutex that is hold over a longer write-out sleep
> > period you potentially limit IO parallelism a lot.
>
> Right. I converted it to be a spinlock and unlock before calling
> ext2_sync_super().
>
> What do you think?
The patch is generally fine. I have just a few minor comments below:

> diff --git a/fs/ext2/super.c b/fs/ext2/super.c
> index 5af1775..70c326c 100644
> --- a/fs/ext2/super.c
> +++ b/fs/ext2/super.c
> @@ -52,8 +52,10 @@ void ext2_error (struct super_block * sb, const char * function,
> struct ext2_super_block *es = sbi->s_es;
>
> if (!(sb->s_flags & MS_RDONLY)) {
> + spin_lock(&sbi->s_lock);
> sbi->s_mount_state |= EXT2_ERROR_FS;
> es->s_state |= cpu_to_le16(EXT2_ERROR_FS);
> + /* drops sbi->s_lock */
> ext2_sync_super(sb, es);
I don't like this dropping of spinlock inside ext2_sync_super. Can we
just drop it here and retake it in ext2_sync_super? It's by far not a
performance critical path so it should not really matter.

> diff --git a/include/linux/ext2_fs_sb.h b/include/linux/ext2_fs_sb.h
> index 1cdb663..0d20278 100644
> --- a/include/linux/ext2_fs_sb.h
> +++ b/include/linux/ext2_fs_sb.h
> @@ -106,6 +106,8 @@ struct ext2_sb_info {
> spinlock_t s_rsv_window_lock;
> struct rb_root s_rsv_window_root;
> struct ext2_reserve_window_node s_rsv_window_head;
> + /* protect against concurrent modifications of this structure */
> + spinlock_t s_lock;
> };
As I'm reading the code s_lock protects some of the fieds but definitely
not all. I'd say it protects s_mount_state, s_blocks_last, s_overhead_last,
and a content of superblock's buffer pointed to by sbi->s_es. The rest just
either does not change during lifetime of the filesystem or has different
locks (either s_umount semaphore or other spinlocks).

Honza

--
Jan Kara <jack@suse.cz>
SUSE Labs, CR