On Fri 14-09-12 15:45:04, Fernando Luis Vázquez Cao wrote:
> iterate_supers() calls a function provided by the caller with the s_umount
> semaphore taken in read mode. However, there may be cases where write mode
> is preferable, so we add __iterate_supers(), which lets one
> specify the mode of the lock, and replace iterate_supers with two helpers
> around __iterate_supers(), iterate_supers_read() and iterate_supers_write().
>
> This will be used to fix the emergency thaw (filesystem unfreeze) code, which
> iterates over the list of superblocks but needs to hold the s_umount semaphore
> in _write_ mode bebore carrying out the actual thaw operation.
>
> This patch introduces no semantic changes since iterate_supers() users become
> iterate_supers_read() which is equivalent.
>
> Cc: Josef Bacik <jbacik@xxxxxxxxxxxx>
> Cc: Eric Sandeen <sandeen@xxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: Dave Chinner <dchinner@xxxxxxxxxx>
> Signed-off-by: Fernando Luis Vazquez Cao <fernando@xxxxxxxxxxxxx>
> ---
...
> diff -urNp linux-3.6-rc5-orig/fs/super.c linux-3.6-rc5/fs/super.c
> --- linux-3.6-rc5-orig/fs/super.c 2012-09-14 11:53:43.416703312 +0900
> +++ linux-3.6-rc5/fs/super.c 2012-09-14 12:30:52.188833193 +0900
> @@ -537,14 +537,22 @@ void drop_super(struct super_block *sb)
> EXPORT_SYMBOL(drop_super);
>
> /**
> - * iterate_supers - call function for all active superblocks
> + * __iterate_supers - call function for all active superblocks
> * @f: function to call
> * @arg: argument to pass to it
> + * @wlock: mode of superblock lock (false->read lock, true->write lock)
> *
> * Scans the superblock list and calls given function, passing it
> * locked superblock and given argument.
> + *
> + * When the caller asks for the superblock lock (s_umount semaphore) to be
> + * taken in write mode, the lock is taken but not released because the
> + * function provided by the caller may deactivate the superblock itself.
> + * It is that function's job to unlock the superblock as needed in such a
> + * case.
> */
> -void iterate_supers(void (*f)(struct super_block *, void *), void *arg)
> +static void __iterate_supers(void (*f)(struct super_block *, void *), void *arg,
> + bool wlock)
> {
> struct super_block *sb, *p = NULL;
>
> @@ -555,10 +563,19 @@ void iterate_supers(void (*f)(struct sup
> sb->s_count++;
> spin_unlock(&sb_lock);
>
> - down_read(&sb->s_umount);
> + if (wlock)
> + down_write(&sb->s_umount);
> + else
> + down_read(&sb->s_umount);
> +
> if (sb->s_root && (sb->s_flags & MS_BORN))
> f(sb, arg);
> - up_read(&sb->s_umount);
> +
> + /* When the semaphore was taken in write mode the function
> + * provided by the caller takes care of unlocking it as
> + * needed. See explanation above for details. */
> + if (!wlock)
> + up_read(&sb->s_umount);
>
> spin_lock(&sb_lock);
> if (p)
These locking rules are ugly and counterintuitive. People will easily
get them wrong and create bugs. I'd rather see emergency thaw retake the
s_umount semaphore so that iterate_supers() can drop it...
Honza
--
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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