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    SubjectRe: [PATCH 05/11] fs: add iterate_supers_excl() and iterate_supers_reverse_excl()
    On Thu, Nov 30, 2017 at 12:23 AM, Luis R. Rodriguez <mcgrof@kernel.org> wrote:
    > There are use cases where we wish to traverse the superblock list
    > but also capture errors, and in which case we want to avoid having
    > our callers issue a lock themselves since we can do the locking for
    > the callers. Provide a iterate_supers_excl() which calls a function
    > with the write lock held. If an error occurs we capture it and
    > propagate it.
    >
    > Likewise there are use cases where we wish to traverse the superblock
    > list but in reverse order. The new iterate_supers_reverse_excl() helpers
    > does this but also also captures any errors encountered.
    >
    > Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
    > ---
    > fs/super.c | 91 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
    > include/linux/fs.h | 2 ++
    > 2 files changed, 93 insertions(+)
    >
    > diff --git a/fs/super.c b/fs/super.c
    > index a63513d187e8..885711c1d35b 100644
    > --- a/fs/super.c
    > +++ b/fs/super.c
    > @@ -605,6 +605,97 @@ void iterate_supers(void (*f)(struct super_block *, void *), void *arg)
    > spin_unlock(&sb_lock);
    > }
    >
    > +/**
    > + * iterate_supers_excl - exclusively call func for all active superblocks
    > + * @f: function to call
    > + * @arg: argument to pass to it
    > + *
    > + * Scans the superblock list and calls given function, passing it
    > + * locked superblock and given argument. Returns 0 unless an error
    > + * occurred on calling the function on any superblock.
    > + */
    > +int iterate_supers_excl(int (*f)(struct super_block *, void *), void *arg)
    > +{
    > + struct super_block *sb, *p = NULL;
    > + int error = 0;
    > +
    > + spin_lock(&sb_lock);
    > + list_for_each_entry(sb, &super_blocks, s_list) {
    > + if (hlist_unhashed(&sb->s_instances))
    > + continue;
    > + sb->s_count++;
    > + spin_unlock(&sb_lock);

    Can anything bad happen if the list is modified at this point by a
    concurrent thread?

    > +
    > + down_write(&sb->s_umount);
    > + if (sb->s_root && (sb->s_flags & SB_BORN)) {
    > + error = f(sb, arg);
    > + if (error) {
    > + up_write(&sb->s_umount);
    > + spin_lock(&sb_lock);
    > + __put_super(sb);
    > + break;
    > + }
    > + }
    > + up_write(&sb->s_umount);
    > +
    > + spin_lock(&sb_lock);
    > + if (p)
    > + __put_super(p);
    > + p = sb;
    > + }
    > + if (p)
    > + __put_super(p);
    > + spin_unlock(&sb_lock);
    > +
    > + return error;
    > +}
    > +

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