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    SubjectRe: [RFC][PATCH 03/20] Add vfsmount writer count
    On Fri, Jun 16, 2006 at 04:12:15PM -0700, Dave Hansen wrote:
    > +/*
    > + * Must be called under write lock on mnt->mnt_sb->s_umount,
    > + * this prevents concurrent decrements which could make the
    > + * value -1, and test in mnt_want_write() wrongly succeed
    > + */
    > +static inline int mnt_make_readonly(struct vfsmount *mnt)
    > +{
    > + if (!atomic_dec_and_test(&mnt->mnt_writers)) {
    > + atomic_inc(&mnt->mnt_writers);
    > + return -EBUSY;
    > + }
    > + return 0;
    > +}

    Check in faccessat() could get screwed by that, if you just lose the
    race with final writer going away. Then mnt_make_readonly() will
    fail (as it should) and access(2) give -EROFS.
    -
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