• Al Viro's avatar
    RCU'd vfsmounts · 48a066e7
    Al Viro authored
    * RCU-delayed freeing of vfsmounts
    * vfsmount_lock replaced with a seqlock (mount_lock)
    * sequence number from mount_lock is stored in nameidata->m_seq and
    used when we exit RCU mode
    * new vfsmount flag - MNT_SYNC_UMOUNT.  Set by umount_tree() when its
    caller knows that vfsmount will have no surviving references.
    * synchronize_rcu() done between unlocking namespace_sem in namespace_unlock()
    and doing pending mntput().
    * new helper: legitimize_mnt(mnt, seq).  Checks the mount_lock sequence
    number against seq, then grabs reference to mnt.  Then it rechecks mount_lock
    again to close the race and either returns success or drops the reference it
    has acquired.  The subtle point is that in case of MNT_SYNC_UMOUNT we can
    simply decrement the refcount and sod off - aforementioned synchronize_rcu()
    makes sure that final mntput() won't come until we leave RCU mode.  We need
    that, since we don't want to end up with some lazy pathwalk racing with
    umount() and stealing the final mntput() from it - caller of umount() may
    expect it to return only once the fs is shut down and we don't want to break
    that.  In other cases (i.e. with MNT_SYNC_UMOUNT absent) we have to do
    full-blown mntput() in case of mount_lock sequence number mismatch happening
    just as we'd grabbed the reference, but in those cases we won't be stealing
    the final mntput() from anything that would care.
    * mntput_no_expire() doesn't lock anything on the fast path now.  Incidentally,
    SMP and UP cases are handled the same way - no ifdefs there.
    * normal pathname resolution does *not* do any writes to mount_lock.  It does,
    of course, bump the refcounts of vfsmount and dentry in the very end, but that's
    it.
    Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
    48a066e7
mount.h 2.81 KB