• Al Viro's avatar
    sanitize vfsmount refcounting changes · f03c6599
    Al Viro authored
    Instead of splitting refcount between (per-cpu) mnt_count
    and (SMP-only) mnt_longrefs, make all references contribute
    to mnt_count again and keep track of how many are longterm
    ones.
    
    Accounting rules for longterm count:
    	* 1 for each fs_struct.root.mnt
    	* 1 for each fs_struct.pwd.mnt
    	* 1 for having non-NULL ->mnt_ns
    	* decrement to 0 happens only under vfsmount lock exclusive
    
    That allows nice common case for mntput() - since we can't drop the
    final reference until after mnt_longterm has reached 0 due to the rules
    above, mntput() can grab vfsmount lock shared and check mnt_longterm.
    If it turns out to be non-zero (which is the common case), we know
    that this is not the final mntput() and can just blindly decrement
    percpu mnt_count.  Otherwise we grab vfsmount lock exclusive and
    do usual decrement-and-check of percpu mnt_count.
    
    For fs_struct.c we have mnt_make_longterm() and mnt_make_shortterm();
    namespace.c uses the latter in places where we don't already hold
    vfsmount lock exclusive and opencodes a few remaining spots where
    we need to manipulate mnt_longterm.
    
    Note that we mostly revert the code outside of fs/namespace.c back
    to what we used to have; in particular, normal code doesn't need
    to care about two kinds of references, etc.  And we get to keep
    the optimization Nick's variant had bought us...
    Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
    f03c6599
path.h 400 Bytes