• Al Viro's avatar
    missing barriers in some of unix_sock ->addr and ->path accesses · ae3b5641
    Al Viro authored
    Several u->addr and u->path users are not holding any locks in
    common with unix_bind().  unix_state_lock() is useless for those
    purposes.
    
    u->addr is assign-once and *(u->addr) is fully set up by the time
    we set u->addr (all under unix_table_lock).  u->path is also
    set in the same critical area, also before setting u->addr, and
    any unix_sock with ->path filled will have non-NULL ->addr.
    
    So setting ->addr with smp_store_release() is all we need for those
    "lockless" users - just have them fetch ->addr with smp_load_acquire()
    and don't even bother looking at ->path if they see NULL ->addr.
    
    Users of ->addr and ->path fall into several classes now:
        1) ones that do smp_load_acquire(u->addr) and access *(u->addr)
    and u->path only if smp_load_acquire() has returned non-NULL.
        2) places holding unix_table_lock.  These are guaranteed that
    *(u->addr) is seen fully initialized.  If unix_sock is in one of the
    "bound" chains, so's ->path.
        3) unix_sock_destructor() using ->addr is safe.  All places
    that set u->addr are guaranteed to have seen all stores *(u->addr)
    while holding a reference to u and unix_sock_destructor() is called
    when (atomic) refcount hits zero.
        4) unix_release_sock() using ->path is safe.  unix_bind()
    is serialized wrt unix_release() (normally - by struct file
    refcount), and for the instances that had ->path set by unix_bind()
    unix_release_sock() comes from unix_release(), so they are fine.
    Instances that had it set in unix_stream_connect() either end up
    attached to a socket (in unix_accept()), in which case the call
    chain to unix_release_sock() and serialization are the same as in
    the previous case, or they never get accept'ed and unix_release_sock()
    is called when the listener is shut down and its queue gets purged.
    In that case the listener's queue lock provides the barriers needed -
    unix_stream_connect() shoves our unix_sock into listener's queue
    under that lock right after having set ->path and eventual
    unix_release_sock() caller picks them from that queue under the
    same lock right before calling unix_release_sock().
        5) unix_find_other() use of ->path is pointless, but safe -
    it happens with successful lookup by (abstract) name, so ->path.dentry
    is guaranteed to be NULL there.
    earlier-variant-reviewed-by: default avatar"Paul E. McKenney" <paulmck@linux.ibm.com>
    Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    ae3b5641
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