exec: kill unsafe BUG_ON(sig->count) checks

de_thread:

	if (atomic_read(&oldsighand->count) <= 1)
		BUG_ON(atomic_read(&sig->count) != 1);

This is not safe without the rmb() in between.  The results of two
correctly ordered __exit_signal()->atomic_dec_and_test()'s could be seen
out of order on our CPU.

The same is true for the "thread_group_empty()" case, __unhash_process()'s
changes could be seen before atomic_dec_and_test(&sig->count).

On some platforms (including i386) atomic_read() doesn't provide even the
compiler barrier, in that case these checks are simply racy.

Remove these BUG_ON()'s. Alternatively, we can do something like

	BUG_ON( ({ smp_rmb(); atomic_read(&sig->count) != 1; }) );

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/exec.c b/fs/exec.c
index af4361c927a9250978dcd0dabd98295b481c75c7..c21a8cc0627709db512597ff2af82ab7c094e67d 100644 (file)
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -784,7 +784,6 @@ static int de_thread(struct task_struct *tsk)
         * and we can just re-use it all.
         */
        if (atomic_read(&oldsighand->count) <= 1) {
-               BUG_ON(atomic_read(&sig->count) != 1);
                signalfd_detach(tsk);
                exit_itimers(sig);
                return 0;
@@ -929,8 +928,6 @@ no_thread_group:
        if (leader)
                release_task(leader);
 
-       BUG_ON(atomic_read(&sig->count) != 1);
-
        if (atomic_read(&oldsighand->count) == 1) {
                /*
                 * Now that we nuked the rest of the thread group,