• Oleg Nesterov's avatar
    kernel: is_current_single_threaded: don't use ->mmap_sem · 967cc537
    Oleg Nesterov authored
    is_current_single_threaded() can safely miss a freshly forked CLONE_VM
    task, but in this case it must not miss its parent. That is why we take
    mm->mmap_sem for writing to make sure a thread/task with the same ->mm
    can't pass exit_mm() and disappear.
    
    However we can avoid ->mmap_sem and rely on rcu/barriers:
    
    	- if we do not see the exiting parent on thread/process list
    	  we see the result of list_del_rcu(), in this case we must
    	  also see the result of list_add_rcu() which does wmb().
    
    	- if we do see the parent but its ->mm == NULL, we need rmb()
    	  to make sure we can't miss the child.
    Signed-off-by: 's avatarOleg Nesterov <oleg@redhat.com>
    Acked-by: 's avatarDavid Howells <dhowells@redhat.com>
    Signed-off-by: 's avatarJames Morris <jmorris@namei.org>
    967cc537
is_single_threaded.c 1.33 KB