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kernel: is_current_single_threaded: don't use ->mmap_sem
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: Oleg Nesterov <[email protected]> Acked-by: David Howells <[email protected]> Signed-off-by: James Morris <[email protected]>
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