From: Oleg Nesterov Date: Thu, 9 Jul 2009 21:28:49 +0000 (+0200) Subject: kernel: is_current_single_threaded: don't use ->mmap_sem X-Git-Tag: v2.6.32-rc1~737^2^2~41 X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=967cc5371113f9806b39a2ebb2174af2883d96fe;p=pandora-kernel.git 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 Acked-by: David Howells Signed-off-by: James Morris --- Reading git-diff-tree failed