sched, signals: fix the racy usage of ->signal in account_group_xxx/run_posix_cpu_timers
Impact: fix potential NULL dereference Contrary to ad474cac changelog, other acct_group_xxx() helpers can be called after exit_notify() by timer tick. Thanks to Roland for pointing out this. Somehow I missed this simple fact when I read the original patch, and I am afraid I confused Frank during the discussion. Sorry. Fortunately, these helpers work with current, we can check ->exit_state to ensure that ->signal can't go away under us. Also, add the comment and compiler barrier to account_group_exec_runtime(), to make sure we load ->signal only once. Signed-off-by: NOleg Nesterov <oleg@redhat.com> Signed-off-by: NIngo Molnar <mingo@elte.hu>
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