Commit 9c368b5b authored by Rik van Riel's avatar Rik van Riel Committed by Ingo Molnar

sched, time: Fix lock inversion in thread_group_cputime()

The sig->stats_lock nests inside the tasklist_lock and the
sighand->siglock in __exit_signal and wait_task_zombie.

However, both of those locks can be taken from irq context,
which means we need to use the interrupt safe variant of
read_seqbegin_or_lock. This blocks interrupts when the "lock"
branch is taken (seq is odd), preventing the lock inversion.

On the first (lockless) pass through the loop, irqs are not
Reported-by: default avatarStanislaw Gruszka <>
Signed-off-by: default avatarRik van Riel <>
Signed-off-by: default avatarPeter Zijlstra (Intel) <>
Cc: Linus Torvalds <>
Link: default avatarIngo Molnar <>
parent ef8ac063
......@@ -289,13 +289,14 @@ void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times)
cputime_t utime, stime;
struct task_struct *t;
unsigned int seq, nextseq;
unsigned long flags;
/* Attempt a lockless read on the first round. */
nextseq = 0;
do {
seq = nextseq;
read_seqbegin_or_lock(&sig->stats_lock, &seq);
flags = read_seqbegin_or_lock_irqsave(&sig->stats_lock, &seq);
times->utime = sig->utime;
times->stime = sig->stime;
times->sum_exec_runtime = sig->sum_sched_runtime;
......@@ -309,7 +310,7 @@ void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times)
/* If lockless access failed, take the lock. */
nextseq = 1;
} while (need_seqretry(&sig->stats_lock, seq));
done_seqretry(&sig->stats_lock, seq);
done_seqretry_irqrestore(&sig->stats_lock, seq, flags);
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