rcu: Mark accesses to ->rcu_urgent_qs and ->rcu_need_heavy_qs

rcu_all_qs() and rcu_note_context_switch() read/clear the per-CPU
->rcu_urgent_qs and ->rcu_need_heavy_qs flags with plain raw_cpu_read()
and this_cpu_write(), while the RCU core clears them with WRITE_ONCE() in
rcu_disable_urgency_upon_qs().  KCSAN flags the resulting same-CPU race:

  BUG: KCSAN: data-race in rcu_all_qs / rcu_disable_urgency_upon_qs

It is benign -- the flags are advisory and rcu_all_qs() re-reads
->rcu_urgent_qs with smp_load_acquire() before acting on it -- but these
are the last unmarked accesses to the two flags; every other access
already uses READ_ONCE()/WRITE_ONCE()/smp_*.  Mark them to match.  No
functional change.

Reproduced on a PREEMPT_NONE, CONFIG_KCSAN_INTERRUPT_WATCHER=y kernel with
a pthreads program whose threads (two per CPU) loop reading a large file:

	for (;;) {
		int fd = open("/proc/kallsyms", O_RDONLY);
		while (read(fd, buf, sizeof(buf)) > 0)
			;
		close(fd);
	}

The read()s drive cond_resched() -> rcu_all_qs() while the busy CPUs keep
the grace period urgent, so the RCU core clears the flags concurrently.

Fixes: 2dba13f0b6c2 ("rcu: Switch urgent quiescent-state requests to rcu_data structure")
Signed-off-by: Itai Handler <itai.handler@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

0 files changed, 0 insertions, 0 deletions