From c0135d07b013fa8f7ba9ec91b4369c372e6a28cb Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Thu, 22 Jan 2015 22:47:14 -0800 Subject: rcu: Clear need_qs flag to prevent splat If the scheduling-clock interrupt sets the current tasks need_qs flag, but if the current CPU passes through a quiescent state in the meantime, then rcu_preempt_qs() will fail to clear the need_qs flag, which can fool RCU into thinking that additional rcu_read_unlock_special() processing is needed. This commit therefore clears the need_qs flag before checking for additional processing. For this problem to occur, we need rcu_preempt_data.passed_quiesce equal to true and current->rcu_read_unlock_special.b.need_qs also equal to true. This condition can occur as follows: 1. CPU 0 is aware of the current preemptible RCU grace period, but has not yet passed through a quiescent state. Among other things, this means that rcu_preempt_data.passed_quiesce is false. 2. Task A running on CPU 0 enters a preemptible RCU read-side critical section. 3. CPU 0 takes a scheduling-clock interrupt, which notices the RCU read-side critical section and the need for a quiescent state, and thus sets current->rcu_read_unlock_special.b.need_qs to true. 4. Task A is preempted, enters the scheduler, eventually invoking rcu_preempt_note_context_switch() which in turn invokes rcu_preempt_qs(). Because rcu_preempt_data.passed_quiesce is false, control enters the body of the "if" statement, which sets rcu_preempt_data.passed_quiesce to true. 5. At this point, CPU 0 takes an interrupt. The interrupt handler contains an RCU read-side critical section, and the rcu_read_unlock() notes that current->rcu_read_unlock_special is nonzero, and thus invokes rcu_read_unlock_special(). 6. Once in rcu_read_unlock_special(), the fact that current->rcu_read_unlock_special.b.need_qs is true becomes apparent, so rcu_read_unlock_special() invokes rcu_preempt_qs(). Recursively, given that we interrupted out of that same function in the preceding step. 7. Because rcu_preempt_data.passed_quiesce is now true, rcu_preempt_qs() does nothing, and simply returns. 8. Upon return to rcu_read_unlock_special(), it is noted that current->rcu_read_unlock_special is still nonzero (because the interrupted rcu_preempt_qs() had not yet gotten around to clearing current->rcu_read_unlock_special.b.need_qs). 9. Execution proceeds to the WARN_ON_ONCE(), which notes that we are in an interrupt handler and thus duly splats. The solution, as noted above, is to make rcu_read_unlock_special() clear out current->rcu_read_unlock_special.b.need_qs after calling rcu_preempt_qs(). The interrupted rcu_preempt_qs() will clear it again, but this is harmless. The worst that happens is that we clobber another attempt to set this field, but this is not a problem because we just got done reporting a quiescent state. Reported-by: Sasha Levin Signed-off-by: Paul E. McKenney [ paulmck: Fix embarrassing build bug noted by Sasha Levin. ] Tested-by: Sasha Levin --- kernel/rcu/tree_plugin.h | 1 + 1 file changed, 1 insertion(+) (limited to 'kernel/rcu/tree_plugin.h') diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 2e850a51bb8f..bca28b00f7e6 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -327,6 +327,7 @@ void rcu_read_unlock_special(struct task_struct *t) special = t->rcu_read_unlock_special; if (special.b.need_qs) { rcu_preempt_qs(); + t->rcu_read_unlock_special.b.need_qs = false; if (!t->rcu_read_unlock_special.s) { local_irq_restore(flags); return; -- cgit v1.2.3 From ad853b48cb4650285e8544eebbba5bbd9274ee15 Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Fri, 13 Feb 2015 14:37:25 -0800 Subject: rcu: use %*pb[l] to print bitmaps including cpumasks and nodemasks printk and friends can now format bitmaps using '%*pb[l]'. cpumask and nodemask also provide cpumask_pr_args() and nodemask_pr_args() respectively which can be used to generate the two printf arguments necessary to format the specified cpu/nodemask. Signed-off-by: Tejun Heo Cc: "Paul E. McKenney" Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- kernel/rcu/tree_plugin.h | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-) (limited to 'kernel/rcu/tree_plugin.h') diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 2e850a51bb8f..0d7bbe3095ad 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -49,7 +49,6 @@ DEFINE_PER_CPU(char, rcu_cpu_has_work); static cpumask_var_t rcu_nocb_mask; /* CPUs to have callbacks offloaded. */ static bool have_rcu_nocb_mask; /* Was rcu_nocb_mask allocated? */ static bool __read_mostly rcu_nocb_poll; /* Offload kthread are to poll. */ -static char __initdata nocb_buf[NR_CPUS * 5]; #endif /* #ifdef CONFIG_RCU_NOCB_CPU */ /* @@ -2386,8 +2385,8 @@ void __init rcu_init_nohz(void) cpumask_and(rcu_nocb_mask, cpu_possible_mask, rcu_nocb_mask); } - cpulist_scnprintf(nocb_buf, sizeof(nocb_buf), rcu_nocb_mask); - pr_info("\tOffload RCU callbacks from CPUs: %s.\n", nocb_buf); + pr_info("\tOffload RCU callbacks from CPUs: %*pbl.\n", + cpumask_pr_args(rcu_nocb_mask)); if (rcu_nocb_poll) pr_info("\tPoll for callbacks from no-CBs CPUs.\n"); -- cgit v1.2.3