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Subject[PATCH v3 3/4] rcu: Do not call rcu_nocb_gp_cleanup() while holding rnp->lock
Date
rcu_nocb_gp_cleanup() is called while holding rnp->lock. Currently,
this is okay because the wake_up_all() in rcu_nocb_gp_cleanup() will
not enable the IRQs. lockdep is happy.

By switching over using swait this is not true anymore. swake_up_all()
enables the IRQs while processing the waiters. __do_softirq() can now
run and will eventually call rcu_process_callbacks() which wants to
grap nrp->lock.

Let's move the rcu_nocb_gp_cleanup() call outside the lock before we
switch over to swait.

If we would hold the rnp->lock and use swait, lockdep reports
following:

=================================
[ INFO: inconsistent lock state ]
4.2.0-rc5-00025-g9a73ba0 #136 Not tainted
---------------------------------
inconsistent {IN-SOFTIRQ-W} -> {SOFTIRQ-ON-W} usage.
rcu_preempt/8 [HC0[0]:SC0[0]:HE1:SE1] takes:
(rcu_node_1){+.?...}, at: [<ffffffff811387c7>] rcu_gp_kthread+0xb97/0xeb0
{IN-SOFTIRQ-W} state was registered at:
[<ffffffff81109b9f>] __lock_acquire+0xd5f/0x21e0
[<ffffffff8110be0f>] lock_acquire+0xdf/0x2b0
[<ffffffff81841cc9>] _raw_spin_lock_irqsave+0x59/0xa0
[<ffffffff81136991>] rcu_process_callbacks+0x141/0x3c0
[<ffffffff810b1a9d>] __do_softirq+0x14d/0x670
[<ffffffff810b2214>] irq_exit+0x104/0x110
[<ffffffff81844e96>] smp_apic_timer_interrupt+0x46/0x60
[<ffffffff81842e70>] apic_timer_interrupt+0x70/0x80
[<ffffffff810dba66>] rq_attach_root+0xa6/0x100
[<ffffffff810dbc2d>] cpu_attach_domain+0x16d/0x650
[<ffffffff810e4b42>] build_sched_domains+0x942/0xb00
[<ffffffff821777c2>] sched_init_smp+0x509/0x5c1
[<ffffffff821551e3>] kernel_init_freeable+0x172/0x28f
[<ffffffff8182cdce>] kernel_init+0xe/0xe0
[<ffffffff8184231f>] ret_from_fork+0x3f/0x70
irq event stamp: 76
hardirqs last enabled at (75): [<ffffffff81841330>] _raw_spin_unlock_irq+0x30/0x60
hardirqs last disabled at (76): [<ffffffff8184116f>] _raw_spin_lock_irq+0x1f/0x90
softirqs last enabled at (0): [<ffffffff810a8df2>] copy_process.part.26+0x602/0x1cf0
softirqs last disabled at (0): [< (null)>] (null)
other info that might help us debug this:
Possible unsafe locking scenario:
CPU0
----
lock(rcu_node_1);
<Interrupt>
lock(rcu_node_1);
*** DEADLOCK ***
1 lock held by rcu_preempt/8:
#0: (rcu_node_1){+.?...}, at: [<ffffffff811387c7>] rcu_gp_kthread+0xb97/0xeb0
stack backtrace:
CPU: 0 PID: 8 Comm: rcu_preempt Not tainted 4.2.0-rc5-00025-g9a73ba0 #136
Hardware name: Dell Inc. PowerEdge R820/066N7P, BIOS 2.0.20 01/16/2014
0000000000000000 000000006d7e67d8 ffff881fb081fbd8 ffffffff818379e0
0000000000000000 ffff881fb0812a00 ffff881fb081fc38 ffffffff8110813b
0000000000000000 0000000000000001 ffff881f00000001 ffffffff8102fa4f
Call Trace:
[<ffffffff818379e0>] dump_stack+0x4f/0x7b
[<ffffffff8110813b>] print_usage_bug+0x1db/0x1e0
[<ffffffff8102fa4f>] ? save_stack_trace+0x2f/0x50
[<ffffffff811087ad>] mark_lock+0x66d/0x6e0
[<ffffffff81107790>] ? check_usage_forwards+0x150/0x150
[<ffffffff81108898>] mark_held_locks+0x78/0xa0
[<ffffffff81841330>] ? _raw_spin_unlock_irq+0x30/0x60
[<ffffffff81108a28>] trace_hardirqs_on_caller+0x168/0x220
[<ffffffff81108aed>] trace_hardirqs_on+0xd/0x10
[<ffffffff81841330>] _raw_spin_unlock_irq+0x30/0x60
[<ffffffff810fd1c7>] swake_up_all+0xb7/0xe0
[<ffffffff811386e1>] rcu_gp_kthread+0xab1/0xeb0
[<ffffffff811089bf>] ? trace_hardirqs_on_caller+0xff/0x220
[<ffffffff81841341>] ? _raw_spin_unlock_irq+0x41/0x60
[<ffffffff81137c30>] ? rcu_barrier+0x20/0x20
[<ffffffff810d2014>] kthread+0x104/0x120
[<ffffffff81841330>] ? _raw_spin_unlock_irq+0x30/0x60
[<ffffffff810d1f10>] ? kthread_create_on_node+0x260/0x260
[<ffffffff8184231f>] ret_from_fork+0x3f/0x70
[<ffffffff810d1f10>] ? kthread_create_on_node+0x260/0x260

Signed-off-by: Daniel Wagner <daniel.wagner@bmw-carit.de>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
---
kernel/rcu/tree.c | 4 +++-
kernel/rcu/tree.h | 3 ++-
kernel/rcu/tree_plugin.h | 16 +++++++++++++---
3 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 775d36c..952536d 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1568,7 +1568,6 @@ static int rcu_future_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
int needmore;
struct rcu_data *rdp = this_cpu_ptr(rsp->rda);

- rcu_nocb_gp_cleanup(rsp, rnp);
rnp->need_future_gp[c & 0x1] = 0;
needmore = rnp->need_future_gp[(c + 1) & 0x1];
trace_rcu_future_gp(rnp, rdp, c,
@@ -1972,6 +1971,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
int nocb = 0;
struct rcu_data *rdp;
struct rcu_node *rnp = rcu_get_root(rsp);
+ struct swait_queue_head *sq;

WRITE_ONCE(rsp->gp_activity, jiffies);
raw_spin_lock_irq(&rnp->lock);
@@ -2010,7 +2010,9 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
needgp = __note_gp_changes(rsp, rnp, rdp) || needgp;
/* smp_mb() provided by prior unlock-lock pair. */
nocb += rcu_future_gp_cleanup(rsp, rnp);
+ sq = rcu_nocb_gp_get(rnp);
raw_spin_unlock_irq(&rnp->lock);
+ rcu_nocb_gp_cleanup(sq);
cond_resched_rcu_qs();
WRITE_ONCE(rsp->gp_activity, jiffies);
rcu_gp_slow(rsp, gp_cleanup_delay);
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 2e991f8..3dcf6368 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -608,7 +608,8 @@ static void zero_cpu_stall_ticks(struct rcu_data *rdp);
static void increment_cpu_stall_ticks(void);
static bool rcu_nocb_cpu_needs_barrier(struct rcu_state *rsp, int cpu);
static void rcu_nocb_gp_set(struct rcu_node *rnp, int nrq);
-static void rcu_nocb_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp);
+static struct swait_queue_head *rcu_nocb_gp_get(struct rcu_node *rnp);
+static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq);
static void rcu_init_one_nocb(struct rcu_node *rnp);
static bool __call_rcu_nocb(struct rcu_data *rdp, struct rcu_head *rhp,
bool lazy, unsigned long flags);
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index b2bf396..db4f357 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1777,9 +1777,9 @@ early_param("rcu_nocb_poll", parse_rcu_nocb_poll);
* Wake up any no-CBs CPUs' kthreads that were waiting on the just-ended
* grace period.
*/
-static void rcu_nocb_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
+static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq)
{
- wake_up_all(&rnp->nocb_gp_wq[rnp->completed & 0x1]);
+ wake_up_all(sq);
}

/*
@@ -1795,6 +1795,11 @@ static void rcu_nocb_gp_set(struct rcu_node *rnp, int nrq)
rnp->need_future_gp[(rnp->completed + 1) & 0x1] += nrq;
}

+static struct swait_queue_head *rcu_nocb_gp_get(struct rcu_node *rnp)
+{
+ return &rnp->nocb_gp_wq[rnp->completed & 0x1];
+}
+
static void rcu_init_one_nocb(struct rcu_node *rnp)
{
init_waitqueue_head(&rnp->nocb_gp_wq[0]);
@@ -2469,7 +2474,7 @@ static bool rcu_nocb_cpu_needs_barrier(struct rcu_state *rsp, int cpu)
return false;
}

-static void rcu_nocb_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
+static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq)
{
}

@@ -2477,6 +2482,11 @@ static void rcu_nocb_gp_set(struct rcu_node *rnp, int nrq)
{
}

+static struct swait_queue_head *rcu_nocb_gp_get(struct rcu_node *rnp)
+{
+ return NULL;
+}
+
static void rcu_init_one_nocb(struct rcu_node *rnp)
{
}
--
2.4.3


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