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Message-ID: <ZxewiAfsMmufpwbV@localhost.localdomain>
Date: Tue, 22 Oct 2024 16:02:48 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Zqiang <qiang.zhang1211@...il.com>
Cc: paulmck@...nel.org, neeraj.upadhyay@...nel.org, joel@...lfernandes.org,
urezki@...il.com, boqun.feng@...il.com, rcu@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] rcu/nocb: Fix the WARN_ON_ONCE() in
rcu_nocb_rdp_deoffload()
Le Tue, Oct 22, 2024 at 11:41:17AM +0800, Zqiang a écrit :
> Currently, running rcutorture test with torture_type=rcu fwd_progress=8
> n_barrier_cbs=8 nocbs_nthreads=8 nocbs_toggle=100 onoff_interval=60
> test_boost=2, will trigger the following warning:
>
> WARNING: CPU: 19 PID: 100 at kernel/rcu/tree_nocb.h:1061 rcu_nocb_rdp_deoffload+0x292/0x2a0
> RIP: 0010:rcu_nocb_rdp_deoffload+0x292/0x2a0
> [18839.537322] Call Trace:
> [18839.538006] <TASK>
> [18839.538596] ? __warn+0x7e/0x120
> [18839.539491] ? rcu_nocb_rdp_deoffload+0x292/0x2a0
> [18839.540757] ? report_bug+0x18e/0x1a0
> [18839.541805] ? handle_bug+0x3d/0x70
> [18839.542837] ? exc_invalid_op+0x18/0x70
> [18839.543959] ? asm_exc_invalid_op+0x1a/0x20
> [18839.545165] ? rcu_nocb_rdp_deoffload+0x292/0x2a0
> [18839.546547] rcu_nocb_cpu_deoffload+0x70/0xa0
> [18839.547814] rcu_nocb_toggle+0x136/0x1c0
> [18839.548960] ? __pfx_rcu_nocb_toggle+0x10/0x10
> [18839.550073] kthread+0xd1/0x100
> [18839.550958] ? __pfx_kthread+0x10/0x10
> [18839.552008] ret_from_fork+0x2f/0x50
> [18839.553002] ? __pfx_kthread+0x10/0x10
> [18839.553968] ret_from_fork_asm+0x1a/0x30
> [18839.555038] </TASK>
>
> CPU0 CPU2 CPU3
> //rcu_nocb_toggle //nocb_cb_wait //rcutorture
>
> // deoffload CPU1 // process CPU1's rdp
> rcu_barrier()
> rcu_segcblist_entrain()
> rcu_segcblist_add_len(1);
> // len == 2
> // enqueue barrier
> // callback to CPU1's
> // rdp->cblist
> rcu_do_batch()
> // invoke CPU1's rdp->cblist
> // callback
> rcu_barrier_callback()
> rcu_barrier()
> mutex_lock(&rcu_state.barrier_mutex);
> // still see len == 2
> // enqueue barrier callback
> // to CPU1's rdp->cblist
> rcu_segcblist_entrain()
> rcu_segcblist_add_len(1);
> // len == 3
> // decrement len
> rcu_segcblist_add_len(-2);
> kthread_parkme()
>
> // CPU1's rdp->cblist len == 1
> // Warn because there is
> // still a pending barrier
> // trigger warning
> WARN_ON_ONCE(rcu_segcblist_n_cbs(&rdp->cblist));
> cpus_read_unlock();
>
> // wait CPU1 comes online
> // invoke barrier callback on
> // CPU1 rdp's->cblist
> wait_for_completion(&rcu_state.barrier_completion);
> // deoffload CPU4
> cpus_read_lock()
> rcu_barrier()
> mutex_lock(&rcu_state.barrier_mutex);
> // block on barrier_mutex
> // wait rcu_barrier() on
> // CPU3 to unlock barrier_mutex
> // but CPU3 unlock barrier_mutex
> // need to wait CPU1 comes online
> // when CPU1 going online will block on cpus_write_lock
>
> The above scenario will not only trigger WARN_ON_ONCE(), but also
> trigger deadlock, this commit therefore check rdp->nocb_cb_sleep
> flags before invoke kthread_parkme(), and the kthread_parkme() is
> not invoke until there are no pending callbacks and set
> rdp->nocb_cb_sleep is true.
>
> Fixes: 1fcb932c8b5c ("rcu/nocb: Simplify (de-)offloading state machine")
> Suggested-by: Frederic Weisbecker <frederic@...nel.org>
> Signed-off-by: Zqiang <qiang.zhang1211@...il.com>
Applied with the below wordsmithing, thanks a lot!
---
From: Zqiang <qiang.zhang1211@...il.com>
Date: Tue, 22 Oct 2024 11:41:17 +0800
Subject: [PATCH] rcu/nocb: Fix missed RCU barrier on deoffloading
Currently, running rcutorture test with torture_type=rcu fwd_progress=8
n_barrier_cbs=8 nocbs_nthreads=8 nocbs_toggle=100 onoff_interval=60
test_boost=2, will trigger the following warning:
WARNING: CPU: 19 PID: 100 at kernel/rcu/tree_nocb.h:1061 rcu_nocb_rdp_deoffload+0x292/0x2a0
RIP: 0010:rcu_nocb_rdp_deoffload+0x292/0x2a0
Call Trace:
<TASK>
? __warn+0x7e/0x120
? rcu_nocb_rdp_deoffload+0x292/0x2a0
? report_bug+0x18e/0x1a0
? handle_bug+0x3d/0x70
? exc_invalid_op+0x18/0x70
? asm_exc_invalid_op+0x1a/0x20
? rcu_nocb_rdp_deoffload+0x292/0x2a0
rcu_nocb_cpu_deoffload+0x70/0xa0
rcu_nocb_toggle+0x136/0x1c0
? __pfx_rcu_nocb_toggle+0x10/0x10
kthread+0xd1/0x100
? __pfx_kthread+0x10/0x10
ret_from_fork+0x2f/0x50
? __pfx_kthread+0x10/0x10
ret_from_fork_asm+0x1a/0x30
</TASK>
CPU0 CPU2 CPU3
//rcu_nocb_toggle //nocb_cb_wait //rcutorture
// deoffload CPU1 // process CPU1's rdp
rcu_barrier()
rcu_segcblist_entrain()
rcu_segcblist_add_len(1);
// len == 2
// enqueue barrier
// callback to CPU1's
// rdp->cblist
rcu_do_batch()
// invoke CPU1's rdp->cblist
// callback
rcu_barrier_callback()
rcu_barrier()
mutex_lock(&rcu_state.barrier_mutex);
// still see len == 2
// enqueue barrier callback
// to CPU1's rdp->cblist
rcu_segcblist_entrain()
rcu_segcblist_add_len(1);
// len == 3
// decrement len
rcu_segcblist_add_len(-2);
kthread_parkme()
// CPU1's rdp->cblist len == 1
// Warn because there is
// still a pending barrier
// trigger warning
WARN_ON_ONCE(rcu_segcblist_n_cbs(&rdp->cblist));
cpus_read_unlock();
// wait CPU1 to comes online and
// invoke barrier callback on
// CPU1 rdp's->cblist
wait_for_completion(&rcu_state.barrier_completion);
// deoffload CPU4
cpus_read_lock()
rcu_barrier()
mutex_lock(&rcu_state.barrier_mutex);
// block on barrier_mutex
// wait rcu_barrier() on
// CPU3 to unlock barrier_mutex
// but CPU3 unlock barrier_mutex
// need to wait CPU1 comes online
// when CPU1 going online will block on cpus_write_lock
The above scenario will not only trigger a WARN_ON_ONCE(), but also
trigger a deadlock.
Thanks to nocb locking, a second racing rcu_barrier() on an offline CPU
will either observe the decremented callback counter down to 0 and spare
the callback enqueue, or rcuo will observe the new callback and keep
rdp->nocb_cb_sleep to false.
Therefore check rdp->nocb_cb_sleep before parking to make sure no
further rcu_barrier() is waiting on the rdp.
Fixes: 1fcb932c8b5c ("rcu/nocb: Simplify (de-)offloading state machine")
Suggested-by: Frederic Weisbecker <frederic@...nel.org>
Signed-off-by: Zqiang <qiang.zhang1211@...il.com>
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
---
kernel/rcu/tree_nocb.h | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 16865475120b..2605dd234a13 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -891,7 +891,18 @@ static void nocb_cb_wait(struct rcu_data *rdp)
swait_event_interruptible_exclusive(rdp->nocb_cb_wq,
nocb_cb_wait_cond(rdp));
if (kthread_should_park()) {
- kthread_parkme();
+ /*
+ * kthread_park() must be preceded by an rcu_barrier().
+ * But yet another rcu_barrier() might have sneaked in between
+ * the barrier callback execution and the callbacks counter
+ * decrement.
+ */
+ if (rdp->nocb_cb_sleep) {
+ rcu_nocb_lock_irqsave(rdp, flags);
+ WARN_ON_ONCE(rcu_segcblist_n_cbs(&rdp->cblist));
+ rcu_nocb_unlock_irqrestore(rdp, flags);
+ kthread_parkme();
+ }
} else if (READ_ONCE(rdp->nocb_cb_sleep)) {
WARN_ON(signal_pending(current));
trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WokeEmpty"));
--
2.46.0
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