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Date: Wed, 17 Jan 2024 09:58:02 -0500
From: Waiman Long <longman@...hat.com>
To: Xuewen Yan <xuewen.yan94@...il.com>, Boqun Feng <boqun.feng@...il.com>
Cc: Zhiguo Niu <zhiguo.niu@...soc.com>, peterz@...radead.org,
mingo@...hat.com, will@...nel.org, linux-kernel@...r.kernel.org,
niuzhiguo84@...il.com, ke.wang@...soc.com, xuewen.yan@...soc.com
Subject: Re: [PATCH] lockdep: fix deadlock issue between lockdep and rcu
On 1/16/24 23:35, Xuewen Yan wrote:
> On Wed, Jan 17, 2024 at 1:47 AM Boqun Feng <boqun.feng@...il.com> wrote:
>> On Mon, Jan 15, 2024 at 04:53:16PM +0800, Zhiguo Niu wrote:
>>> There is a deadlock scenario between lockdep and rcu when
>>> rcu nocb feature is enabled, just as following call stack:
>>>
>>> rcuop/x
>>> -000|queued_spin_lock_slowpath(lock = 0xFFFFFF817F2A8A80, val = ?)
>>> -001|queued_spin_lock(inline) // try to hold nocb_gp_lock
>>> -001|do_raw_spin_lock(lock = 0xFFFFFF817F2A8A80)
>>> -002|__raw_spin_lock_irqsave(inline)
>>> -002|_raw_spin_lock_irqsave(lock = 0xFFFFFF817F2A8A80)
>>> -003|wake_nocb_gp_defer(inline)
>>> -003|__call_rcu_nocb_wake(rdp = 0xFFFFFF817F30B680)
>>> -004|__call_rcu_common(inline)
>>> -004|call_rcu(head = 0xFFFFFFC082EECC28, func = ?)
>>> -005|call_rcu_zapped(inline)
>>> -005|free_zapped_rcu(ch = ?)// hold graph lock
>>> -006|rcu_do_batch(rdp = 0xFFFFFF817F245680)
>>> -007|nocb_cb_wait(inline)
>>> -007|rcu_nocb_cb_kthread(arg = 0xFFFFFF817F245680)
>>> -008|kthread(_create = 0xFFFFFF80803122C0)
>>> -009|ret_from_fork(asm)
>>>
>>> rcuop/y
>>> -000|queued_spin_lock_slowpath(lock = 0xFFFFFFC08291BBC8, val = 0)
>>> -001|queued_spin_lock()
>>> -001|lockdep_lock()
>>> -001|graph_lock() // try to hold graph lock
>>> -002|lookup_chain_cache_add()
>>> -002|validate_chain()
>>> -003|lock_acquire
>>> -004|_raw_spin_lock_irqsave(lock = 0xFFFFFF817F211D80)
>>> -005|lock_timer_base(inline)
>>> -006|mod_timer(inline)
>>> -006|wake_nocb_gp_defer(inline)// hold nocb_gp_lock
>>> -006|__call_rcu_nocb_wake(rdp = 0xFFFFFF817F2A8680)
>>> -007|__call_rcu_common(inline)
>>> -007|call_rcu(head = 0xFFFFFFC0822E0B58, func = ?)
>>> -008|call_rcu_hurry(inline)
>>> -008|rcu_sync_call(inline)
>>> -008|rcu_sync_func(rhp = 0xFFFFFFC0822E0B58)
>>> -009|rcu_do_batch(rdp = 0xFFFFFF817F266680)
>>> -010|nocb_cb_wait(inline)
>>> -010|rcu_nocb_cb_kthread(arg = 0xFFFFFF817F266680)
>>> -011|kthread(_create = 0xFFFFFF8080363740)
>>> -012|ret_from_fork(asm)
>>>
>>> rcuop/x and rcuop/y are rcu nocb threads with the same nocb gp thread.
>>>
>> Nice! Looks like you find the root cause ;-) nocb_gp_lock and graph_lock
>> have an ABBA deadlock due to lockdep's dependency on RCU. I assume this
>> actually fixes the problem you saw?
>>
>> However, I want to suggest a different fix, please see below:
>>
>>> This patch release the graph lock before lockdep call_rcu.
>>>
>>> Signed-off-by: Zhiguo Niu <zhiguo.niu@...soc.com>
>>> Signed-off-by: Xuewen Yan <xuewen.yan@...soc.com>
>>> ---
>>> kernel/locking/lockdep.c | 38 +++++++++++++++++++++++++-------------
>>> 1 file changed, 25 insertions(+), 13 deletions(-)
>>>
>>> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
>>> index 151bd3d..c1d432a 100644
>>> --- a/kernel/locking/lockdep.c
>>> +++ b/kernel/locking/lockdep.c
>>> @@ -6186,23 +6186,29 @@ static struct pending_free *get_pending_free(void)
>>> /*
>>> * Schedule an RCU callback if no RCU callback is pending. Must be called with
>>> * the graph lock held.
>>> + *
>>> + * Return true if graph lock need be released by the caller, otherwise false
>>> + * means graph lock is released by itself.
>>> */
>>> -static void call_rcu_zapped(struct pending_free *pf)
>>> +static bool call_rcu_zapped(struct pending_free *pf)
>>> {
>>> WARN_ON_ONCE(inside_selftest());
>>>
>>> if (list_empty(&pf->zapped))
>>> - return;
>>> + return true;
>>>
>>> if (delayed_free.scheduled)
>>> - return;
>>> + return true;
>>>
>>> delayed_free.scheduled = true;
>>>
>>> WARN_ON_ONCE(delayed_free.pf + delayed_free.index != pf);
>>> delayed_free.index ^= 1;
>>>
>>> + lockdep_unlock();
>>> call_rcu(&delayed_free.rcu_head, free_zapped_rcu);
>>> +
>>> + return false;
>>> }
>>>
>>> /* The caller must hold the graph lock. May be called from RCU context. */
>>> @@ -6228,6 +6234,7 @@ static void free_zapped_rcu(struct rcu_head *ch)
>>> {
>>> struct pending_free *pf;
>>> unsigned long flags;
>>> + bool need_unlock;
>>>
>>> if (WARN_ON_ONCE(ch != &delayed_free.rcu_head))
>>> return;
>>> @@ -6243,9 +6250,9 @@ static void free_zapped_rcu(struct rcu_head *ch)
>>> /*
>>> * If there's anything on the open list, close and start a new callback.
>>> */
>>> - call_rcu_zapped(delayed_free.pf + delayed_free.index);
>>> -
>>> - lockdep_unlock();
>>> + need_unlock = call_rcu_zapped(delayed_free.pf + delayed_free.index);
>>> + if (need_unlock)
>>> + lockdep_unlock();
>> Instead of returning a bool to control the unlock, I think it's better
>> that we refactor the call_rcu_zapped() a bit, so it becomes a
>> prepare_call_rcu_zapped():
>>
>> // See if we need to queue an RCU callback, must called with
>> // the lockdep lock held, returns false if either we don't have
>> // any pending free or the callback is already scheduled.
>> // Otherwise, a call_rcu() must follow this function call.
>> static bool prepare_call_rcu_zapped(struct pending_free *pf)
>> {
>> WARN_ON_ONCE(inside_selftest());
>>
>> if (list_empty(&pf->zapped))
>> return false;
>>
>> if (delayed_free.scheduled)
>> return false;
>>
>> delayed_free.scheduled = true;
>>
>> WARN_ON_ONCE(delayed_free.pf + delayed_free.index != pf);
>> delayed_free.index ^= 1;
>>
>> return true;
>> }
>>
>> , and here we can:
>>
>> <lockdep_lock() is called previous>
>> need_callback = prepare_call_rcu_zapped(...);
>> lockdep_unlock();
>> raw_local_irq_restore(flags);
>>
>> if (need_callback)
>> call_rcu(&delayed_free.rcu_head, free_zapped_rcu);
> Would there be any problems if call_rcu is placed outside the shutdown
> interrupt?
call_rcu() doesn't need to be called with interrupt disabled. In fact,
it calls local_irq_save() itself when necessary. So that is perfectly fine.
Cheers,
Longman
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