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Date: Tue, 16 Jan 2024 09:47:08 -0800
From: Boqun Feng <boqun.feng@...il.com>
To: Zhiguo Niu <zhiguo.niu@...soc.com>
Cc: peterz@...radead.org, mingo@...hat.com, will@...nel.org,
longman@...hat.com, 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 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);
compared to your fix, we don't have a special logic where
call_rcu_zapped() can be an unlock in some conditions, which prevents
local correctness reasoning.
Thoughts?
Regards,
Boqun
> raw_local_irq_restore(flags);
> }
>
[...]
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