Re: [PATCH] lockdep: fix deadlock issue between lockdep and rcu
From: Waiman Long
Date: Wed Jan 17 2024 - 09:58:19 EST
On 1/16/24 23:35, Xuewen Yan wrote:
On Wed, Jan 17, 2024 at 1:47 AM Boqun Feng <boqun.feng@xxxxxxxxx> 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@xxxxxxxxxx>
Signed-off-by: Xuewen Yan <xuewen.yan@xxxxxxxxxx>
---
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