Re: [PATCH] lock/lockdep: Add missing graph_unlock in validate_chain

From: Xuewen Yan
Date: Tue Jan 09 2024 - 01:12:16 EST

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In reply to: Boqun Feng: "Re: [PATCH] lock/lockdep: Add missing graph_unlock in validate_chain"
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Hi Boqun

On Tue, Jan 9, 2024 at 1:35 PM Boqun Feng <boqun.feng@xxxxxxxxx> wrote:
>
> On Tue, Jan 09, 2024 at 10:55:11AM +0800, Xuewen Yan wrote:
> > Hi boqun
> >
> > On Tue, Jan 9, 2024 at 12:30 AM Boqun Feng <boqun.feng@xxxxxxxxx> wrote:
> > >
> > > On Fri, Jan 05, 2024 at 12:46:36PM +0800, Xuewen Yan wrote:
> > > [...]
> > > > >
> > > > > Are you hitting a real issue or this is found by code reading?
> > > >
> > > > Indeed, we hit a real issue:
> > > > One cpu did not call graph_unlock, as a result, caused a deadlock with
> > > > other cpus,
> > > > because any cpu calling raw_spin_lock would get the graph_lock first.
> > > >
> > >
> > > Could you share more details about the real issue you hit? For example,
> > > serial log? I asked because although the graph_unlock() makes logical
> > > sense, but that path should really not hit if lockdep works correctly.
> >
> > The following is our stack:
> >
> > first, there is a scenario in our kernel tree:
> > get mutexA lock-->get rcu lock-->get mutexB lock
> > As a result, there is a warning about the chain:
> >
> > [ 7.344848][ T1@C0] reboot: Restarting system with command
> > 'userrequested,recovery'
> > [ 7.354028][ T1@C0]
> > [ 7.354358][ T1@C0] =============================
> > [ 7.354967][ T1@C0] [ BUG: Invalid wait context ]
> > [ 7.355580][ T1@C0] 6.6.5-0-g53d4f6ea5e56-dirty-ab000013 #1
> > Tainted: G W
> > [ 7.356720][ T1@C0] -----------------------------
> > [ 7.357326][ T1@C0] init/1 is trying to lock:
> > [ 7.357894][ T1@C0] ffffffc07b0b5310 (kmsg_buf_lock){+.+.}-{3:3},
> > at: last_kmsg_handler+0x60/0xb8
> > [ 7.359244][ T1@C0] other info that might help us debug this:
> > [ 7.359982][ T1@C0] context-{4:4}
> > [ 7.360420][ T1@C0] 2 locks held by init/1:
> > [ 7.360967][ T1@C0] #0: ffffffc08234d0b8
> > (system_transition_mutex){+.+.}-{3:3}, at:
> > __arm64_sys_reboot+0x130/0x27c
> > [ 7.362353][ T1@C0] #1: ffffffc0823c9a18
> > (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire+0x0/0x38
> > [ 7.363568][ T1@C0] stack backtrace:
> > [ 7.364038][ T1@C0] CPU: 0 PID: 1 Comm: init Tainted: G W
> > 6.6.5-android14-0-g53d4f6ea5e56-dirty-ab000013 #1
> > [ 7.365453][ T1@C0] Hardware name: Unisoc ***-base Board (DT)
> > [ 7.366238][ T1@C0] Call trace:
> > [ 7.366652][ T1@C0] dump_backtrace+0xf8/0x148
> > [ 7.367244][ T1@C0] show_stack+0x20/0x30
> > [ 7.367779][ T1@C0] dump_stack_lvl+0x60/0x84
> > [ 7.368372][ T1@C0] __lock_acquire+0xc2c/0x3288
> > [ 7.368982][ T1@C0] lock_acquire+0x124/0x2b0
> > [ 7.369556][ T1@C0] __mutex_lock+0xa0/0xbfc
> > [ 7.370122][ T1@C0] mutex_lock_nested+0x2c/0x38
> > [ 7.370730][ T1@C0] last_kmsg_handler+0x60/0xb8 <<<<get mutex B
> > [ 7.371494][ T1@C0] kmsg_dump+0xf0/0x16c <<<<rcu lock
> > [ 7.372028][ T1@C0] kernel_restart+0x100/0x11c
> > [ 7.372626][ T1@C0] __arm64_sys_reboot+0x1a8/0x27c
> > [ 7.373270][ T1@C0] invoke_syscall+0x60/0x11c
> > [ 7.373857][ T1@C0] el0_svc_common+0xb4/0xf0
> > [ 7.374434][ T1@C0] do_el0_svc+0x24/0x30
> > [ 7.374967][ T1@C0] el0_svc+0x50/0xe4
> > [ 7.375467][ T1@C0] el0t_64_sync_handler+0x68/0xbc
> > [ 7.376109][ T1@C0] el0t_64_sync+0x1a8/0x1ac
> >
> > This warning should be caused by the order of taking the lock.
> > But when CONFIG_PREEMPT_RCU is turned on, I think it is reasonable to
> > take the mutex after taking the rcu-lock.
>
> Thanks for the information, but...
>
> No, the rule of CONFIG_PREEMPT_RCU is allowing only implicit preemption
> in RCU read-side critical sections. So calling mutex after taking RCU
> read_lock() is illegal.
>
> See the comments before rcu_read_lock():
>
> * In preemptible RCU implementations (PREEMPT_RCU) in CONFIG_PREEMPTION
> * kernel builds, RCU read-side critical sections may be preempted,
> * but explicit blocking is illegal.

Thank you for your patient reply！And we would fix it in our tree.

>
> > Maybe the logic of check_wait_context needs to be modified？
> >
> > And then it occurs the following stack:
> >
> > core0 get rdp_gp->nocb_gp_lock and then try to get graph_lock:
> >
> > -000|queued_spin_lock_slowpath(lock = 0xFFFFFFC08291BBC8, val = 0)
> > -001|queued_spin_lock()
> > -001|lockdep_lock()
> > -001|graph_lock()
> > -002|lookup_chain_cache_add()
> > -002|validate_chain()
> > -003|lock_acquire(lock = 0xFFFFFF817F211D98, subclass = 0, trylock =
> > 0, read = 0, check = 1, nest_lock = 0x0, ip = 18446743800981533176)
> > -004|__raw_spin_lock_irqsave()
> > | lock = 0xFFFFFF817F211D80 -> (
> > | raw_lock = (
> > | val = (counter = 0),
> > | locked = 0,
> > | pending = 0,
> > | locked_pending = 0,
> > | tail = 0),
> > | magic = 3735899821,
> > | owner_cpu = 4294967295,
> > | owner = 0xFFFFFFFFFFFFFFFF,
> > | dep_map = (key = 0xFFFFFFC082F12C88, class_cache =
> > (0xFFFFFFC0825378E0, 0x0), name = 0xFFFFFFC081541039, wait_type_outer
> > = 0, wait_type_inner = 2, lock_type = 0))
> > | flags = 0
> > |
> > 110|
> > -004|_raw_spin_lock_irqsave(lock = 0xFFFFFF817F211D80)
> > -005|lock_timer_base(inline)
> > -005|__mod_timer.llvm.7968396489078322347(timer = 0xFFFFFF817F2A88D0,
> > expires = 4294893461, options = 0)
> > -006|mod_timer(inline)
> > -006|wake_nocb_gp_defer(inline)
> > -006|__call_rcu_nocb_wake(rdp = 0xFFFFFF817F2A8680, was_alldone = ?, flags = ?)
> > -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)
> >
> > However, the grapg_lock is owned by core1, and it try to get
> > rdp_gp->nocb_gp_lock, and it caused the deadlock.
> > But we do not see where the core do not unlock the graph_lock:
> >
>
> Right, this is the key point. There is a static variable tracking the
> owner task of graph-lock:
>
> static struct task_struct *__owner;
>
> are you able to find some information from the coredump?
>
> > -000|queued_spin_lock_slowpath(lock = 0xFFFFFF817F2A8A80, val = ?)
> > -001|queued_spin_lock(inline)
> > -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, was_alldone = ?, flags = ?)
> > -004|__call_rcu_common(inline)
> > -004|call_rcu(head = 0xFFFFFFC082EECC28, func = ?)
> > -005|call_rcu_zapped(inline)
> > -005|free_zapped_rcu(ch = ?)
> > -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)
> >
> > Based on this, we read the lockdep code, and found the check_prev_add's logic.
> >
> > But now we reproduce the scenario although we add the graph_unlock in
> > check_prev_add().
>
> If you have a reliable reproduce then could you try the following debug
> code?
>
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index e85b5ad3e206..a313fcc78e8e 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -466,6 +466,8 @@ static __always_inline void lockdep_recursion_finish(void)
> {
> if (WARN_ON_ONCE(__this_cpu_dec_return(lockdep_recursion)))
> __this_cpu_write(lockdep_recursion, 0);
> +
> + BUG_ON(__owner == current);
> }
>
> void lockdep_set_selftest_task(struct task_struct *task)
>
> This may tell you which code path in lockdep forgot to unlock the graph
> lock.

Thanks! We will try it!

>
> > So this is not the root-cause of our problem. And we are also still debugging.
>
> That matches my expectation: that "return 0" should never hit. So the
> problem may be somewhere any else.

We are still reading the code of lockdep and found that the following
path does not seem to release the lock?

lookup_chain_cache_add
add_chain_cache():
if (lockdep_assert_locked())
return 0;
In lockdep_assert_locked(), I do not see the graph_unlock(), did I
miss something?
Or should we also add graph_unlock?

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 8b665ba90ad0..0acb64ede7cd 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -3710,8 +3710,10 @@ static inline int add_chain_cache(struct
task_struct *curr,
* disabled to make this an IRQ-safe lock.. for recursion reasons
* lockdep won't complain about its own locking errors.
*/
- if (lockdep_assert_locked())
+ if (lockdep_assert_locked()) {
+ graph_unlock();
return 0;
+ }

chain = alloc_lock_chain();
if (!chain) {

Thanks!
---
BRs
xuewen

>
> Regards,
> BOqun
>
> > We would be very grateful if you could give us some suggestions.
> >
> > Thanks!
> >
> > ---
> > BRs
> > xuewen
> >
> >
> > > Thanks!
> > >
> > > Regards,
> > > Boqun
> > >
> > > > Thanks!

Next message: Rafał Miłecki: "Re: [PATCH 1/2] dt-bindings: arm64: mediatek: Add MT7988A and BPI-R4"
Previous message: Peng Fan: "Re: [PATCH] clk: imx: scu: Use common error handling code in imx_clk_scu_alloc_dev()"
In reply to: Boqun Feng: "Re: [PATCH] lock/lockdep: Add missing graph_unlock in validate_chain"
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