Re: [PATCH v3 00/21] DEPT(Dependency Tracker)

From: Byungchul Park
Date: Thu Mar 03 2022 - 04:48:54 EST

Next message: Peter Geis: "Re: [PATCH] mmc: host: dw-mmc-rockchip: fix handling invalid clock rates"
Previous message: Stephen Rothwell: "linux-next: Signed-off-by missing for commit in the tip tree"
In reply to: Hyeonggon Yoo: "Re: [PATCH v3 00/21] DEPT(Dependency Tracker)"
Next in thread: Hyeonggon Yoo: "Re: [PATCH v3 00/21] DEPT(Dependency Tracker)"
Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]

On Thu, Mar 03, 2022 at 08:03:21AM +0000, Hyeonggon Yoo wrote:
> On Thu, Mar 03, 2022 at 09:18:13AM +0900, Byungchul Park wrote:
> > Hi Hyeonggon,
> >
> > Dept also allows the following scenario when an user guarantees that
> > each lock instance is different from another at a different depth:
> >
> > lock A0 with depth
> > lock A1 with depth + 1
> > lock A2 with depth + 2
> > lock A3 with depth + 3
> > (and so on)
> > ..
> > unlock A3
> > unlock A2
> > unlock A1
> > unlock A0

Look at this. Dept allows object->lock -> other_object->lock (with a
different depth using *_lock_nested()) so won't report it.

> > However, Dept does not allow the following scenario where another lock
> > class cuts in the dependency chain:
> >
> > lock A0 with depth
> > lock B
> > lock A1 with depth + 1
> > lock A2 with depth + 2
> > lock A3 with depth + 3
> > (and so on)
> > ..
> > unlock A3
> > unlock A2
> > unlock A1
> > unlock B
> > unlock A0
> >
> > This scenario is clearly problematic. What do you think is going to
> > happen with another context running the following?
> >
>
> First of all, I want to say I'm not expert at locking primitives.
> I may be wrong.

It's okay. Thanks anyway for your feedback.

> > > 45 * scan_mutex [-> object->lock] -> kmemleak_lock -> other_object->lock (SINGLE_DEPTH_NESTING)
> > > 46 *
> > > 47 * No kmemleak_lock and object->lock nesting is allowed outside scan_mutex
> > > 48 * regions.
>
> lock order in kmemleak is described above.
>
> and DEPT detects two cases as deadlock:
>
> 1) object->lock -> other_object->lock

It's not a deadlock *IF* two have different depth using *_lock_nested().
Dept also allows this case. So Dept wouldn't report it.

> 2) object->lock -> kmemleak_lock, kmemleak_lock -> other_object->lock

But this usage is risky. I already explained it in the mail you replied
to. I copied it. See the below.

context A
> > lock A0 with depth
> > lock B
> > lock A1 with depth + 1
> > lock A2 with depth + 2
> > lock A3 with depth + 3
> > (and so on)
> > ..
> > unlock A3
> > unlock A2
> > unlock A1
> > unlock B
> > unlock A0

...

context B
> > lock A1 with depth
> > lock B
> > lock A2 with depth + 1
> > lock A3 with depth + 2
> > (and so on)
> > ..
> > unlock A3
> > unlock A2
> > unlock B
> > unlock A1

where Ax : object->lock, B : kmemleak_lock.

A deadlock might occur if the two contexts run at the same time.

> And in kmemleak case, 1) and 2) is not possible because it must hold
> scan_mutex first.

This is another issue. Let's focus on whether the order is okay for now.

> I think the author of kmemleak intended lockdep to treat object->lock
> and other_object->lock as different class, using raw_spin_lock_nested().

Yes. The author meant to assign a different class according to its depth
using a Lockdep API. Strictly speaking, those are the same class anyway
but we assign a different class to each depth to avoid Lockdep splats
*IF* the user guarantees the nesting lock usage is safe, IOW, guarantees
each lock instance is different at a different depth.

I was fundamentally asking you... so... is the nesting lock usage safe
for real? I hope you distinguish between the safe case and the risky
case when *_lock_nested() is involved. Thoughts?

Thanks,
Byungchul

> Am I missing something?
>
> Thanks.
>
> > lock A1 with depth
> > lock B
> > lock A2 with depth + 1
> > lock A3 with depth + 2
> > (and so on)
> > ..
> > unlock A3
> > unlock A2
> > unlock B
> > unlock A1
> >
> > It's a deadlock. That's why Dept reports this case as a problem. Or am I
> > missing something?
> >
> > Thanks,
> > Byungchul
> >
> > > ---------------------------------------------------
> > > context A's detail
> > > ---------------------------------------------------
> > > context A
> > > [S] __raw_spin_lock_irqsave(&object->lock:0)
> > > [W] __raw_spin_lock_irqsave(kmemleak_lock:0)
> > > [E] spin_unlock(&object->lock:0)
> > >
> > > [S] __raw_spin_lock_irqsave(&object->lock:0):
> > > [<ffffffc00810302c>] scan_gray_list+0x84/0x13c
> > > stacktrace:
> > > dept_ecxt_enter+0x88/0xf4
> > > _raw_spin_lock_irqsave+0xf0/0x1c4
> > > scan_gray_list+0x84/0x13c
> > > kmemleak_scan+0x2d8/0x54c
> > > kmemleak_scan_thread+0xac/0xd4
> > > kthread+0xd4/0xe4
> > > ret_from_fork+0x10/0x20
> > >
> > > [W] __raw_spin_lock_irqsave(kmemleak_lock:0):
> > > [<ffffffc008102ebc>] scan_block+0x3c/0x128
> > > stacktrace:
> > > __dept_wait+0x8c/0xa4
> > > dept_wait+0x6c/0x88
> > > _raw_spin_lock_irqsave+0xb8/0x1c4
> > > scan_block+0x3c/0x128
> > > scan_gray_list+0xc4/0x13c
> > > kmemleak_scan+0x2d8/0x54c
> > > kmemleak_scan_thread+0xac/0xd4
> > > kthread+0xd4/0xe4
> > > ret_from_fork+0x10/0x20
> > >
> > > [E] spin_unlock(&object->lock:0):
> > > [<ffffffc008102ee0>] scan_block+0x60/0x128
> > >
> > > ---------------------------------------------------
> > > context B's detail
> > > ---------------------------------------------------
> > > context B
> > > [S] __raw_spin_lock_irqsave(kmemleak_lock:0)
> > > [W] _raw_spin_lock_nested(&object->lock:0)
> > > [E] spin_unlock(kmemleak_lock:0)
> > >
> > > [S] __raw_spin_lock_irqsave(kmemleak_lock:0):
> > > [<ffffffc008102ebc>] scan_block+0x3c/0x128
> > > stacktrace:
> > > dept_ecxt_enter+0x88/0xf4
> > > _raw_spin_lock_irqsave+0xf0/0x1c4
> > > scan_block+0x3c/0x128
> > > kmemleak_scan+0x19c/0x54c
> > > kmemleak_scan_thread+0xac/0xd4
> > > kthread+0xd4/0xe4
> > > ret_from_fork+0x10/0x20
> > >
> > > [W] _raw_spin_lock_nested(&object->lock:0):
> > > [<ffffffc008102f34>] scan_block+0xb4/0x128
> > > stacktrace:
> > > dept_wait+0x74/0x88
> > > _raw_spin_lock_nested+0xa8/0x1b0
> > > scan_block+0xb4/0x128
> > > kmemleak_scan+0x19c/0x54c
> > > kmemleak_scan_thread+0xac/0xd4
> > > kthread+0xd4/0xe4
> > > ret_from_fork+0x10/0x20
> > > [E] spin_unlock(kmemleak_lock:0):
> > > [<ffffffc008102ee0>] scan_block+0x60/0x128
> > > stacktrace:
> > > dept_event+0x7c/0xfc
> > > _raw_spin_unlock_irqrestore+0x8c/0x120
> > > scan_block+0x60/0x128
> > > kmemleak_scan+0x19c/0x54c
> > > kmemleak_scan_thread+0xac/0xd4
> > > kthread+0xd4/0xe4
> > > ret_from_fork+0x10/0x20
> > > ---------------------------------------------------
> > > information that might be helpful
> > > ---------------------------------------------------
> > > CPU: 1 PID: 38 Comm: kmemleak Tainted: G W 5.17.0-rc1+ #1
> > > Hardware name: linux,dummy-virt (DT)
> > > Call trace:
> > > dump_backtrace.part.0+0x9c/0xc4
> > > show_stack+0x14/0x28
> > > dump_stack_lvl+0x9c/0xcc
> > > dump_stack+0x14/0x2c
> > > print_circle+0x2d4/0x438
> > > cb_check_dl+0x6c/0x70
> > > bfs+0xc0/0x168
> > > add_dep+0x88/0x11c
> > > add_wait+0x2d0/0x2dc
> > > __dept_wait+0x8c/0xa4
> > > dept_wait+0x6c/0x88
> > > _raw_spin_lock_irqsave+0xb8/0x1c4
> > > scan_block+0x3c/0x128
> > > scan_gray_list+0xc4/0x13c
> > > kmemleak_scan+0x2d8/0x54c
> > > kmemleak_scan_thread+0xac/0xd4
> > > kthread+0xd4/0xe4
> > > ret_from_fork+0x10/0x20
> > >
> > > > ===================================================
> > > > DEPT: Circular dependency has been detected.
> > > > 5.17.0-rc1+ #1 Tainted: G W
> > > > ---------------------------------------------------
> > > > summary
> > > > ---------------------------------------------------
> > > > *** AA DEADLOCK ***
> > > >
> > > > context A
> > > > [S] __raw_spin_lock_irqsave(&object->lock:0)
> > > > [W] _raw_spin_lock_nested(&object->lock:0)
> > > > [E] spin_unlock(&object->lock:0)
> > > >
> > > > [S]: start of the event context
> > > > [W]: the wait blocked
> > > > [E]: the event not reachable
> > > > ---------------------------------------------------
> > > > context A's detail
> > > > ---------------------------------------------------
> > > > context A
> > > > [S] __raw_spin_lock_irqsave(&object->lock:0)
> > > > [W] _raw_spin_lock_nested(&object->lock:0)
> > > > [E] spin_unlock(&object->lock:0)
> > > >
> > > > [S] __raw_spin_lock_irqsave(&object->lock:0):
> > > > [<ffffffc00810302c>] scan_gray_list+0x84/0x13c
> > > > stacktrace:
> > > > dept_ecxt_enter+0x88/0xf4
> > > > _raw_spin_lock_irqsave+0xf0/0x1c4
> > > > scan_gray_list+0x84/0x13c
> > > > kmemleak_scan+0x2d8/0x54c
> > > > kmemleak_scan_thread+0xac/0xd4
> > > > kthread+0xd4/0xe4
> > > > ret_from_fork+0x10/0x20
> > > >
> > > > [E] spin_unlock(&object->lock:0):
> > > > [<ffffffc008102ee0>] scan_block+0x60/0x128
> > > > ---------------------------------------------------
> > > > information that might be helpful
> > > > ---------------------------------------------------
> > > > CPU: 1 PID: 38 Comm: kmemleak Tainted: G W 5.17.0-rc1+ #1
> > > > Hardware name: linux,dummy-virt (DT)
> > > > Call trace:
> > > > dump_backtrace.part.0+0x9c/0xc4
> > > > show_stack+0x14/0x28
> > > > dump_stack_lvl+0x9c/0xcc
> > > > dump_stack+0x14/0x2c
> > > > print_circle+0x2d4/0x438
> > > > cb_check_dl+0x44/0x70
> > > > bfs+0x60/0x168
> > > > add_dep+0x88/0x11c
> > > > add_wait+0x2d0/0x2dc
> > > > __dept_wait+0x8c/0xa4
> > > > dept_wait+0x6c/0x88
> > > > _raw_spin_lock_nested+0xa8/0x1b0
> > > > scan_block+0xb4/0x128
> > > > scan_gray_list+0xc4/0x13c
> > > > kmemleak_scan+0x2d8/0x54c
> > > > kmemleak_scan_thread+0xac/0xd4
> > > > kthread+0xd4/0xe4
> > > > ret_from_fork+0x10/0x20
> > > >
> > > [...]
> > >
> > > --
> > > Thank you, You are awesome!
> > > Hyeonggon :-)
>
> --
> Thank you, You are awesome!
> Hyeonggon :-)

Next message: Peter Geis: "Re: [PATCH] mmc: host: dw-mmc-rockchip: fix handling invalid clock rates"
Previous message: Stephen Rothwell: "linux-next: Signed-off-by missing for commit in the tip tree"
In reply to: Hyeonggon Yoo: "Re: [PATCH v3 00/21] DEPT(Dependency Tracker)"
Next in thread: Hyeonggon Yoo: "Re: [PATCH v3 00/21] DEPT(Dependency Tracker)"
Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]