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Date:   Thu, 3 Mar 2022 18:48:24 +0900
From:   Byungchul Park <byungchul.park@lge.com>
To:     Hyeonggon Yoo <42.hyeyoo@gmail.com>
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Subject: Re: [PATCH v3 00/21] DEPT(Dependency Tracker)
Message-ID: <20220303094824.GA24977@X58A-UD3R>
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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 :-)