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 :-)
