* Guenter Roeck linux@roeck-us.net [240904 00:27]:
On 9/3/24 20:36, Liam R. Howlett wrote:
- Guenter Roeck linux@roeck-us.net [240903 22:38]:
On 9/3/24 19:31, Liam R. Howlett wrote:
- SeongJae Park sj@kernel.org [240903 21:18]:
On Tue, 3 Sep 2024 17:58:15 -0700 SeongJae Park sj@kernel.org wrote:
On Tue, 3 Sep 2024 20:48:53 -0400 "Liam R. Howlett" Liam.Howlett@oracle.com wrote:
> * SeongJae Park sj@kernel.org [240903 20:45]: > > damon_test_three_regions_in_vmas() initializes a maple tree with > > MM_MT_FLAGS. The flags contains MT_FLAGS_LOCK_EXTERN, which means > > mt_lock of the maple tree will not be used. And therefore the maple > > tree initialization code skips initialization of the mt_lock. However, > > __link_vmas(), which adds vmas for test to the maple tree, uses the > > mt_lock. In other words, the uninitialized spinlock is used. The > > problem becomes celar when spinlock debugging is turned on, since it > > reports spinlock bad magic bug. Fix the issue by not using the mt_lock > > as promised. > > You can't do this, lockdep will tell you this is wrong.
Hmm, but lockdep was silence on my setup?
> We need a lock and to use the lock for writes.
This code is executed by a single-thread test code. Do we still need the lock?
> > I'd suggest using different flags so the spinlock is used.
The reporter mentioned simply dropping MT_FLAGS_LOCK_EXTERN from the flags causes suspicious RCU usage message. May I ask if you have a suggestion of better flags?
That would be the lockdep complaining, so that's good.
I was actually thinking replacing the mt_init_flags() with mt_init(), which same to mt_init_flags() with zero flag, like below.
Yes. This will use the spinlock which should fix your issue, but it will use a different style of maple tree.
Perhaps use MT_FLAGS_ALLOC_RANGE to use the same type of maple tree, if you ever add threading you will want the rcu flag as well (MT_FLAGS_USE_RCU).
I would recommend those two and just use the spinlock.
I tried that (MT_FLAGS_ALLOC_RANGE | MT_FLAGS_USE_RCU). it also triggers the suspicious RCU usage message.
I am running ./tools/testing/kunit/kunit.py run '*damon*' --arch x86_64 --raw with: CONFIG_LOCKDEP=y CONFIG_DEBUG_SPINLOCK=y
and I don't have any issue with locking in the existing code. How do I recreate this issue?
I tested again, and I still see
[ 6.233483] ok 4 damon [ 6.234190] KTAP version 1 [ 6.234263] # Subtest: damon-operations [ 6.234335] # module: vaddr [ 6.234384] 1..6 [ 6.235726] [ 6.235931] ============================= [ 6.236018] WARNING: suspicious RCU usage [ 6.236280] 6.11.0-rc6-00029-gda66250b210f-dirty #1 Tainted: G N [ 6.236398] ----------------------------- [ 6.236474] lib/maple_tree.c:832 suspicious rcu_dereference_check() usage! [ 6.236579] [ 6.236579] other info that might help us debug this: [ 6.236579] [ 6.236738] [ 6.236738] rcu_scheduler_active = 2, debug_locks = 1 [ 6.237039] no locks held by kunit_try_catch/208. [ 6.237166] [ 6.237166] stack backtrace: [ 6.237385] CPU: 0 UID: 0 PID: 208 Comm: kunit_try_catch Tainted: G N 6.11.0-rc6-00029-gda66250b210f-dirty #1 [ 6.237629] Tainted: [N]=TEST [ 6.237714] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014 [ 6.238065] Call Trace: [ 6.238233] <TASK> [ 6.238547] dump_stack_lvl+0x9e/0xe0 [ 6.239473] lockdep_rcu_suspicious+0x145/0x1b0 [ 6.239621] mas_walk+0x19f/0x1d0 [ 6.239765] mas_find+0xb5/0x150 [ 6.239873] __damon_va_three_regions+0x7e/0x130
This function isn't taking the rcu read lock while iterating the tree.
Try this:
diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c index b0e8b361891d..08cfd22b5249 100644 --- a/mm/damon/vaddr.c +++ b/mm/damon/vaddr.c @@ -126,6 +126,7 @@ static int __damon_va_three_regions(struct mm_struct *mm, * If this is too slow, it can be optimised to examine the maple * tree gaps. */ + rcu_read_lock(); for_each_vma(vmi, vma) { unsigned long gap;
@@ -146,6 +147,7 @@ static int __damon_va_three_regions(struct mm_struct *mm, next: prev = vma; } + rcu_read_unlock();
if (!sz_range(&second_gap) || !sz_range(&first_gap)) return -EINVAL;