* Guenter Roeck <linux@xxxxxxxxxxxx> [240904 00:27]:
> On 9/3/24 20:36, Liam R. Howlett wrote:
> > * Guenter Roeck <linux@xxxxxxxxxxxx> [240903 22:38]:
> > > On 9/3/24 19:31, Liam R. Howlett wrote:
> > > > * SeongJae Park <sj@xxxxxxxxxx> [240903 21:18]:
> > > > > On Tue, 3 Sep 2024 17:58:15 -0700 SeongJae Park <sj@xxxxxxxxxx> wrote:
> > > > >
> > > > > > On Tue, 3 Sep 2024 20:48:53 -0400 "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx> wrote:
> > > > > >
> > > > > > > * SeongJae Park <sj@xxxxxxxxxx> [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;
