The following commit has been merged into the locking/urgent branch of tip:

Commit-ID:     e703b7e247503b8bf87b62c02a4392749b09eca8
Gitweb:        https://git.kernel.org/tip/e703b7e247503b8bf87b62c02a4392749b09eca8
Author:        Thomas Gleixner <tglx@linutronix.de>
AuthorDate:    Wed, 30 Jul 2025 21:44:55 +02:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Sat, 02 Aug 2025 15:11:52 +02:00

futex: Move futex cleanup to __mmdrop()

Futex hash allocations are done in mm_init() and the cleanup happens in
__mmput(). That works most of the time, but there are mm instances which
are instantiated via mm_alloc() and freed via mmdrop(), which causes the
futex hash to be leaked.

Move the cleanup to __mmdrop().

Fixes: 56180dd20c19 ("futex: Use RCU-based per-CPU reference counting instead of rcuref_t")
Reported-by: André Draszik <andre.draszik@linaro.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: André Draszik <andre.draszik@linaro.org>
Link: https://lore.kernel.org/all/87ldo5ihu0.ffs@tglx
Closes: https://lore.kernel.org/all/0c8cc83bb73abf080faf584f319008b67d0931db.camel@linaro.org

---
 kernel/fork.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index f82b77e..1b0535e 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -686,6 +686,7 @@ void __mmdrop(struct mm_struct *mm)
 	mm_pasid_drop(mm);
 	mm_destroy_cid(mm);
 	percpu_counter_destroy_many(mm->rss_stat, NR_MM_COUNTERS);
+	futex_hash_free(mm);
 
 	free_mm(mm);
 }
@@ -1133,7 +1134,6 @@ static inline void __mmput(struct mm_struct *mm)
 	if (mm->binfmt)
 		module_put(mm->binfmt->module);
 	lru_gen_del_mm(mm);
-	futex_hash_free(mm);
 	mmdrop(mm);
 }
 

On Sat, Aug 02, 2025 at 01:22:10PM +0000, tip-bot2 for Thomas Gleixner wrote:
> The following commit has been merged into the locking/urgent branch of tip:
> 
> Commit-ID:     e703b7e247503b8bf87b62c02a4392749b09eca8
> Gitweb:        https://git.kernel.org/tip/e703b7e247503b8bf87b62c02a4392749b09eca8
> Author:        Thomas Gleixner <tglx@linutronix.de>
> AuthorDate:    Wed, 30 Jul 2025 21:44:55 +02:00
> Committer:     Thomas Gleixner <tglx@linutronix.de>
> CommitterDate: Sat, 02 Aug 2025 15:11:52 +02:00
> 
> futex: Move futex cleanup to __mmdrop()
> 
> Futex hash allocations are done in mm_init() and the cleanup happens in
> __mmput(). That works most of the time, but there are mm instances which
> are instantiated via mm_alloc() and freed via mmdrop(), which causes the
> futex hash to be leaked.
> 
> Move the cleanup to __mmdrop().
> 
> Fixes: 56180dd20c19 ("futex: Use RCU-based per-CPU reference counting instead of rcuref_t")
> Reported-by: André Draszik <andre.draszik@linaro.org>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Tested-by: André Draszik <andre.draszik@linaro.org>
> Link: https://lore.kernel.org/all/87ldo5ihu0.ffs@tglx
> Closes: https://lore.kernel.org/all/0c8cc83bb73abf080faf584f319008b67d0931db.camel@linaro.org

Thomas,

it seems this change caused the bug being reported here:

https://lore.kernel.org/all/20250818131902.5039-1-hdanton@sina.com/