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kernfs: Relax constraint in draining guard

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[ Upstream commit 071d8e4c2a3b0999a9b822e2eb8854784a350f8a ]

The active reference lifecycle provides the break/unbreak mechanism but
the active reference is not truly active after unbreak -- callers don't
use it afterwards but it's important for proper pairing of kn->active
counting. Assuming this mechanism is in place, the WARN check in
kernfs_should_drain_open_files() is too sensitive -- it may transiently
catch those (rightful) callers between
kernfs_unbreak_active_protection() and kernfs_put_active() as found out by Chen
Ridong:

	kernfs_remove_by_name_ns	kernfs_get_active // active=1
	__kernfs_remove					  // active=0x80000002
	kernfs_drain			...
	wait_event
	//waiting (active == 0x80000001)
					kernfs_break_active_protection
					// active = 0x80000001
	// continue
					kernfs_unbreak_active_protection
					// active = 0x80000002
	...
	kernfs_should_drain_open_files
	// warning occurs
					kernfs_put_active

To avoid the false positives (mind panic_on_warn) remove the check altogether.
(This is meant as quick fix, I think active reference break/unbreak may be
simplified with larger rework.)

Fixes: bdb2fd7fc5 ("kernfs: Skip kernfs_drain_open_files() more aggressively")
Link: https://lore.kernel.org/r/kmmrseckjctb4gxcx2rdminrjnq2b4ipf7562nvfd432ld5v5m@2byj5eedkb2o/

Cc: Chen Ridong <chenridong@huawei.com>
Signed-off-by: Michal Koutný <mkoutny@suse.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20250505121201.879823-1-mkoutny@suse.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
This commit is contained in:
Michal Koutný 2025-05-05 14:12:00 +02:00 committed by Greg Kroah-Hartman
parent 713b9bfa93
commit 6c81f1c781
2 changed files with 5 additions and 3 deletions
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5
fs/kernfs/dir.c
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@ -1560,8 +1560,9 @@ void kernfs_break_active_protection(struct kernfs_node *kn)
* invoked before finishing the kernfs operation. Note that while this
* function restores the active reference, it doesn't and can't actually
* restore the active protection - @kn may already or be in the process of
* being removed. Once kernfs_break_active_protection() is invoked, that
* protection is irreversibly gone for the kernfs operation instance.
* being drained and removed. Once kernfs_break_active_protection() is
* invoked, that protection is irreversibly gone for the kernfs operation
* instance.
*
* While this function may be called at any point after
* kernfs_break_active_protection() is invoked, its most useful location

3
fs/kernfs/file.c
View File

@ -820,8 +820,9 @@ bool kernfs_should_drain_open_files(struct kernfs_node *kn)
/*
* @kn being deactivated guarantees that @kn->attr.open can't change
* beneath us making the lockless test below safe.
* Callers post kernfs_unbreak_active_protection may be counted in
* kn->active by now, do not WARN_ON because of them.
*/
WARN_ON_ONCE(atomic_read(&kn->active) != KN_DEACTIVATED_BIAS);
rcu_read_lock();
on = rcu_dereference(kn->attr.open);