6.6-stable review patch. If anyone has any objections, please let me know.
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From: Michal Koutný mkoutny@suse.com
[ 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: bdb2fd7fc56e1 ("kernfs: Skip kernfs_drain_open_files() more aggressively") Link: https://lore.kernel.org/r/kmmrseckjctb4gxcx2rdminrjnq2b4ipf7562nvfd432ld5v5m...
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 --- fs/kernfs/dir.c | 5 +++-- fs/kernfs/file.c | 3 ++- 2 files changed, 5 insertions(+), 3 deletions(-)
diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c index b068ed32d7b32..f6e2a4523f7e6 100644 --- a/fs/kernfs/dir.c +++ b/fs/kernfs/dir.c @@ -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 diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c index 332d08d2fe0d5..6b90fea6cca20 100644 --- a/fs/kernfs/file.c +++ b/fs/kernfs/file.c @@ -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);