Re: [PATCH] kernfs: Relax constraint in draining guard

[Tejun Heo <tj@kernel.org>]

On Mon, May 05, 2025 at 02:12:00PM +0200, Michal Koutný wrote:
> 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@2byj5eedkb2o/
> 
> Cc: Chen Ridong <chenridong@huawei.com>
> Signed-off-by: Michal Koutný <mkoutny@suse.com>

Acked-by: Tejun Heo <tj@kernel.org>

Thanks.

-- 
tejun