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Message-ID: <aBlJlqfAlayxfMc7@slm.duckdns.org>
Date: Mon, 5 May 2025 13:28:22 -1000
From: Tejun Heo <tj@...nel.org>
To: Michal Koutný <mkoutny@...e.com>
Cc: Chengming Zhou <zhouchengming@...edance.com>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
linux-kernel@...r.kernel.org, Chen Ridong <chenridong@...wei.com>
Subject: Re: [PATCH] kernfs: Relax constraint in draining guard
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@...wei.com>
> Signed-off-by: Michal Koutný <mkoutny@...e.com>
Acked-by: Tejun Heo <tj@...nel.org>
Thanks.
--
tejun
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