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Date: Tue, 30 Nov 2021 09:00:36 +0100
From: Marek Szyprowski <m.szyprowski@...sung.com>
To: Minchan Kim <minchan@...nel.org>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Tejun Heo <tj@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kernfs: switch global kernfs_rwsem lock to per-fs lock
On 29.11.2021 17:58, Minchan Kim wrote:
> On Fri, Nov 26, 2021 at 12:54:45PM +0100, Marek Szyprowski wrote:
>> Hi,
>>
>> On 19.11.2021 00:00, Minchan Kim wrote:
>>> The kernfs implementation has big lock granularity(kernfs_rwsem) so
>>> every kernfs-based(e.g., sysfs, cgroup) fs are able to compete the
>>> lock. It makes trouble for some cases to wait the global lock
>>> for a long time even though they are totally independent contexts
>>> each other.
>>>
>>> A general example is process A goes under direct reclaim with holding
>>> the lock when it accessed the file in sysfs and process B is waiting
>>> the lock with exclusive mode and then process C is waiting the lock
>>> until process B could finish the job after it gets the lock from
>>> process A.
>>>
>>> This patch switches the global kernfs_rwsem to per-fs lock, which
>>> put the rwsem into kernfs_root.
>>>
>>> Suggested-by: Tejun Heo <tj@...nel.org>
>>> Signed-off-by: Minchan Kim <minchan@...nel.org>
>> This patch landed recently in linux-next (20211126) as commit
>> 393c3714081a ("kernfs: switch global kernfs_rwsem lock to per-fs lock").
>> In my tests I've found that it causes the following warning during the
>> system reboot:
>>
>> =========================
>> WARNING: held lock freed!
>> 5.16.0-rc2+ #10984 Not tainted
>> -------------------------
>> kworker/1:0/18 is freeing memory ffff00004034e200-ffff00004034e3ff,
>> with a lock still held there!
>> ffff00004034e348 (&root->kernfs_rwsem){++++}-{3:3}, at:
>> __kernfs_remove+0x310/0x37c
>> 3 locks held by kworker/1:0/18:
>> #0: ffff000040107938 ((wq_completion)cgroup_destroy){+.+.}-{0:0}, at:
>> process_one_work+0x1f0/0x6f0
>> #1: ffff80000b55bdc0
>> ((work_completion)(&(&css->destroy_rwork)->work)){+.+.}-{0:0}, at:
>> process_one_work+0x1f0/0x6f0
>> #2: ffff00004034e348 (&root->kernfs_rwsem){++++}-{3:3}, at:
>> __kernfs_remove+0x310/0x37c
>>
>> stack backtrace:
>> CPU: 1 PID: 18 Comm: kworker/1:0 Not tainted 5.16.0-rc2+ #10984
>> Hardware name: Raspberry Pi 4 Model B (DT)
>> Workqueue: cgroup_destroy css_free_rwork_fn
>> Call trace:
>> dump_backtrace+0x0/0x1ac
>> show_stack+0x18/0x24
>> dump_stack_lvl+0x8c/0xb8
>> dump_stack+0x18/0x34
>> debug_check_no_locks_freed+0x124/0x140
>> kfree+0xf0/0x3a4
>> kernfs_put+0x1f8/0x224
>> __kernfs_remove+0x1b8/0x37c
>> kernfs_destroy_root+0x38/0x50
>> css_free_rwork_fn+0x288/0x3d4
>> process_one_work+0x288/0x6f0
>> worker_thread+0x74/0x470
>> kthread+0x188/0x194
>> ret_from_fork+0x10/0x20
>>
>> Let me know if you need more information or help in reproducing this issue.
> Hi Marek,
>
> Thanks for the report. Could you try this one?
>
> >From 68741aa598ffd4a407d6d5f6b58bc7e7281e6f81 Mon Sep 17 00:00:00 2001
> From: Minchan Kim <minchan@...nel.org>
> Date: Mon, 29 Nov 2021 08:40:15 -0800
> Subject: [PATCH] kernfs: prevent early freeing of root node
>
> Marek reported the warning below.
>
> =========================
> WARNING: held lock freed!
> 5.16.0-rc2+ #10984 Not tainted
> -------------------------
> kworker/1:0/18 is freeing memory ffff00004034e200-ffff00004034e3ff,
> with a lock still held there!
> ffff00004034e348 (&root->kernfs_rwsem){++++}-{3:3}, at:
> __kernfs_remove+0x310/0x37c
> 3 locks held by kworker/1:0/18:
> #0: ffff000040107938 ((wq_completion)cgroup_destroy){+.+.}-{0:0}, at:
> process_one_work+0x1f0/0x6f0
> #1: ffff80000b55bdc0
> ((work_completion)(&(&css->destroy_rwork)->work)){+.+.}-{0:0}, at:
> process_one_work+0x1f0/0x6f0
> #2: ffff00004034e348 (&root->kernfs_rwsem){++++}-{3:3}, at:
> __kernfs_remove+0x310/0x37c
>
> stack backtrace:
> CPU: 1 PID: 18 Comm: kworker/1:0 Not tainted 5.16.0-rc2+ #10984
> Hardware name: Raspberry Pi 4 Model B (DT)
> Workqueue: cgroup_destroy css_free_rwork_fn
> Call trace:
> dump_backtrace+0x0/0x1ac
> show_stack+0x18/0x24
> dump_stack_lvl+0x8c/0xb8
> dump_stack+0x18/0x34
> debug_check_no_locks_freed+0x124/0x140
> kfree+0xf0/0x3a4
> kernfs_put+0x1f8/0x224
> __kernfs_remove+0x1b8/0x37c
> kernfs_destroy_root+0x38/0x50
> css_free_rwork_fn+0x288/0x3d4
> process_one_work+0x288/0x6f0
> worker_thread+0x74/0x470
> kthread+0x188/0x194
> ret_from_fork+0x10/0x20
>
> Since kernfs moves the kernfs_rwsem lock into root, it couldn't hold
> the lock when the root node is tearing down. Thus, get the refcount
> of root node.
>
> Reported-by: Marek Szyprowski <m.szyprowski@...sung.com>
> Signed-off-by: Minchan Kim <minchan@...nel.org>
Right, this fixes the issue.
Tested-by: Marek Szyprowski <m.szyprowski@...sung.com>
> ---
> fs/kernfs/dir.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
> index 13cae0ccce74..e6d9772ddb4c 100644
> --- a/fs/kernfs/dir.c
> +++ b/fs/kernfs/dir.c
> @@ -961,7 +961,13 @@ struct kernfs_root *kernfs_create_root(struct kernfs_syscall_ops *scops,
> */
> void kernfs_destroy_root(struct kernfs_root *root)
> {
> - kernfs_remove(root->kn); /* will also free @root */
> + /*
> + * kernfs_remove holds kernfs_rwsem from the root so the root
> + * shouldn't be freed during the operation.
> + */
> + kernfs_get(root->kn);
> + kernfs_remove(root->kn);
> + kernfs_put(root->kn); /* will also free @root */
> }
>
> /**
Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland
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