linux-kernel - Re: [PATCH] kernfs: switch global kernfs

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Date:   Mon, 29 Nov 2021 08:58:26 -0800
From:   Minchan Kim <minchan@...nel.org>
To:     Marek Szyprowski <m.szyprowski@...sung.com>
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 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>
---
 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 */
 }
 
 /**
-- 
2.34.0.rc2.393.gf8c9666880-goog