Messages in this thread | | | Date | Tue, 30 Nov 2021 09:00:36 +0100 | Subject | Re: [PATCH] kernfs: switch global kernfs_rwsem lock to per-fs lock | From | Marek Szyprowski <> |
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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@kernel.org> >>> Signed-off-by: Minchan Kim <minchan@kernel.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@kernel.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@samsung.com> > Signed-off-by: Minchan Kim <minchan@kernel.org>
Right, this fixes the issue.
Tested-by: Marek Szyprowski <m.szyprowski@samsung.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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