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    SubjectRe: [PATCH] sysfs: handle duplicate removal attempts in sysfs_remove_group()
    [+cc Rafael, James]

    On Tue, Nov 19, 2013 at 11:18 PM, Tejun Heo <tj@kernel.org> wrote:
    > (cc'ing Bjorn)
    >
    > Hello,
    >
    > On Tue, Nov 19, 2013 at 03:09:58PM +0200, Mika Westerberg wrote:
    >> Commit bcdde7e221a8 (sysfs: make __sysfs_remove_dir() recursive) changed
    >> the behavior so that directory removals will be done recursively. This
    >> means that the sysfs group might already be removed if its parent directory
    >> has been removed.
    >>
    >> The current code outputs warnings similar to following log snippet when it
    >> detects that there is no group for the given kobject:
    >>
    >> WARNING: CPU: 0 PID: 4 at fs/sysfs/group.c:214 sysfs_remove_group+0xc6/0xd0()
    >> sysfs group ffffffff81c6f1e0 not found for kobject 'host7'
    >> Modules linked in:
    >> CPU: 0 PID: 4 Comm: kworker/0:0 Not tainted 3.12.0+ #13
    >> Hardware name: /D33217CK, BIOS GKPPT10H.86A.0042.2013.0422.1439 04/22/2013
    >> Workqueue: kacpi_hotplug acpi_hotplug_work_fn
    >> 0000000000000009 ffff8801002459b0 ffffffff817daab1 ffff8801002459f8
    >> ffff8801002459e8 ffffffff810436b8 0000000000000000 ffffffff81c6f1e0
    >> ffff88006d440358 ffff88006d440188 ffff88006e8b4c28 ffff880100245a48
    >> Call Trace:
    >> [<ffffffff817daab1>] dump_stack+0x45/0x56
    >> [<ffffffff810436b8>] warn_slowpath_common+0x78/0xa0
    >> [<ffffffff81043727>] warn_slowpath_fmt+0x47/0x50
    >> [<ffffffff811ae526>] sysfs_remove_group+0xc6/0xd0
    >> [<ffffffff81432f7e>] dpm_sysfs_remove+0x3e/0x50
    >> [<ffffffff8142a0d0>] device_del+0x40/0x1b0
    >> [<ffffffff8142a24d>] device_unregister+0xd/0x20
    >> [<ffffffff8144131a>] scsi_remove_host+0xba/0x110
    >> [<ffffffff8145f526>] ata_host_detach+0xc6/0x100
    >> [<ffffffff8145f578>] ata_pci_remove_one+0x18/0x20
    >> [<ffffffff812e8f48>] pci_device_remove+0x28/0x60
    >> [<ffffffff8142d854>] __device_release_driver+0x64/0xd0
    >> [<ffffffff8142d8de>] device_release_driver+0x1e/0x30
    >> [<ffffffff8142d257>] bus_remove_device+0xf7/0x140
    >> [<ffffffff8142a1b1>] device_del+0x121/0x1b0
    >> [<ffffffff812e43d4>] pci_stop_bus_device+0x94/0xa0
    >> [<ffffffff812e437b>] pci_stop_bus_device+0x3b/0xa0
    >> [<ffffffff812e437b>] pci_stop_bus_device+0x3b/0xa0
    >> [<ffffffff812e44dd>] pci_stop_and_remove_bus_device+0xd/0x20
    >> [<ffffffff812fc743>] trim_stale_devices+0x73/0xe0
    >> [<ffffffff812fc78b>] trim_stale_devices+0xbb/0xe0
    >> [<ffffffff812fc78b>] trim_stale_devices+0xbb/0xe0
    >> [<ffffffff812fcb6e>] acpiphp_check_bridge+0x7e/0xd0
    >> [<ffffffff812fd90d>] hotplug_event+0xcd/0x160
    >> [<ffffffff812fd9c5>] hotplug_event_work+0x25/0x60
    >> [<ffffffff81316749>] acpi_hotplug_work_fn+0x17/0x22
    >> [<ffffffff8105cf3a>] process_one_work+0x17a/0x430
    >> [<ffffffff8105db29>] worker_thread+0x119/0x390
    >> [<ffffffff81063a5d>] kthread+0xcd/0xf0
    >> [<ffffffff817eb33c>] ret_from_fork+0x7c/0xb0
    >
    > So, we do have cases where the parent is removed before the child. I
    > suppose the parent pci bridge is removed already? AFAICS this
    > shouldn't break anything but people did seem to expect the removals to
    > be ordered from child to parent. Bjorn, is this something you expect
    > to happened?

    I do not expect a PCI bridge to be removed before the devices below
    it. We should be removing all the children before removing the parent
    bridge.

    But is this related to PCI? I don't see the connection yet. I tried
    to look into this a bit (my notes are at
    https://bugzilla.kernel.org/show_bug.cgi?id=65281), but I haven't
    figured out the big-picture problem yet.

    I don't have warm fuzzies that adding a "have we already removed this"
    check is the best resolution, but maybe that's just because I don't
    understand the problem.

    Bjorn

    >> I'm not 100% sure that this is the correct solution. It seem to fix my case
    >> but I might be missing something as I'm not that familiar with sysfs.
    >
    > Yeah, looks okay to me for now. One nit at the end tho.
    >
    > I find requiring removal of each sysfs attribute when the whole node
    > is going away rather weird. It forced us to have extra code which
    > does whole bunch of hash table lookups and deletion operations and the
    > only thing that achieved was either triggering warning if somebody did
    > it in the wrong order or spuriously, or leaking memory if somebody
    > forgot some without any way to find out about them.
    >
    > Now, all those are harmlessly unnecessary and we're adding more logic
    > to suppress warnings on specific cases. In the longer term, we
    > probably just wanna drop all the unnecessary removal logics and
    > warnings.
    >
    >> + /*
    >> + * Sysfs directories are now removed recursively by
    >> + * sysfs_remove_dir(). This means that this function can be called
    >> + * multiple times on the same group. If the parent directory is
    >> + * already removed we don't do anything here.
    >> + */
    >
    > The function won't be called multiple times but may be called on a
    > group whose kobj whose sysfs entry is already removed in which case
    > all its groups are guaranteed to be already removed.
    >
    > Can you please update the comment to reflect the above?
    >
    > With that,
    >
    > Acked-by: Tejun Heo <tj@kernel.org>
    >
    > Thanks.
    >
    > --
    > tejun


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