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SubjectRe: [PATCH 01/10] sysfs: don't use global workqueue in sysfs_schedule_callback()
On Fri, 17 Apr 2009 12:04:25 -0700
Greg Kroah-Hartman <gregkh@suse.de> wrote:

> From: Alex Chiang <achiang@hp.com>
>
> A sysfs attribute using sysfs_schedule_callback() to commit suicide
> may end up calling device_unregister(), which will eventually call
> a driver's ->remove function.
>
> Drivers may call flush_scheduled_work() in their shutdown routines,
> in which case lockdep will complain with something like the following:
>
> =============================================
> [ INFO: possible recursive locking detected ]
> 2.6.29-rc8-kk #1
> ---------------------------------------------
> events/4/56 is trying to acquire lock:
> (events){--..}, at: [<ffffffff80257fc0>] flush_workqueue+0x0/0xa0
>
> but task is already holding lock:
> (events){--..}, at: [<ffffffff80257648>] run_workqueue+0x108/0x230
>
> other info that might help us debug this:
> 3 locks held by events/4/56:
> #0: (events){--..}, at: [<ffffffff80257648>] run_workqueue+0x108/0x230
> #1: (&ss->work){--..}, at: [<ffffffff80257648>] run_workqueue+0x108/0x230
> #2: (pci_remove_rescan_mutex){--..}, at: [<ffffffff803c10d1>] remove_callback+0x21/0x40
>
> stack backtrace:
> Pid: 56, comm: events/4 Not tainted 2.6.29-rc8-kk #1
> Call Trace:
> [<ffffffff8026dfcd>] validate_chain+0xb7d/0x1260
> [<ffffffff8026eade>] __lock_acquire+0x42e/0xa40
> [<ffffffff8026f148>] lock_acquire+0x58/0x80
> [<ffffffff80257fc0>] ? flush_workqueue+0x0/0xa0
> [<ffffffff8025800d>] flush_workqueue+0x4d/0xa0
> [<ffffffff80257fc0>] ? flush_workqueue+0x0/0xa0
> [<ffffffff80258070>] flush_scheduled_work+0x10/0x20
> [<ffffffffa0144065>] e1000_remove+0x55/0xfe [e1000e]
> [<ffffffff8033ee30>] ? sysfs_schedule_callback_work+0x0/0x50
> [<ffffffff803bfeb2>] pci_device_remove+0x32/0x70
> [<ffffffff80441da9>] __device_release_driver+0x59/0x90
> [<ffffffff80441edb>] device_release_driver+0x2b/0x40
> [<ffffffff804419d6>] bus_remove_device+0xa6/0x120
> [<ffffffff8043e46b>] device_del+0x12b/0x190
> [<ffffffff8043e4f6>] device_unregister+0x26/0x70
> [<ffffffff803ba969>] pci_stop_dev+0x49/0x60
> [<ffffffff803baab0>] pci_remove_bus_device+0x40/0xc0
> [<ffffffff803c10d9>] remove_callback+0x29/0x40
> [<ffffffff8033ee4f>] sysfs_schedule_callback_work+0x1f/0x50
> [<ffffffff8025769a>] run_workqueue+0x15a/0x230
> [<ffffffff80257648>] ? run_workqueue+0x108/0x230
> [<ffffffff8025846f>] worker_thread+0x9f/0x100
> [<ffffffff8025bce0>] ? autoremove_wake_function+0x0/0x40
> [<ffffffff802583d0>] ? worker_thread+0x0/0x100
> [<ffffffff8025b89d>] kthread+0x4d/0x80
> [<ffffffff8020d4ba>] child_rip+0xa/0x20
> [<ffffffff8020cebc>] ? restore_args+0x0/0x30
> [<ffffffff8025b850>] ? kthread+0x0/0x80
> [<ffffffff8020d4b0>] ? child_rip+0x0/0x20
>
> Although we know that the device_unregister path will never acquire
> a lock that a driver might try to acquire in its ->remove, in general
> we should never attempt to flush a workqueue from within the same
> workqueue, and lockdep rightly complains.
>
> So as long as sysfs attributes cannot commit suicide directly and we
> are stuck with this callback mechanism, put the sysfs callbacks on
> their own workqueue instead of the global one.
>
> This has the side benefit that if a suicidal sysfs attribute kicks
> off a long chain of ->remove callbacks, we no longer induce a long
> delay on the global queue.

I still don't know why I merged

: commit 2355b70fd59cb5be7de2052a9edeee7afb7ff099
: Author: Lai Jiangshan <laijs@cn.fujitsu.com>
: Date: Thu Apr 2 16:58:24 2009 -0700
:
: workqueue: avoid recursion in run_workqueue()

there was nothing wrong with permitting limited recursion into
run_workqueue(). It never deadlocked and the three-deep-recursion
warning never triggered.

> + if (sysfs_workqueue == NULL) {
> + sysfs_workqueue = create_workqueue("sysfsd");
> + if (sysfs_workqueue == NULL) {
> + module_put(owner);
> + return -ENOMEM;
> + }
> + }

This will create a kernel thread per CPU. Surely
create_singlethread_workqueue() will suffice?



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