lkml.org 
[lkml]   [2014]   [Apr]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    SubjectRe: [RFC PATCH v3] Use kernfs_break_active_protection() for device online store callbacks
    From
    Date
    On Tue, 2014-04-15 at 10:50 -0400, Tejun Heo wrote:
    > Hello,
    >
    > On Tue, Apr 15, 2014 at 10:44:37AM +0800, Li Zhong wrote:
    > > / *
    > > * This process might deadlock with another process trying to
    > > * remove this device:
    > > * This process holding the s_active of "online" attribute, and tries
    > > * to online/offline the device with some locks protecting hotplug.
    > > * Device removing process holding some locks protecting hotplug, and
    > > * tries to remove the "online" attribute, waiting for the s_active to
    > > * be released.
    > > *
    > > * The deadlock described above should be solved with
    > > * lock_device_hotplug_sysfs(). We temporarily drop the active
    > > * protection here to avoid some lockdep warnings.
    > > *
    > > * If device_hotplug_lock is forgotten to be used when removing
    > > * device(possibly some very simple device even don't need this lock?),
    > > * @dev could go away any time after dropping the active protection.
    > > * So increase its ref count before dropping active protection.
    > > * Though invoking device_{on|off}line() on a removed device seems
    > > * unreasonable, it should be less disastrous than playing with freed
    > > * @dev. Also, we might be able to have some mechanism abort
    > > * device_{on|off}line() if @dev already removed.
    > > */
    >
    > Hmmm... I'm not sure I fully understand the problem. Does the code
    > ever try to remove "online" while holding cpu_add_remove_lock and,
    > when written 0, online knob grabs cpu_add_remove_lock?

    Yes.

    In acpi_processor_remove(), cpu_maps_update_begin() is called to hold
    cpu_add_remove_lock, and then arch_unregister_cpu calls
    unregister_cpu(), which will try to remove dir cpu1 including "online".

    while written 0 to online, cpu_down() will also try to grab
    cpu_add_remove_lock with cpu_maps_update_begin().

    > If so, that is
    > an actually possible deadlock, no?

    Yes, but it seems to me that it is solved in commit 5e33bc41, which uses
    lock_device_hotplug_sysfs() to return a restart syscall error if not
    able to try lock the device_hotplug_lock. That also requires the device
    removing code path to take the device_hotplug_lock.

    Thanks, Zhong

    >
    > Thanks.
    >




    \
     
     \ /
      Last update: 2014-04-16 04:01    [W:4.135 / U:0.180 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site