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    Subject[ 038/102] x86/microcode: Add local mutex to fix physical CPU hot-add deadlock
    Date
    3.9-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

    commit 074d72ff57f65de779e2f70d5906964c0ba1c123 upstream.

    This can easily be triggered if a new CPU is added (via
    ACPI hotplug mechanism) and from user-space you do:

    echo 1 > /sys/devices/system/cpu/cpu3/online

    (or wait for UDEV to do it) on a newly appeared physical CPU.

    The deadlock is that the "store_online" in drivers/base/cpu.c
    takes the cpu_hotplug_driver_lock() lock, then calls "cpu_up".
    "cpu_up" eventually ends up calling "save_mc_for_early"
    which also takes the cpu_hotplug_driver_lock() lock.

    And here is that lockdep thinks of it:

    smpboot: Stack at about ffff880075c39f44
    smpboot: CPU3: has booted.
    microcode: CPU3 sig=0x206a7, pf=0x2, revision=0x25

    =============================================
    [ INFO: possible recursive locking detected ]
    3.9.0upstream-10129-g167af0e #1 Not tainted
    ---------------------------------------------
    sh/2487 is trying to acquire lock:
    (x86_cpu_hotplug_driver_mutex){+.+.+.}, at: [<ffffffff81075512>] cpu_hotplug_driver_lock+0x12/0x20

    but task is already holding lock:
    (x86_cpu_hotplug_driver_mutex){+.+.+.}, at: [<ffffffff81075512>] cpu_hotplug_driver_lock+0x12/0x20

    other info that might help us debug this:
    Possible unsafe locking scenario:

    CPU0
    ----
    lock(x86_cpu_hotplug_driver_mutex);
    lock(x86_cpu_hotplug_driver_mutex);

    *** DEADLOCK ***

    May be due to missing lock nesting notation

    6 locks held by sh/2487:
    #0: (sb_writers#5){.+.+.+}, at: [<ffffffff811ca48d>] vfs_write+0x17d/0x190
    #1: (&buffer->mutex){+.+.+.}, at: [<ffffffff812464ef>] sysfs_write_file+0x3f/0x160
    #2: (s_active#20){.+.+.+}, at: [<ffffffff81246578>] sysfs_write_file+0xc8/0x160
    #3: (x86_cpu_hotplug_driver_mutex){+.+.+.}, at: [<ffffffff81075512>] cpu_hotplug_driver_lock+0x12/0x20
    #4: (cpu_add_remove_lock){+.+.+.}, at: [<ffffffff810961c2>] cpu_maps_update_begin+0x12/0x20
    #5: (cpu_hotplug.lock){+.+.+.}, at: [<ffffffff810962a7>] cpu_hotplug_begin+0x27/0x60

    Suggested-and-Acked-by: Borislav Petkov <bp@alien8.de>
    Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
    Cc: fenghua.yu@intel.com
    Cc: xen-devel@lists.xensource.com
    Link: http://lkml.kernel.org/r/1368029583-23337-1-git-send-email-konrad.wilk@oracle.com
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    arch/x86/kernel/microcode_intel_early.c | 5 +++--
    1 file changed, 3 insertions(+), 2 deletions(-)

    --- a/arch/x86/kernel/microcode_intel_early.c
    +++ b/arch/x86/kernel/microcode_intel_early.c
    @@ -487,6 +487,7 @@ static inline void show_saved_mc(void)
    #endif

    #if defined(CONFIG_MICROCODE_INTEL_EARLY) && defined(CONFIG_HOTPLUG_CPU)
    +static DEFINE_MUTEX(x86_cpu_microcode_mutex);
    /*
    * Save this mc into mc_saved_data. So it will be loaded early when a CPU is
    * hot added or resumes.
    @@ -507,7 +508,7 @@ int save_mc_for_early(u8 *mc)
    * Hold hotplug lock so mc_saved_data is not accessed by a CPU in
    * hotplug.
    */
    - cpu_hotplug_driver_lock();
    + mutex_lock(&x86_cpu_microcode_mutex);

    mc_saved_count_init = mc_saved_data.mc_saved_count;
    mc_saved_count = mc_saved_data.mc_saved_count;
    @@ -544,7 +545,7 @@ int save_mc_for_early(u8 *mc)
    }

    out:
    - cpu_hotplug_driver_unlock();
    + mutex_unlock(&x86_cpu_microcode_mutex);

    return ret;
    }



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