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    Subject[patch 04/20] Fix timer race in dst GC code
    -stable review patch.  If anyone has any objections, please let us know.

    ------------------
    From: Dmitry Mishin <dim@openvz.org>

    [NET]: add_timer -> mod_timer() in dst_run_gc()

    Patch from Dmitry Mishin <dim@openvz.org>:

    Replace add_timer() by mod_timer() in dst_run_gc
    in order to avoid BUG message.

    CPU1 CPU2
    dst_run_gc() entered dst_run_gc() entered
    spin_lock(&dst_lock) .....
    del_timer(&dst_gc_timer) fail to get lock
    .... mod_timer() <--- puts
    timer back
    to the list
    add_timer(&dst_gc_timer) <--- BUG because timer is in list already.

    Found during OpenVZ internal testing.

    At first we thought that it is OpenVZ specific as we
    added dst_run_gc(0) call in dst_dev_event(),
    but as Alexey pointed to me it is possible to trigger
    this condition in mainstream kernel.

    F.e. timer has fired on CPU2, but the handler was preeempted
    by an irq before dst_lock is tried.
    Meanwhile, someone on CPU1 adds an entry to gc list and
    starts the timer.
    If CPU2 was preempted long enough, this timer can expire
    simultaneously with resuming timer handler on CPU1, arriving
    exactly to the situation described.

    Signed-off-by: Dmitry Mishin <dim@openvz.org>
    Signed-off-by: Kirill Korotaev <dev@openvz.org>
    Signed-off-by: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

    ---
    net/core/dst.c | 3 +--
    1 file changed, 1 insertion(+), 2 deletions(-)

    --- linux-2.6.17.8.orig/net/core/dst.c
    +++ linux-2.6.17.8/net/core/dst.c
    @@ -95,12 +95,11 @@ static void dst_run_gc(unsigned long dum
    dst_gc_timer_inc = DST_GC_INC;
    dst_gc_timer_expires = DST_GC_MIN;
    }
    - dst_gc_timer.expires = jiffies + dst_gc_timer_expires;
    #if RT_CACHE_DEBUG >= 2
    printk("dst_total: %d/%d %ld\n",
    atomic_read(&dst_total), delayed, dst_gc_timer_expires);
    #endif
    - add_timer(&dst_gc_timer);
    + mod_timer(&dst_gc_timer, jiffies + dst_gc_timer_expires);

    out:
    spin_unlock(&dst_lock);
    --
    -
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