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    Subject[PATCH] net: sched: dev_deactivate_many(): use msleep(1) instead of yield() to wait for outstanding qdisc_run calls
    Date
    On PREEMPT_RT enabled systems the interrupt handler run as threads at prio 50
    (by default). If a high priority userspace process tries to shut down a busy
    network interface it might spin in a yield loop waiting for the device to
    become idle. With the interrupt thread having a lower priority than the
    looping process it might never be scheduled and so result in a deadlock on UP
    systems.

    With Magic SysRq the following backtrace can be produced:

    > test_app R running 0 174 168 0x00000000
    > [<c02c7070>] (__schedule+0x220/0x3fc) from [<c02c7870>] (preempt_schedule_irq+0x48/0x80)
    > [<c02c7870>] (preempt_schedule_irq+0x48/0x80) from [<c0008fa8>] (svc_preempt+0x8/0x20)
    > [<c0008fa8>] (svc_preempt+0x8/0x20) from [<c001a984>] (local_bh_enable+0x18/0x88)
    > [<c001a984>] (local_bh_enable+0x18/0x88) from [<c025316c>] (dev_deactivate_many+0x220/0x264)
    > [<c025316c>] (dev_deactivate_many+0x220/0x264) from [<c023be04>] (__dev_close_many+0x64/0xd4)
    > [<c023be04>] (__dev_close_many+0x64/0xd4) from [<c023be9c>] (__dev_close+0x28/0x3c)
    > [<c023be9c>] (__dev_close+0x28/0x3c) from [<c023f7f0>] (__dev_change_flags+0x88/0x130)
    > [<c023f7f0>] (__dev_change_flags+0x88/0x130) from [<c023f904>] (dev_change_flags+0x10/0x48)
    > [<c023f904>] (dev_change_flags+0x10/0x48) from [<c024c140>] (do_setlink+0x370/0x7ec)
    > [<c024c140>] (do_setlink+0x370/0x7ec) from [<c024d2f0>] (rtnl_newlink+0x2b4/0x450)
    > [<c024d2f0>] (rtnl_newlink+0x2b4/0x450) from [<c024cfa0>] (rtnetlink_rcv_msg+0x158/0x1f4)
    > [<c024cfa0>] (rtnetlink_rcv_msg+0x158/0x1f4) from [<c0256740>] (netlink_rcv_skb+0xac/0xc0)
    > [<c0256740>] (netlink_rcv_skb+0xac/0xc0) from [<c024bbd8>] (rtnetlink_rcv+0x18/0x24)
    > [<c024bbd8>] (rtnetlink_rcv+0x18/0x24) from [<c02561b8>] (netlink_unicast+0x13c/0x198)
    > [<c02561b8>] (netlink_unicast+0x13c/0x198) from [<c025651c>] (netlink_sendmsg+0x264/0x2e0)
    > [<c025651c>] (netlink_sendmsg+0x264/0x2e0) from [<c022af98>] (sock_sendmsg+0x78/0x98)
    > [<c022af98>] (sock_sendmsg+0x78/0x98) from [<c022bb50>] (___sys_sendmsg.part.25+0x268/0x278)
    > [<c022bb50>] (___sys_sendmsg.part.25+0x268/0x278) from [<c022cf08>] (__sys_sendmsg+0x48/0x78)
    > [<c022cf08>] (__sys_sendmsg+0x48/0x78) from [<c0009320>] (ret_fast_syscall+0x0/0x2c)

    This patch works around the problem by replacing yield() by msleep(1), giving
    the interrupt thread time to finish, similar to other changes contained in the
    rt patch set. Using wait_for_completion() instead would probably be a better
    solution.

    Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
    ---
    Hello,

    this patch is intended for -rt, as the problem probably doesn't occur on non rt
    systems. However, I put netdev on Cc, as they probably can come up with a
    proper solution.

    regards,
    Marc

    net/sched/sch_generic.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
    index a7f838b..d5b00ad 100644
    --- a/net/sched/sch_generic.c
    +++ b/net/sched/sch_generic.c
    @@ -839,7 +839,7 @@ void dev_deactivate_many(struct list_head *head)
    /* Wait for outstanding qdisc_run calls. */
    list_for_each_entry(dev, head, unreg_list)
    while (some_qdisc_is_busy(dev))
    - yield();
    + msleep(1)
    }

    void dev_deactivate(struct net_device *dev)
    --
    1.9.0


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