taprio: fix panic while hw offload sched list swap

taprio: fix panic while hw offload sched list swap

Don't swap oper and admin schedules too early, it's not correct and
causes crash.

Steps to reproduce:

1)
tc qdisc replace dev eth0 parent root handle 100 taprio \
    num_tc 3 \
    map 2 2 1 0 2 2 2 2 2 2 2 2 2 2 2 2 \
    queues 1@0 1@1 1@2 \
    base-time $SOME_BASE_TIME \
    sched-entry S 01 80000 \
    sched-entry S 02 15000 \
    sched-entry S 04 40000 \
    flags 2

2)
tc qdisc replace dev eth0 parent root handle 100 taprio \
    base-time $SOME_BASE_TIME \
    sched-entry S 01 90000 \
    sched-entry S 02 20000 \
    sched-entry S 04 40000 \
    flags 2

3)
tc qdisc replace dev eth0 parent root handle 100 taprio \
    base-time $SOME_BASE_TIME \
    sched-entry S 01 150000 \
    sched-entry S 02 200000 \
    sched-entry S 04 40000 \
    flags 2

Do 2 3 2 .. steps  more times if not happens and observe:

[  305.832319] Unable to handle kernel write to read-only memory at
virtual address ffff0000087ce7f0
[  305.910887] CPU: 0 PID: 0 Comm: swapper/0 Not tainted
[  305.919306] Hardware name: Texas Instruments AM654 Base Board (DT)

[...]

[  306.017119] x1 : ffff800848031d88 x0 : ffff800848031d80
[  306.022422] Call trace:
[  306.024866]  taprio_free_sched_cb+0x4c/0x98
[  306.029040]  rcu_process_callbacks+0x25c/0x410
[  306.033476]  __do_softirq+0x10c/0x208
[  306.037132]  irq_exit+0xb8/0xc8
[  306.040267]  __handle_domain_irq+0x64/0xb8
[  306.044352]  gic_handle_irq+0x7c/0x178
[  306.048092]  el1_irq+0xb0/0x128
[  306.051227]  arch_cpu_idle+0x10/0x18
[  306.054795]  do_idle+0x120/0x138
[  306.058015]  cpu_startup_entry+0x20/0x28
[  306.061931]  rest_init+0xcc/0xd8
[  306.065154]  start_kernel+0x3bc/0x3e4
[  306.068810] Code: f2fbd5b7 f2fbd5b6 d503201f f9400422 (f9000662)
[  306.074900] ---[ end trace 96c8e2284a9d9d6e ]---
[  306.079507] Kernel panic - not syncing: Fatal exception in interrupt
[  306.085847] SMP: stopping secondary CPUs
[  306.089765] Kernel Offset: disabled

Try to explain one of the possible crash cases:

The "real" admin list is assigned when admin_sched is set to
new_admin, it happens after "swap", that assigns to oper_sched NULL.
Thus if call qdisc show it can crash.

Farther, next second time, when sched list is updated, the admin_sched
is not NULL and becomes the oper_sched, previous oper_sched was NULL so
just skipped. But then admin_sched is assigned new_admin, but schedules
to free previous assigned admin_sched (that already became oper_sched).

Farther, next third time, when sched list is updated,
while one more swap, oper_sched is not null, but it was happy to be
freed already (while prev. admin update), so while try to free
oper_sched the kernel panic happens at taprio_free_sched_cb().

So, move the "swap emulation" where it should be according to function
comment from code.

Fixes: 9c66d15646760e ("taprio: Add support for hardware offloading")
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Acked-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Tested-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>