Re: [PATCH 3/4] sparc64: convert spinlock_t to raw_spinlock_t in mmu_context_t

[David Miller <davem@davemloft.net>]

From: Kirill Tkhai <tkhai@yandex.ru>
Date: Wed, 12 Feb 2014 12:33:58 +0400

> 12.02.2014, 11:48, "Allen Pais" <allen.pais@oracle.com>:
> 
>>  On Wednesday 12 February 2014 02:43 AM, Kirill Tkhai wrote:
>>>   06.01.2014, 07:56, "Allen Pais" <allen.pais@oracle.com>:
>>>>   In the attempt of get PREEMPT_RT working on sparc64 using
>>>>   linux-stable-rt version 3.10.22-rt19+, the kernel crash
>>>>   with the following trace:
>>>>
>>>>   [ 1487.027884] I7: <rt_mutex_setprio+0x3c/0x2c0>
>>>>   [ 1487.027885] Call Trace:
>>>>   [ 1487.027887]  [00000000004967dc] rt_mutex_setprio+0x3c/0x2c0
>>>>   [ 1487.027892]  [00000000004afe20] task_blocks_on_rt_mutex+0x180/0x200
>>>>   [ 1487.027895]  [0000000000819114] rt_spin_lock_slowlock+0x94/0x300
>>>>   [ 1487.027897]  [0000000000817ebc] __schedule+0x39c/0x53c
>>>>   [ 1487.027899]  [00000000008185fc] schedule+0x1c/0xc0
>>>>   [ 1487.027908]  [000000000048fff4] smpboot_thread_fn+0x154/0x2e0
>>>>   [ 1487.027913]  [000000000048753c] kthread+0x7c/0xa0
>>>>   [ 1487.027920]  [00000000004060c4] ret_from_syscall+0x1c/0x2c
>>>>   [ 1487.027922]  [0000000000000000]           (null)
>>  Now, consistently I've been getting sun4v_data_access_exception.
>>  Here's the trace:
>>  [ 4673.360121] sun4v_data_access_exception: ADDR[0000080000000000] CTX[0000] TYPE[0004], going.
> 
> I've never dived at sparc's tlb before, but it seems now I'm understanding.
> 
> arch_enter_lazy_mmu_mode() makes possible delayed tlb flushing. In !RT kernel
> you collect flush requests before you really flush all of them.
> 
> In RT you collect them too, but you are able to be preempted in any moment.
> So, you may switch to other process with unflushed tlb, which is very bad.
> 
> Try to not to set tb->active = 1; in arch_enter_lazy_mmu_mode(). Set it to zero.
> We will look if this robust fix helps.

Sorry for coming into this discussion so late.

Indeed, the pending flushes are per-cpu and we must flush them out in the
event of a preemption.

PowerPC does the same exact thing with arch_enter_lazy_mmu_mode(), in
fact that's where I copied the logic from.  Does PowerPC not work with
-rt? :-)
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