Re: [Qemu-devel] Status of my hacks on the MTTCG WIP branch

["Alex Bennée" <alex.bennee@linaro.org>]

Alex Bennée <alex.bennee@linaro.org> writes:

> alvise rigo <a.rigo@virtualopensystems.com> writes:
>
>> On Fri, Jan 15, 2016 at 4:25 PM, Alex Bennée <alex.bennee@linaro.org> wrote:
>>>
>>> alvise rigo <a.rigo@virtualopensystems.com> writes:
>>>
>>>> On Fri, Jan 15, 2016 at 3:51 PM, Alex Bennée <alex.bennee@linaro.org> wrote:
>>>>>
>>>>> alvise rigo <a.rigo@virtualopensystems.com> writes:
>>>>>
<snip>
>>>> Keep in mind that Linux on arm64 uses the LDXP/STXP instructions that
>>>> exist solely in aarch64.
>>>> These instructions are purely emulated now and can potentially write
>>>> 128 bits of data in a non-atomic fashion.
>>>
>>> Sure, but I doubt they are the reason for this hang as the kernel
>>> doesn't use them.
>>
>> The kernel does use them for __cmpxchg_double in
>> arch/arm64/include/asm/atomic_ll_sc.h.
>
> I take it back, if I'd have grepped for "ldxp" instead of "stxp" I would
> have seen it, sorry about that ;-)
>
>> In any case, the normal exclusive instructions are also emulated in
>> target-arm/translate-a64.c.
>
> I'll check on them on Monday. I'd assumed all the stuff was in the
> helpers as I scanned through and missed the translate.c changes Fred
> made. Hopefully that will be the last hurdle.

I'm pleased to confirm you were right. I hacked up Fred's helper based
solution for aarch64 including the ldxp/stxp stuff. It's not
semantically correct because:

  result = atomic_bool_cmpxchg(p, oldval, (uint8_t)newval) &&
           atomic_bool_cmpxchg(&p[1], oldval2, (uint8_t)newval2);

won't leave the system as it was before if the race causes the second
cmpxchg to fail. I assume this won't be a problem in the LL/SC world as
we'll be able to serialise all accesses to the exclusive page properly?

See:

https://github.com/stsquad/qemu/tree/mttcg/multi_tcg_v8_wip_ajb_fix_locks-r2

>
> In the meantime if I'm not booting Jessie I can get MTTCG aarch64
> working with a initrd based rootfs. Once I've gone through those I'm
> planning on giving it a good stress test with -fsantize=threads.

My first pass with this threw up a bunch of errors with the RCU code
like this:

WARNING: ThreadSanitizer: data race (pid=15387)
  Atomic write of size 4 at 0x7f59efa51d48 by main thread (mutexes: write M172):
    #0 __tsan_atomic32_fetch_add <null> (libtsan.so.0+0x000000058e8f)
    #1 call_rcu1 util/rcu.c:288 (qemu-system-aarch64+0x0000006c3bd0)
    #2 address_space_update_topology /home/alex/lsrc/qemu/qemu.git/memory.c:806 (qemu-system-aarch64+0x0000001ed9ca)
    #3 memory_region_transaction_commit /home/alex/lsrc/qemu/qemu.git/memory.c:842 (qemu-system-aarch64+0x0000001ed9ca)
    #4 address_space_init /home/alex/lsrc/qemu/qemu.git/memory.c:2136 (qemu-system-aarch64+0x0000001f1fa6)
    #5 memory_map_init /home/alex/lsrc/qemu/qemu.git/exec.c:2344 (qemu-system-aarch64+0x000000196607)
    #6 cpu_exec_init_all /home/alex/lsrc/qemu/qemu.git/exec.c:2795 (qemu-system-aarch64+0x000000196607)
    #7 main /home/alex/lsrc/qemu/qemu.git/vl.c:4083 (qemu-system-aarch64+0x0000001829aa)

  Previous read of size 4 at 0x7f59efa51d48 by thread T1:
    #0 call_rcu_thread util/rcu.c:242 (qemu-system-aarch64+0x0000006c3d92)
    #1 <null> <null> (libtsan.so.0+0x0000000235f9)

  Location is global 'rcu_call_count' of size 4 at 0x7f59efa51d48 (qemu-system-aarch64+0x0000010f1d48)

  Mutex M172 (0x7f59ef6254e0) created at:
    #0 pthread_mutex_init <null> (libtsan.so.0+0x000000027ee5)
    #1 qemu_mutex_init util/qemu-thread-posix.c:55 (qemu-system-aarch64+0x0000006ad747)
    #2 qemu_init_cpu_loop /home/alex/lsrc/qemu/qemu.git/cpus.c:890 (qemu-system-aarch64+0x0000001d4166)
    #3 main /home/alex/lsrc/qemu/qemu.git/vl.c:3005 (qemu-system-aarch64+0x0000001820ac)

  Thread T1 (tid=15389, running) created by main thread at:
    #0 pthread_create <null> (libtsan.so.0+0x0000000274c7)
    #1 qemu_thread_create util/qemu-thread-posix.c:525 (qemu-system-aarch64+0x0000006ae04d)
    #2 rcu_init_complete util/rcu.c:320 (qemu-system-aarch64+0x0000006c3d52)
    #3 rcu_init util/rcu.c:351 (qemu-system-aarch64+0x00000018e288)
    #4 __libc_csu_init <null> (qemu-system-aarch64+0x0000006c63ec)


but I don't know how many are false positives so I'm going to look in more
detail now.

<snip>

--
Alex Bennée