Re: [Qemu-devel] strange crash in tracked_request_begin

[Christian Borntraeger]

On 03/07/2016 06:01 PM, Stefan Hajnoczi wrote:
> On Mon, Mar 07, 2016 at 01:29:08PM +0100, Christian Borntraeger wrote:
>> Folks,
>>
>> I had a crash of a qemu guest in tracked_request_begin.
>> The testcase was a guest with ramdisk/kernel that reboots in a 
>> loop. (about 10 times per second) with a single null-co disk 
>> attached. No idea how to reproduce this, seems to be a lucky hit.
>>
>> (gdb) bt
>> #0  0x00000000101db5ba in tracked_request_begin (address@hidden, 
>> address@hidden, address@hidden, address@hidden, address@hidden)
>>     at /home/cborntra/REPOS/qemu/block/io.c:390
>> #1  0x00000000101de91e in bdrv_co_do_preadv (bs=0x42a39190, offset=0, 
>> bytes=4096, qiov=0x3ff7400cbd8, flags=<optimized out>, 
>> address@hidden(unknown: 0))
>>     at /home/cborntra/REPOS/qemu/block/io.c:1001
>> #2  0x00000000101dfc3e in bdrv_co_do_readv (flags=(unknown: 0), 
>> qiov=<optimized out>, nb_sectors=<optimized out>, sector_num=<optimized 
>> out>, bs=<optimized out>)
>>     at /home/cborntra/REPOS/qemu/block/io.c:1024
>> #3  bdrv_co_do_rw (opaque=0x3ff7400e370) at 
>> /home/cborntra/REPOS/qemu/block/io.c:2173
>> #4  0x000000001022d8f6 in coroutine_trampoline (i0=<optimized out>, 
>> i1=-1946150928) at /home/cborntra/REPOS/qemu/util/coroutine-ucontext.c:79
>> #5  0x000003ff95ed150a in __makecontext_ret () from /lib64/libc.so.6
>>
>> looking at the code we are at
>>
>> QLIST_INSERT_HEAD(&bs->tracked_requests, req, list);
>> which translates to
>>
>> if (((req)->list.le_next = (&bs->tracked_requests)->lh_first) != NULL) 
>>     (&bs->tracked_requests)->lh_first->list.le_prev = &(req)->list.le_next;
>> (&bs->tracked_requests)->lh_first = (req);                       
>> (req)->list.le_prev = &(&bs->tracked_requests)->lh_first;
>>
>> gdb says, that (&bs->tracked_requests)->lh_first) is zero in the corefile
>> (gdb) print /x bs->tracked_requests
>> $6 = {lh_first = 0x0}
>>
>> Now looking at the code I am asking myself if this can happen in parallel
>> to another code that touches tracked_requests, because gcc seems to read
>> &bs->tracked_requests)->lh_first twice (first to check the value, then
>> to use it as pointer)
> 
> tracked_requests is protected by AioContext.  Perhaps something is doing
> I/O without acquiring AioContext?

Hmm, the guest was rebooting, which resets all devices. Maybe something
in that code is still not right? I will have a look.
> 
> Luckily there is only 1 place where items are added and removed from
> tracked_requests.  This might make debugging somewhat easier.

I have trouble reproducing the issue, which makes it hard :-/
 


>>
>> 388      qemu_co_queue_init(&req->wait_queue);
>>    0x00000000101db594 <+76>: la      %r2,72(%r13)
>>    0x00000000101db598 <+80>: brasl   %r14,0x1022cdc0 <qemu_co_queue_init>
>>
>> 389  
>> 390      QLIST_INSERT_HEAD(&bs->tracked_requests, req, list);
>>    0x00000000101db59e <+86>: lg      %r1,12744(%r12)         # r1 = 
>> (&bs->tracked_requests)->lh_first)
>>    0x00000000101db5a4 <+92>: stg     %r1,48(%r13)            # 
>> (req)->list.le_next = r1
>>    0x00000000101db5aa <+98>: cgij    %r1,0,8,0x101db5c0 ---+ # if r1==0 goto
>>    0x00000000101db5b0 <+104>:        lg      %r1,12744(%r12)       | # r1 = 
>> (&bs->tracked_requests)->lh_first) (again!!)
>>    0x00000000101db5b6 <+110>:        la      %r2,48(%r13)          | 
>> => 0x00000000101db5ba <+114>:        stg     %r2,56(%r1)           | # r1==0 
>> bang
>>    0x00000000101db5c0 <+120>:        stg     %r13,12744(%r12)<-----+
>>    0x00000000101db5c6 <+126>:        lay     %r12,12744(%r12)
>>    0x00000000101db5cc <+132>:        stg     %r12,56(%r13)
>>
>>
>> Christian
>>