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From: "Denis V. Lunev" <den@openvz.org>
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Date: Thu, 14 Apr 2016 16:40:31 +0300
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Subject: Re: [Qemu-devel] [PATCH for 2.6 1/1] nbd: fix assert() on qemu-nbd
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To: Paolo Bonzini <pbonzini@redhat.com>, qemu-devel@nongnu.org
Cc: Pavel Butsykin <pbutsykin@virtuozzo.com>, Max Reitz <mreitz@redhat.com>, Kevin Wolf <kwolf@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>

On 04/14/2016 04:23 PM, Paolo Bonzini wrote:
>
> On 14/04/2016 12:20, Denis V. Lunev wrote:
>> From: Pavel Butsykin <pbutsykin@virtuozzo.com>
>>
>>  From time to time qemu-nbd is crashing on the following assert:
>>      assert(state == TERMINATING);
>>      nbd_export_closed
>>      nbd_export_put
>>      main
>> and the state at the moment of the crash is evaluated to TERMINATE.
>>
>> During shutdown process of the client the nbd_client_thread thread sends
>> SIGTERM signal and the main thread calls the nbd_client_closed callback.
>> If the SIGTERM callback will be executed after change the state to
>> TERMINATING, then the state will once again be TERMINATE.
>>
>> To solve the issue, we must change the state to TERMINATE only if the state
>> is RUNNING. In the other case we are shutting down already.
>>
>> Signed-off-by: Pavel Butsykin <pbutsykin@virtuozzo.com>
>> Signed-off-by: Denis V. Lunev <den@openvz.org>
>> CC: Paolo Bonzini <pbonzini@redhat.com>
>> ---
>>   qemu-nbd.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/qemu-nbd.c b/qemu-nbd.c
>> index ca4a724..956013f 100644
>> --- a/qemu-nbd.c
>> +++ b/qemu-nbd.c
>> @@ -213,7 +213,7 @@ static int find_partition(BlockBackend *blk, int partition,
>>   
>>   static void termsig_handler(int signum)
>>   {
>> -    state = TERMINATE;
>> +    atomic_cmpxchg(&state, RUNNING, TERMINATE);
> Just a simple "if" is enough (the rest of the file is not using any of
> atomic{mb_,}{read,set}.
>
> I'm not able to send further pull requests for 2.6, can the block
> maintainers help?
>
> Paolo

unfortunately, if () would be not enough. The race is with different
thread which can run on the another CPU. Thus this OP should be
atomic or some locking is required.

Den

P.S. Added more block guys...