There found 2 solutions for this:
1. In ngx_http_file_cache_update the call of
ngx_shmtx_lock(&cache->shpool->mutex); may be moved before
rc = ngx_ext_rename_file(&tf->file.name, &c->file.name, &ext);
I should work but there will be another syscall when the resource is
locked.

2. When cache file is read from cache directory its binary header may be
compared with shared memory structure and if shared mem structure is
different it should replaced with the last one read from the file.

I have patched the approach 1 and I am starting the automated test now.


On 09/30/2011 12:15 PM, Maxim Dounin wrote:
> Hello!
>
> On Thu, Sep 29, 2011 at 01:00:20PM +0300, Anatoli Marinov wrote:
>
>> I invested more time to investigate the issue and I found something.
>>
>> Let's assume there is 2 nginx workers with pids 6362 and 6364 (they
>> are actually 4 but only these 2 are interested).
>> There are 3 parallel requests 10102, 10188 and 10232 for one and the
>> same file.
>>
>> See the scenario. It describes the issue:
>>
>> --- context switch to 6362---
>> 6362 - 10102 - begin (connection accepted)
>> 6362 - 10102 - file do not exist in the cache
>> 6362 - 10102 - upstream peer found
>>
>> --- context switch to 6364 ---
>> 6364 - 10188 - begin (connection accepted)
>> 6364 - 10188 - file do not exist in the cache
>> 6364 - 10188 - upstream peer found
>> 6364 - 10188 - header received (header size 369 bytes)
>> 6364 - 10188 - file received
>> 6364 - 10188 - file renamed (with header size 369 bytes)
>> 6364 - 10188 - shared mem updated - header size 369 bytes
>> 6364 - 10188 - success end
>>
>> --- context switch to 6362 ---
>> 6362 - 10102 - header received (header size 373 bytes)
>> 6362 - 10102 - file received
>> 6362 - 10102 - file renamed (with header size 373 bytes)
>> shared memory is not updated because the cpu context is switched
>> or the process works in parallel on different cpu
>>
>> --- context switch to 6364 ---
>> 6364 - 10232 - begin (connection accepted)
>> 6364 - 10232 - record is found in shared memory - file exists
>> (header size is 369 from shared mem)
>> 6364 - 10232 - header is read from the file (but only first 369 bytes)
>>
>> --- context switch to 6362 ---
>> 6362 - 10102 - shared mem updated - header size 373 bytes
>> ....
>> ....
>>
>> --- context switch to 6364 ---
>> 6364 - 10232 - http header could not be parsed because there are
>> missing 4 bytes
>> 6364 - 10232 - failed end
>>
>> See the attached file. The scenario it taken from it.
>> In general it is very hard for reproduction.
>> I hit is 2-3 times for 10 000 000 requests. But the issue exists :).
> Thanks for investigating this. I'll take a look how to fix this
> properly.
>
> Maxim Dounin
>
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> nginx-devel@nginx.org
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