[lldb-dev] Windows crash after D9056

Thu May 21 11:43:36 PDT 2015

On Thu, May 21, 2015 at 10:58 AM, Zachary Turner <zturner at google.com> wrote:

> I feel like it should be the other way around.  If LockFile is taking a
> reference then it is implicitly saying that someone else owns the
> underlying File handle.  But who else cares about the contents of a lock
> file other than the LockFile object itself?
>
> It's not the case in current LockFile usage scenario but I can imagine
following use case - somebody wants to synchronize IO operations on file's
content:

   - With multiple readers and awaiting writers
   - Single writer and awaiting readers

After you locked a file with LockFile you can proceed on IO operations and
be confident that these operations are synchronized - e.g., instead of
zero-size lock file we could have used module's file as synchronization
point and to lock a module file first before pouring data into it..

> Why not just change the LockFile constructor to not take a file descriptor
> but instead take the path?  This is even better because now in
> LockFileWindows you don't even need to use an fd you can just store a
> HANDLE directly, and it gives you more flexibility with how you create the
> file because you can use windows-specific options such as
> FILE_FLAG_DELETE_ON_CLOSE (if you want that, anyway).
>
> If you're concerned about the case where the Open operation fails and you
> want to handle that error, then don't let the constructor take anything,
> but instead you could make a LockFile::Open() method or a static LockFile
> &&LockFile::Create() method.  (by returning r-value reference here you
> could make LockFile object moveable but non-copyable, which makes sense for
> something like a lock file.
>
> On Thu, May 21, 2015 at 10:50 AM Oleksiy Vyalov <ovyalov at google.com>
> wrote:
>
>> I'm somewhat concerned about following scenario:
>>
>> File file_obj(...);
>> LockFile lock (file_obj.GetDescriptor ());
>> ..
>>
>> My understanding, that on Windows if LockFile duplicates original handle
>> (i.e. file.GetDescriptor ()) only a duplicated handle will be allowed to
>> call IO functions on this file within locked region, meanwhile file_obj
>> will be blocked on IO.
>> If LockFile just holds a reference to original handle then file_obj can
>> proceed on IO operations in exclusive mode.
>>
>> In order to make it clear that LockFile references File we can make
>>  LockFile to take File& as constructor parameter instead of a raw handle.
>>
>> On Thu, May 21, 2015 at 10:28 AM, Zachary Turner <zturner at google.com>
>> wrote:
>>
>>> Why does someone other than the LockFile object care about holding a raw
>>> handle? Shouldn't it just hold a reference/pointer to the LockFile?
>>> On Thu, May 21, 2015 at 10:06 AM Oleksiy Vyalov <ovyalov at google.com>
>>> wrote:
>>>
>>>> Windows file lock is tied to a file handle - using handle duplication
>>>> effectively applies locking on duplicated handle, not on original, so your
>>>> original handle will be blocked on IO trying to read/write.
>>>> Maybe we can just eliminate CloseHandle call (
>>>> https://github.com/llvm-mirror/lldb/blob/master/source/Host/windows/LockFileWindows.cpp#L50)
>>>> if file will be eventually closed by LockFileWindows caller.
>>>>
>>>> On Thu, May 21, 2015 at 9:58 AM, Zachary Turner <zturner at google.com>
>>>> wrote:
>>>>
>>>>> I don't experience this crash but looking at the proposed fix it
>>>>> certainly seems like a plausible explanation. Seems to me like something
>>>>> should transfer ownership though rather than duplicating the handle
>>>>>
>>>>> On Thu, May 21, 2015 at 9:52 AM Colin Riley <colin at codeplay.com>
>>>>> wrote:
>>>>>
>>>>>>  Windows 7 64, Debug, latest vs13.
>>>>>>
>>>>>>
>>>>>> On 21/05/2015 17:39, Oleksiy Vyalov wrote:
>>>>>>
>>>>>> Hi Colin,
>>>>>>
>>>>>>  could you give more context about crash - what build configuration
>>>>>> do you use (debug, release,..) and which OS?
>>>>>> I'm running this code on Windows 7 and haven't noticed any failures.
>>>>>>
>>>>>>
>>>>>> On Thu, May 21, 2015 at 8:57 AM, Colin Riley <colin at codeplay.com>
>>>>>> wrote:
>>>>>>
>>>>>>>
>>>>>>> Zachary, do you see this on windows at all? Tip for us results in
>>>>>>> crashes when releasing file descriptors without the below fix.
>>>>>>>
>>>>>>> Colin
>>>>>>>
>>>>>>>
>>>>>>> On 19/05/2015 12:52, Aidan Dodds wrote:
>>>>>>>
>>>>>>>  Hi,
>>>>>>>
>>>>>>> We have been seeing a crash on windows when connecting to an android
>>>>>>> target using lldb-server.
>>>>>>> I am not sure if this affects other platforms too.
>>>>>>> I think this was introduced with http://reviews.llvm.org/D9056.
>>>>>>>
>>>>>>> I tracked the crash back to the workings of
>>>>>>> ModuleCache::GetAndPut().
>>>>>>>
>>>>>>> The crash seems to be due to a file descriptor being released twice,
>>>>>>> once by the original "File lock_file" and again by the "LockFile lock" who
>>>>>>> share the same file descriptor.
>>>>>>>
>>>>>>> The file descriptor sharing happens because of this line:
>>>>>>> ModuleCache.cpp @ 164
>>>>>>> LockFile lock (lock_file.GetDescriptor ());
>>>>>>>
>>>>>>> Both destructors attempt to release effectively the same file
>>>>>>> descriptor.  I was able to fix the crash by duplicating the file handle in
>>>>>>> the lock file constructor using _dup().  (patch attached)
>>>>>>> I wasn't sure if this was the right fix however. Has anyone else
>>>>>>> seen this?
>>>>>>> Should "File lock_file" perhaps transfer its file descriptor
>>>>>>> completely rather then share it?
>>>>>>>
>>>>>>> Thanks,
>>>>>>> Aidan
>>>>>>>
>>>>>>>
>>>>>>>  _______________________________________________
>>>>>>> lldb-dev mailing listlldb-dev at cs.uiuc.eduhttp://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
>>>>>>>
>>>>>>>
>>>>>>> --
>>>>>>> - Colin Riley
>>>>>>> Senior Director,
>>>>>>> Parallel/Graphics Debugger Systems
>>>>>>>
>>>>>>> Codeplay Software Ltd
>>>>>>> 45 York Place, Edinburgh, EH1 3HP
>>>>>>> Tel: 0131 466 0503
>>>>>>> Fax: 0131 557 6600
>>>>>>> Website: http://www.codeplay.com
>>>>>>> Twitter: https://twitter.com/codeplaysoft
>>>>>>>
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>>>>>>>
>>>>>>>
>>>>>>
>>>>>>
>>>>>>  --
>>>>>>  Oleksiy Vyalov | Software Engineer | ovyalov at google.com
>>>>>>
>>>>>>
>>>>>> --
>>>>>> - Colin Riley
>>>>>> Senior Director,
>>>>>> Parallel/Graphics Debugger Systems
>>>>>>
>>>>>> Codeplay Software Ltd
>>>>>> 45 York Place, Edinburgh, EH1 3HP
>>>>>> Tel: 0131 466 0503
>>>>>> Fax: 0131 557 6600
>>>>>> Website: http://www.codeplay.com
>>>>>> Twitter: https://twitter.com/codeplaysoft
>>>>>>
>>>>>> This email and any attachments may contain confidential and /or privileged information and is for use by the addressee only. If you are not the intended recipient, please notify Codeplay Software Ltd immediately and delete the message from your computer. You may not copy or forward it,or use or disclose its contents to any other person. Any views or other information in this message which do not relate to our business are not authorized by Codeplay software Ltd, nor does this message form part of any contract unless so stated.
>>>>>> As internet communications are capable of data corruption Codeplay Software Ltd does not accept any responsibility for any changes made to this message after it was sent. Please note that Codeplay Software Ltd does not accept any liability or responsibility for viruses and it is your responsibility to scan any attachments.
>>>>>> Company registered in England and Wales, number: 04567874
>>>>>> Registered office: 81 Linkfield Street, Redhill RH1 6BY
>>>>>>
>>>>>>
>>>>
>>>>
>>>> --
>>>> Oleksiy Vyalov | Software Engineer | ovyalov at google.com
>>>>
>>>
>>
>>
>> --
>> Oleksiy Vyalov | Software Engineer | ovyalov at google.com
>>
>

-- 
Oleksiy Vyalov | Software Engineer | ovyalov at google.com
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