[swift-users] Is this really a race condition?

[Edward Connell]

Not at all, thank you for trying it out.
When I run my code on the Mac in Xcode with thread sanitizer on, I don't
get any warnings or problems at all. I only find problems on Linux. I have
noticed that for the same release, the compiler and libraries are not
identical between Mac and Linux. I am currently using what I believe is the
latest release for both

$ swift --version
Swift version 3.0.2 (swift-3.0.2-RELEASE)
Target: x86_64-unknown-linux-gnu



On Thu, Mar 2, 2017 at 3:32 PM, Zhao Xin <owenzx at gmail.com> wrote:

> I am sorry if this bothers you. I just put the original code in Xcode and
> it works. Anyone knows why this works in Xcode or in macOS? I thought it
> should show the same warnings as Linux.
>
> Zhaoxin
>
> On Fri, Mar 3, 2017 at 2:51 AM, Edward Connell via swift-users <
> swift-users at swift.org> wrote:
>
>> Hi Guillaume,
>> There is still a race condition report, but I think maybe we are almost
>> there. I had to modify your example because it didn't quite build, and a
>> loop to do it twice since it randomly complains if you only do it once.
>> Btw, this is on Linux.
>> The warning is triggered when deinitialize is called. It wasn't in your
>> example, but it seems that it is required since the Elements are not a
>> trivial type like Int.
>>
>> Thanks, Ed
>>
>> for _ in 0..<2 {
>> let count = 1000
>> let items = UnsafeMutablePointer<[UInt8]?>.allocate(capacity: count)
>> items.initialize(to: nil, count: count)
>>
>> DispatchQueue.concurrentPerform(iterations: count) {
>> items[$0] = [UInt8](repeating: 7, count: 10)
>> }
>>
>> items.deinitialize(count: count)
>> items.deallocate(capacity: count)
>> }
>>
>> ==================
>> WARNING: ThreadSanitizer: data race (pid=24076)
>>   Write of size 8 at 0x7d0c00005fa0 by main thread:
>>     #0 free <null> (libtsan.so.0+0x000000025819)
>>     #1 _TwXxOs31_ClosedRangeIndexRepresentation <null>
>> (libswiftCore.so+0x00000027e071)
>>
>>   Previous write of size 8 at 0x7d0c00005fa0 by thread T20:
>>     #0 malloc <null> (libtsan.so.0+0x0000000254a3)
>>     #1 swift_slowAlloc <null> (libswiftCore.so+0x0000002ee3e5)
>>
>>   Thread T20 (tid=24288, running) created by main thread at:
>>     #0 pthread_create <null> (libtsan.so.0+0x000000027577)
>>     #1 manager_workqueue_additem /home/buildnode/disk2/workspac
>> e/oss-swift-3.0-package-linux-ubuntu-16_04/swift-corelibs-
>> libdispatch/libpwq/src/posix/manager.c:815 (libdispatch.so+0x00000007c6b1
>> )
>>
>> SUMMARY: ThreadSanitizer: data race ??:0 __interceptor_free
>> ==================
>>
>>
>> ThreadSanitizer: reported 1 warnings
>>
>>
>>
>> On Wed, Mar 1, 2017 at 6:48 PM, Edward Connell <ewconnell at gmail.com>
>> wrote:
>>
>>> Ahh! thank you. That makes sense.
>>>
>>> On Wed, Mar 1, 2017 at 3:29 PM, Guillaume Lessard <
>>> glessard at tffenterprises.com> wrote:
>>>
>>>>
>>>> > On Mar 1, 2017, at 3:21 PM, Edward Connell via swift-users <
>>>> swift-users at swift.org> wrote:
>>>> >
>>>> > The thread sanitizer on Linux is reporting that I have race
>>>> conditions in libswiftcore. I eliminated enough code down to this trivial
>>>> example. Is there really a race condition here or are these bogus errors?
>>>> >
>>>> >       let count = 1000
>>>> >       var items = [[UInt8]?](repeating: nil, count: count)
>>>> >
>>>> >       DispatchQueue.concurrentPerform(iterations: count) {
>>>> >               items[$0] = [UInt8](repeating: 7, count: 10)
>>>> >       }
>>>> >
>>>> > My real scenario is retrieving data asynchronously, so I just threw
>>>> in a buffer assignment.
>>>>
>>>> The assignments to array elements are where the race lies.
>>>>
>>>> I don’t know about the libswiftcore part, but: assigning to a shared
>>>> Array concurrently from multiple threads won't work, because of Array's
>>>> copy-on-write behaviour. You could do
>>>>
>>>> let items = UnsafeMutablePointer<[UInt8]?>.allocate(capacity: 1)
>>>> items.initialize(to: nil, count: count)
>>>>
>>>> DispatchQueue.concurrentPerform(iterations: count) {
>>>>   items[$0].initialize([UInt8](repeating: 7, count: 10))
>>>> }
>>>>
>>>> // you’ll be able to see here that they’re all initialized
>>>>
>>>> items.deallocate(capacity: count)
>>>>
>>>> Cheers,
>>>> Guillaume Lessard
>>>>
>>>>
>>>
>>
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>>
>>
>
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