[llvm-dev] [ThreadSanitizer] Get deadlocks working

[Vaivaswatha Nagaraj via llvm-dev]

Thanks Kostya and Dmiitry, that was helpful.

  - Vaivaswatha

On Tue, Sep 8, 2015 at 10:49 PM, Kostya Serebryany <kcc at google.com> wrote:

>
>
> On Tue, Sep 8, 2015 at 12:15 AM, Dmitry Vyukov via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> +thread-sanitizer mailing list
>>
>> On Mon, Sep 7, 2015 at 6:41 PM, Vaivaswatha Nagaraj <vn at compilertree.com>
>> wrote:
>> > Hi,
>> >
>> > I am interested in understand the compiler-rt thread sanitizer tool and
>> have
>> > recently started experimenting with it. In particular, I'm interested
>> in the
>> > deadlock detector.
>> >
>> > I see that deadlock detection currently don't work. (I tried with a few
>> > simple deadlocks, as well as the test case "must_deadlock.cc" that is
>> in the
>> > test-suite). I understand from the comments that this is because the
>> real
>> > pthread_mutex_lock() is called before the handler (MutexLock()) in tsan.
>> >
>> > 1. Is there any particular reason that this interceptor is designed this
>> > way? Can there be a callback prior to calling the real
>> pthread_mutex_lock()
>> > that can, for example, detect deadlocks?
>> > 2. Upon debugging a simple test case, I see that there is a worker
>> thread
>> > that is created. Can this worker not check for the deadlock when the
>> actual
>> > threads are deadlocked.
>>
>> Hello Vaivaswatha,
>>
>> Yes, tsan currently detects only _potential_ deadlocks. On actual
>> deadlocks it deadlocks as well.
>> This is just not implemented yet.
>
>
> Correct, I just did not have time to implement this.
> In practice this is not the most important thing to have because
> if the deadlock has actually happened in a test it is easy to find.
> The potential deadlocks are more important.
> Yet I fully agree this needs to be implemented.
>
> --kcc
>
>
>
>> Yes, mutex callback needs to be
>> split into pre and post parts. The deadlock detector already has
>> separate callbacks for these events, it is just a matter of threading
>> these callbacks through tsan code.
>> You can look at the standalone deadlock detector in
>> lib/tsan/dd/dd_interceptors.cc, it does it correctly.
>>
>> --
>> Dmitry Vyukov, Software Engineer, dvyukov at google.com
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>
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