[Mono-bugs] [Bug 662381] C# lock () fails on latest Mono runtimes

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https://bugzilla.novell.com/show_bug.cgi?id=662381

https://bugzilla.novell.com/show_bug.cgi?id=662381#c5


--- Comment #5 from John Costella <john.costella at petermac.org> 2011-01-05 23:58:35 UTC ---
Hi Jérémie,

I don't think you grasp the bug. Let me simplify it in pseudocode. We have
something like:

static Main()
{
  Parallel.For( ... Foo() );
}

static Foo()
{
  lock (locker)
  {
    Bar();
  }
}

static object locker;

static Bar()
{
  Parallel.For( ... nothing that calls Foo() directly or indirectly );
}

OK, let's say Main's Parallel.For() spawns two threads A and B. They both call
Foo(). Let's say A gets there first, and gets the lock. B blocks at the lock.

Then, inside the lock, A calls Bar(). Inside Bar(), A finds a Parallel.For()
that does something irrelevant, unrelated to the above code. Regardless of
whether or not this spawns more threads or just runs on thread A, it should
complete and Bar() return thread A to Foo() before thread A releases the lock
and thread B is unblocked.

What is happening is that when thread A gets to the Parallel.For() in Bar(),
sometimes thread B is immediately unblocked, and starts wandering through the
critical section of code in Foo() while thread A still holds the lock to it.

That simply shouldn't happen. I don't know whether it's TPL or the threading
libraries themselves that possess the bug, but it's a bug.

Thanks 
John

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