[Click] NotifierQueue on multi-threaded Click problem.

[Eddie Kohler]

OK, too bad.

I still doubt that replacing _head and _tail with atomic_uint32_t would fix 
the underlying problem.  The atomic_uint32_t, in kernel, is read with 
atomic_read() and written with atomic_set().  Here's how they are implemented 
on x86 (LINUX/include/asm-i386/atomic.h):

typedef struct { volatile int counter; } atomic_t;
#define atomic_read(v)		((v)->counter)
#define atomic_set(v,i)		(((v)->counter) = (i))

I.e., just with reads and assignments!  Once we declare the _head and _tail 
variables as volatile, it's the same as atomic_uint32_t for reads and 
assignments.  (Atomic_uint32_t DOES make a difference for operations like +=, 
-=, &=, dec_and_test; but _head and _tail are never touched that way.)

I am happy to be proved wrong, as usual; can you explain in more detail why 
atomic operations would help?  I am looking for a sample list of interleaved 
instructions that would cause an error.

-*-

But: There are some remaining places where I can see potential race conditions 
involving notification.  For example:

Initial state:   _head = 0, _tail = 2, _capacity = 1000
Thread 1 push(): int h = _head, t = _tail, nt = next_i(t);
                  => int h = 0, t = 2, nt = 3;
                  => Thread 1 sleeps
Thread 2 pull(): (executes once)
                  => _head = 1, _tail = 2
Thread 2 pull(): (executes once)
                  => _head = 2, _tail = 2
Thread 2 pull(): (executes, returns null, _sleepiness++)
Thread 2 pull(): => _empty_note.sleep()
Thread 1 push(): => wakes up
                  => executes "if (s == 1) _empty_note.wake();"
                  => But s == 3!
                  => So notifier is not woken, the bug happens.

I have attached a diff to address these problems.  Does this do anything?

Eddie





Joonwoo Park wrote:
> 2007/9/8, Eddie Kohler <kohler at cs.ucla.edu>:
>> P.S. Do you still see a problem without the ToHosts?
>>
> 
> Yes I do,
> Problem still occurs without ToHosts and Tees.
> I tried 1652d76b7a7685105bcb244e7f58a55a9226c19e
> 
> Joonwoo Park
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