Re: PI patch against 2.6.16-rt9

On Tue, 28 Mar 2006, Thomas Gleixner wrote:

On Tue, 2006-03-28 at 22:17 +0100, Esben Nielsen wrote:
I think we talk about the situation

No, we talk about existing lock chains L(0) --> L(n).

B locks 1 C locks 2 D locks 3
B locks 2, boosts C and block
A locks 2
A is boost B
A drop it's spinlocks and is preempted
C unlocks 2 and auto unboosts
B is running
B locks 3, boosts C and blocks
A gets a CPU again
A boosts B
A boosts D

Is there anything wrong with that?
And in the case where A==D there indeed is a deadlock which will be
detected.

If you get to L(x) the underlying dependencies might have changed
already as well as the dependencies x ... n. We might get false
positives in the deadlock detection that way, as a deadlock is an
"atomic" state.

As I see it you might detect a circular lock graph "atomically". But is
that a "deadlock"? Yes, if you rule out signals and timeouts, this
situation does indeed deadlock your program.

But if you count in signals and timeouts your algoritm also gives "false
positives": You can detect a circular lock but when you return from
rt_mutex_slowlock(), a signal is delivered and there is no longer a
circular dependency and most important: The program wouldn't be
deadlocked even if you didn't ask for deadlock detection and your task in
that case would block.

I would like to see an examble of a false deadlock. I don't rule them out
in the present code. But they might be simple to fix.

Esben


tglx


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