On 10/25/2014 04:19 PM, Thomas Gleixner wrote:
On Thu, 23 Oct 2014, Chris Friesen wrote:
I recently noticed that when CONFIG_PREEMPT_RT_FULL is enabled we the
semantics change. From "include/linux/rwsem_rt.h":
* Note that the semantics are different from the usual
* Linux rw-sems, in PREEMPT_RT mode we do not allow
* multiple readers to hold the lock at once, we only allow
* a read-lock owner to read-lock recursively. This is
* better for latency, makes the implementation inherently
* fair and makes it simpler as well.
How is this valid? It seems to me that there are any number of code paths
that could depend on having multiple threads of execution be able to hold the
reader lock simultaneously. Something as simple as:
thread A:
take rw_semaphore X for reading
take lock Y, modify data, release lock Y
wake up thread B
wait on conditional protected by lock Y
free rw_semaphore X
thread B:
take rw_semaphore X for reading
wait on conditional protected by lock Y
send message to wake up thread A
free rw_semaphore X
I don't see why B should wake A without changing the conditional. A
won't make progress by being woken by B as the conditional does not
magically change just because B wakes A.
So what you wanted to say is:
thread B:
take rw_semaphore X for reading
wait on conditional protected by lock Y
+ take lock Y, modify data, release lock Y
send message to wake up thread A
free rw_semaphore X
Otherwise your example does not make any sense at all. And that has
some serious non RT related implications.
Yes, your reformulated version is what I meant to say. Sorry for any
confusion.
Does the RT kernel just disallow this sort of algorithm?
Yes. For a good reason. Let's add thread C
A B C
down_read(X)
down_write(X)
lock(Y)
modify data
unlock(Y)
wake(B)
down_read(X)
Due to the mainline rwsem fairness semantics:
A holds X, C is blocked on A and B is blocked on A.
Deadlock, without RT and the single reader restriction being involved.
Crap, I had forgotten about the fairness semantics stuff. That makes
perfect sense.
Thanks for the explanation.
Chris
--
To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
