On Mon, 19 Dec 2005, Ingo Molnar wrote:
>
> in fact, generic mutexes are _more_ fair than struct semaphore in their
> wait logic. In the stock semaphore implementation, when a waiter is
> woken up, it will retry the lock, and if it fails, it goes back to the
> _tail_ of the queue again - waiting one full cycle again.
Ingo, I don't think that is true.
It shouldn't be true, at least. The whole point with the "sleeper" count
was to not have that happen. Of course, bugs happen, so I won't guarantee
that's actually true, but ..
If you are woken up as a waiter on a semaphore, you shouldn't fail to get
it. You will be woken first, and nobody else will get at it, because the
count has been kept negative or zero even by the waiters, so that a
fast-path user shouldn't be able to get the lock without going through the
slow path and adding itself (last) to the list.
But hey, somebody should test it with <n> kernel threads that just do
down()/up() and some make-believe work in between to make sure there
really is contention, and count how many times they actually get the
semaphore. That code has been changed so many times that it may not work
the way it is advertized ;)
[ Oh. I'm looking at the semaphore code, and I realize that we have a
"wake_up(&sem->wait)" in the __down() path because we had some race long
ago that we fixed by band-aiding over it. Which means that we wake up
sleepers that shouldn't be woken up. THAT may well be part of the
performance problem.. The semaphores are really meant to wake up just
one at a time, but because of that race hack they'll wake up _two_ at a
time - once by up(), once by down().
That also destroys the fairness. Does anybody remember why it's that
way? ]
Ho humm.. That's interesting.
Linus
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