Messages in this thread | ![/](/images/icornerl.gif) | | Date | Sat, 21 Apr 2001 16:03:27 +0200 | From | Andrea Arcangeli <> | Subject | Re: x86 rwsem in 2.4.4pre[234] are still buggy [was Re: rwsem benchmarks [Re: generic rwsem [Re: Alpha "process table hang"]]] |
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On Fri, Apr 20, 2001 at 04:45:32PM -0700, Linus Torvalds wrote: > I would suggest the following: > > - the generic semaphores should use the lock that already exists in the > wait-queue as the semaphore spinlock.
Ok, that is what my generic code does.
> - the generic semaphores should _not_ drop the lock. Right now it drops > the semaphore lock when it goes into the slow path, only to re-aquire > it. This is due to bad interfacing with the generic slow-path routines.
My generic code doesn't drop the lock.
> I suspect that this lock-drop is why Andrea sees problems with the > generic semaphores. The changes to "count" and "sleeper" aren't > actually atomic, because we don't hold the lock over them all. And > re-using the lock means that we don't need the two levels of > spinlocking for adding ourselves to the wait queue. Easily done by just > moving the locking _out_ of the wait-queue helper functions, no?
Basically yes, however for the wakeup I wrote a dedicated routine that knows how to do the wake-all-next-readers or wake-next-writer (it is not the same helper function of sched.c).
> - the generic semaphores are entirely out-of-line, and are just declared > universally as regular FASTCALL() functions.
This is what I implemented originally but then I moved the fast path inline for the fast-path benchmark reasons. I think in real life it doesn't matter much if the fast path is inline or not.
> The fast-path x86 code looks ok to me. The debugging stuff makes it less > readable than it should be, I suspect, and is probably not worth it at > this stage. The users of rw-semaphores are so well-defined (and so well > debugged) that the debugging code only makes the code harder to follow > right now.
yes I agree, infact I added the ->magic check only to catch uninitialized semaphores (and this one doesn't hurt readability that much).
> Comments? Andrea? Your patches have looked ok, but I absoutely refuse to > see the non-inlined fast-path for reasonable x86 hardware..
In my last patch the fast path is inline as said above but it is not in asm yet because I couldn't get convinced it was right code. I plan to return looking into the rwsem soon. I also seen David fixed the bug and dropped the buggy rwsem-spin.h, so I suggest to merge his code for now, after a very short look it seems certainly better than pre5.
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