Messages in this thread | | | Date | Sat, 12 May 2007 15:44:19 +0200 (CEST) | From | Esben Nielsen <> | Subject | Re: [PATCH 0/2] convert mmap_sem to a scalable rw_mutex |
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On Sat, 12 May 2007, Peter Zijlstra wrote:
> On Sat, 2007-05-12 at 11:27 +0200, Esben Nielsen wrote: >> >> On Fri, 11 May 2007, Peter Zijlstra wrote: >> >>> >>> I was toying with a scalable rw_mutex and found that it gives ~10% reduction in >>> system time on ebizzy runs (without the MADV_FREE patch). >>> >> >> You break priority enheritance on user space futexes! :-( >> The problems is that the futex waiter have to take the mmap_sem. And as >> your rw_mutex isn't PI enabled you get priority inversions :-( > > Do note that rwsems have no PI either. > PI is not a concern for mainline - yet, I do have ideas here though. > > If PI wasn't a concern for mainline, why is PI futexes merged into the mainline?
I notice that the rwsems used now isn't priority inversion safe (thus destroyingthe perpose of having PI futexes). We thus already have a bug in the mainline.
I suggest making a rw_mutex which does read side PI: A reader boosts the writer, but a writer can't boost the readers, since there can be a large amount of those.
I don't have time to make such a rw_mutex but I have a simple idea for one, where the rt_mutex can be reused.
struct pi_rw_mutex { int count; /* 0 -> unoccupied, >0 -> the number of current readers, Second highest bit: there are a waiting writer -1 -> A writer have it. */ struct rt_mutex mutex; }
Use atomic exchange on count.
When locking:
A writer checks if count <= 0. If so it sets the value to -1 and takes the mutex. When it gets the mutex it rechecks the count and proceeds. If count > 0 the writer sets the second highest bit and add itself to the wait-list in the mutex and sleeps. (The mutex will now be in a state where owner==NULL but there are waiters. It must be cheched if the rt_mutex code can handle this.)
A reader checks if count >= 0. If so it does count++ and proceeds. If count < 0 it takes the rtmutex. When it gets the mutex it sets the count to 1, unlocks the mutex and proceeds.
When unlocking:
The writer sets count to 0 or 0x8000000 (second highest bit) depending on how many waiters the mutex have and unlocks the mutex.
The reader checks if count is 0x80000001. If so it sets count to 0 and wakes up the first waiter on the mutex (if there are any). Otherwise it just do count--.
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