PATCH powerpc Merge asm-ppc*/rwsem.h
paulus at samba.org
Sat Sep 24 10:46:33 EST 2005
David Howells writes:
> rwsems on ppc64 should really be using a 64-bit counter, not a 32-bit counter,
> otherwise you limit the maximum number of processes to 32K-ish.
> The counter should be "signed long" really.
It has long annoyed me that we waste half the bits in the rwsem
counter, just because you assume a lowest-common-denominator set of
atomic ops. IIRC your implementation replaced the earlier ppc
implementation which had a 32-bit counter and didn't have the 32k
I'd have to study it in more detail, but I strongly suspect that with
an atomic operation that did something like
*p = max(*p, limit) + inc
atomically (or alternatively a min), we could increase the limit to at
least 1G processes with a 32-bit counter, without needing to change
your common slow-path implementation.
Such an atomic op is easy to implement with load-and-reserve /
store-conditional instructions. Look at __sem_update_count in
arch/ppc64/kernel/semaphore.c for an example.
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