Messages in this thread | | | Date | Wed, 12 Apr 2006 15:50:27 -0600 | From | Andreas Dilger <> | Subject | Re: [Ext2-devel] Re: [RFC][PATCH 0/3] ext3 percpu counter fixes to suppport for ext3 unsigned long type free blocks counter |
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On Apr 12, 2006 14:28 -0700, Mingming Cao wrote: > where the check for unsigned long overflow is only turned on 32 bit > platforms. > > > Or make the counter s64? so that it stays 64 bit on all arches? > > > > Well, don't we have the problem : 64 bit counter add/dec/update is not > always atomic on all 32 bit platforms? There are risk that we will get > bogus global value.
My thought here is that the per-cpu counter could still be a 32-bit counter and the global value could be a 64-bit value. That way, we don't need to mess with 64-bit math in the common case, and we can still have a 64-bit global value. The minor drawback would be that we can't have a per-cpu delta of more than 2^31 at a time, but I don't think this is a worry here.
> > why not change the global per-cpu counter type to unsigned long (as we > > discussed earlier), so we don't need the extra "ul" flags and interfaces, > > and all arches get a standard unsigned long return type? > > We could also > > do away with percpu_read_positive then no? The applications for per-cpu > > counters is going to be upcounters always methinks...
The "percpu_read_positive" usage is broken in any case, since it doesn't correctly handle the case where there is no space in the filesystem at all. The calling code (ext3_statfs) really needs to just call percpu_read() and then return zero if this is negative.
Cheers, Andreas -- Andreas Dilger Principal Software Engineer Cluster File Systems, Inc.
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