[adding linux-arch]
On Sunday 29 January 2006 01:55, Andrew Morton wrote:
> Benjamin LaHaise <[email protected]> wrote:
> > On Sat, Jan 28, 2006 at 01:28:20AM +0100, Eric Dumazet wrote:
> > > We might use atomic_long_t only (and no spinlocks)
> > > Something like this ?
> >
> > Erk, complex and slow... Try using local_t instead, which is
> > substantially cheaper on the P4 as it doesn't use the lock prefix and act
> > as a memory barrier. See asm/local.h.
>
> local_t isn't much use until we get rid of asm-generic/local.h. Bloaty,
> racy with nested interrupts.
It is just implemented wrong. It should use
local_irq_save()/local_irq_restore() instead. But my bigger problem
with local_t is these few architectures (IA64, PPC64) who implement it
with atomic_t. This means we can't replace local statistics counters
with local_t because it would be regression for them. I haven't done the
benchmarks yet, but I suspect both IA64 and PPC64 really should
just turn off interrupts.
-Andi
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