Re: [PATCH] lib/atomic64_test: do not build on non-atomic64 systems

From: Andrew Morton
Date: Sun Oct 24 2010 - 21:52:23 EST


On Sun, 24 Oct 2010 09:20:34 -0700 Roland Dreier <rdreier@xxxxxxxxx> wrote:

> > That's only part of the problem. The following won't build also:
>
> > drivers/infiniband/hw
>
> Interesting... I hadn't looked at that usage before. Both drivers that
> seem to use atomic64 already depend on 64BIT in Kconfig, so I suspect
> the intersection of 64BIT and !ATOMIC64 is empty?
>
> But eg drivers/infiniband/hw/qib/qib_rc.c does essentially:
>
> atomic64_t *maddr;
> u64 sdata = <something>
> ...
> maddr = (atomic64_t *) qp->r_sge.sge.vaddr;
> atomic64_add_return(sdata, maddr)
>
> is it legit to cast some random address to atomic64_t and expect it to
> work across archs that implement atomic64?

Not really. If someone implements atomic64 on 32-bit they may
do it by putting a spinlock in the atomic64_t. Or they might use
hashed spinlocks, in which case that'll work.

Probably hashed spinlocks, given the (realtively new) convention that
the all-zeroes pattern is a legit way of initialising an atomic_t.

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