This patch is another step towards updating the code to handle the
32-bit kernels which I have not been regularly testing. This changes
do not really impact the common case I'm expected which is the latest
kernel running on an x86_64 arch.
Until the linux-2.6.31 kernel the x86 arch did not have support for
64-bit atomic operations. Additionally, the new atomic_compat.h support
for this case was wrong because it embedded a spinlock in the atomic
variable which must always and only be 64-bits total. To handle these
32-bit issues we now simply fall back to the --enable-atomic-spinlock
implementation if the kernel does not provide the 64-bit atomic funcs.
The second issue this patch addresses is the DEBUG_KMEM assumption that
there will always be atomic64 funcs available. On 32-bit archs this may
not be true, and actually that's just fine. In that case the kernel will
will never be able to allocate more the 32-bits worth anyway. So just
check if atomic64 funcs are available, if they are not it means this
is a 32-bit machine and we can safely use atomic_t's instead.
These functions didn't exist for all archs prior to 2.6.24. This
patch addes an autoconf test to detect this and add them when needed.
The autoconf check is needed instead of just an #ifndef because in
the most modern kernels atomic64_{cmp}xchg are implemented as in
inline function and not a #define.
Previously Solaris style atomic primitives were implemented simply by
wrapping the desired operation in a global spinlock. This was easy to
implement at the time when I wasn't 100% sure I could safely layer the
Solaris atomic primatives on the Linux counterparts. It however was
likely not good for performance.
After more investigation however it does appear the Solaris primitives
can be layered on Linux's fairly safely. The Linux atomic_t type really
just wraps a long so we can simply cast the Solaris unsigned value to
either a atomic_t or atomic64_t. The only lingering problem for both
implementations is that Solaris provides no atomic read function. This
means reading a 64-bit value on a 32-bit arch can (and will) result in
word breaking. I was very concerned about this initially, but upon
further reflection it is a limitation of the Solaris API. So really
we are just being bug-for-bug compatible here.
With this change the default implementation is layered on top of Linux
atomic types. However, because we're assuming a lot about the internal
implementation of those types I've made it easy to fall-back to the
generic approach. Simply build with --enable-atomic_spinlocks if
issues are encountered with the new implementation.
atomic operations will be rewritten anyway with the correct arch
specific assembly. But not today.
git-svn-id: https://outreach.scidac.gov/svn/spl/trunk@65 7e1ea52c-4ff2-0310-8f11-9dd32ca42a1c
what I would call effecient but it does have the advantage
of being correct which is all I need right now. I added
a regression test as well.
git-svn-id: https://outreach.scidac.gov/svn/spl/trunk@57 7e1ea52c-4ff2-0310-8f11-9dd32ca42a1c
Brian Behlendorf
behlendo