On 2023/11/2 12:53, Vineet Gupta wrote:
On 10/29/23 20:41, wuqiang.matt wrote:
arch_cmpxchg_relaxed:
...
switch(sizeof((_p_))) {
case 4:
....
arch_cmpxchg:
...
BUILD_BUG_ON(sizeof(_p_) != 4);
...
_p is the address pointer, so I'm thinking it's a typo but I couldn't
yet confirm. There is not much about arc processors in the web :(
Hmm, indeed. This seems like a bug but it depends on the 'llock %0, [%1]'
can take a 32bit address or 32bit data register. Usually it should
check the size of data, but need to check with ISA manual.
Vineet, can you check this suspicious bug?
ARCv2 is a 32-bit ISA and LLOCK/SCOND work on 32-bit data.
So the pointers will be 32-bit anyways. Is the issue that pointer/cmpxchg
operation could be on a smaller data type ?
For ARCv2 with CONFIG_ARC_HAS_LLSC, better add the data size checking and
only permit 32bit data size. Even for 32-bit system, data should can be
64bit 'long long'.
Right, makes sense. Can you send a patch ?
Okay, I will.
And In the case that CONFIG_ARC_HAS_LLSC is undefined, in arch_cmpxchg: the
pointer size checking is unnecessary, since it's using spinlock internally:
Correct, I had the same thought.
https://elixir.bootlin.com/linux/v6.6-rc7/source/arch/arc/include/asm/cmpxchg.h#L60:
BUILD_BUG_ON(sizeof(_p_) != 4); \
\
/* \
* spin lock/unlock provide the needed smp_mb() before/after \
*/ \
atomic_ops_lock(__flags); \
_prev_ = *_p_; \
if (_prev_ == _o_) \
*_p_ = _n_; \
atomic_ops_unlock(__flags);
Can you do that fix in same patch as well ?
Sure.
Another question about the naming: arch_cmpxchg_relaxed() implemented if
CONFIG_ARC_HAS_LLSC is configured and arch_cmpxchg() defined for the rest.
Are there any reasons for difference names ?
Yes the atomics API consists of _relaxed, _acquire, _release and unadorned.
_relaxed is the most efficient, with rest having some extra barriers.
If arch provides _relaxed, generic code can create the rest - assuming arch
hardware can support the relaxed ones.
Definitely Yes! The generic cmpxchg will prefer arch_cmpxchg_relaxed rather
than arch_cmpxchg.
That is true for LLSC.
However for !LLSC, spinlock versions already have full barriers due to
spinlock, so relaxed variant doesn't make sense.
As I checked, Synopsys has released 64bit ARC processors (HS66/HS68), but
I don't know the status of Linux kernel support.
Yes there's an internal ARC64 port running but it is not ready for upstreaming
yet.
Thanks for the update.
-Vineet
Regards,
wuqiang
