Re: [OpenRISC] [PATCH v6 1/9] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS_XCHG32

On Wed, Apr 07, 2021 at 12:51:56AM +0800, Boqun Feng wrote:

Hi,

On Wed, Mar 31, 2021 at 02:30:32PM +0000, guoren@kernel.org wrote:

quoted

From: Guo Ren <redacted>

Some architectures don't have sub-word swap atomic instruction,
they only have the full word's one.

The sub-word swap only improve the performance when:
NR_CPUS < 16K
 *  0- 7: locked byte
 *     8: pending
 *  9-15: not used
 * 16-17: tail index
 * 18-31: tail cpu (+1)

The 9-15 bits are wasted to use xchg16 in xchg_tail.

Please let architecture select xchg16/xchg32 to implement
xchg_tail.

If the architecture doesn't have sub-word swap atomic, won't it generate
the same/similar code no matter which version xchg_tail() is used? That
is even CONFIG_ARCH_USE_QUEUED_SPINLOCKS_XCHG32=y, xchg_tail() acts
similar to an xchg16() implemented by cmpxchg(), which means we still
don't have forward progress guarantee. So this configuration doesn't
solve the problem.

I think it's OK to introduce this config and don't provide xchg16() for
risc-v. But I don't see the point of converting other architectures to
use it.

Hello,

For OpenRISC I did ack the patch to convert to
CONFIG_ARCH_USE_QUEUED_SPINLOCKS_XCHG32=y.  But I think you are right, the
generic code in xchg_tail and the xchg16 emulation code in produced by OpenRISC
using xchg32 would produce very similar code.  I have not compared instructions,
but it does seem like duplicate functionality.

Why doesn't RISC-V add the xchg16 emulation code similar to OpenRISC?  For
OpenRISC we added xchg16 and xchg8 emulation code to enable qspinlocks.  So
one thought is with CONFIG_ARCH_USE_QUEUED_SPINLOCKS_XCHG32=y, can we remove our
xchg16/xchg8 emulation code?

-Stafford