[PATCH V7 5/5] riscv: Add qspinlock support

guoren
2022-06-28T08:19:32Z

From: Guo Ren <guoren@linux.alibaba.com>

Enable qspinlock by the requirements mentioned in a8ad07e5240c9
("asm-generic: qspinlock: Indicate the use of mixed-size atomics").

- RISC-V atomic_*_release()/atomic_*_acquire() are implemented with
own relaxed version plus acquire/release_fence for RCsc
synchronization.

- RISC-V LR/SC pairs could provide a strong/weak forward guarantee
that depends on micro-architecture. And RISC-V ISA spec has given
out several limitations to let hardware support strict forward
guarantee (RISC-V User ISA - 8.3 Eventual Success of
Store-Conditional Instructions). Some riscv cores such as BOOMv3
& XiangShan could provide strict & strong forward guarantee (The
cache line would be kept in an exclusive state for Backoff cycles,
and only this core's interrupt could break the LR/SC pair).

- RISC-V provides cheap atomic_fetch_or_acquire() with RCsc.

- RISC-V only provides relaxed xchg16 to support qspinlock.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Palmer Dabbelt <palmer@rivosinc.com>
---
arch/riscv/Kconfig | 9 +++++++++
arch/riscv/include/asm/Kbuild | 2 ++
arch/riscv/include/asm/cmpxchg.h | 17 +++++++++++++++++
arch/riscv/kernel/setup.c | 4 ++++
4 files changed, 32 insertions(+)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 32ffef9f6e5b..47e12ab9c822 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -333,6 +333,15 @@ config NODES_SHIFT
Specify the maximum number of NUMA Nodes available on the target
system. Increases memory reserved to accommodate various tables.

+config RISCV_USE_QUEUED_SPINLOCKS
+ bool "Using queued spinlock instead of ticket-lock"
+ depends on SMP && MMU
+ select ARCH_USE_QUEUED_SPINLOCKS
+ default y
+ help
+ Make sure your micro arch LL/SC has a strong forward progress guarantee.
+ Otherwise, stay at ticket-lock.
+
config RISCV_ALTERNATIVE
bool
depends on !XIP_KERNEL
diff --git a/arch/riscv/include/asm/Kbuild b/arch/riscv/include/asm/Kbuild
index 504f8b7e72d4..2cce98c7b653 100644
--- a/arch/riscv/include/asm/Kbuild
+++ b/arch/riscv/include/asm/Kbuild
@@ -2,7 +2,9 @@
generic-y += early_ioremap.h
generic-y += flat.h
generic-y += kvm_para.h
+generic-y += mcs_spinlock.h
generic-y += parport.h
+generic-y += qspinlock.h
generic-y += spinlock.h
generic-y += spinlock_types.h
generic-y += qrwlock.h
diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
index 12debce235e5..492104d45a23 100644
--- a/arch/riscv/include/asm/cmpxchg.h
+++ b/arch/riscv/include/asm/cmpxchg.h
@@ -17,6 +17,23 @@
__typeof__(new) __new = (new); \
__typeof__(*(ptr)) __ret; \
switch (size) { \
+ case 2: { \
+ u32 temp; \
+ u32 shif = ((ulong)__ptr & 2) ? 16 : 0; \
+ u32 mask = 0xffff << shif; \
+ __ptr = (__typeof__(ptr))((ulong)__ptr & ~(ulong)2); \
+ __asm__ __volatile__ ( \
+ "0: lr.w %0, %2\n" \
+ " and %1, %0, %z3\n" \
+ " or %1, %1, %z4\n" \
+ " sc.w %1, %1, %2\n" \
+ " bnez %1, 0b\n" \
+ : "=&r" (__ret), "=&r" (temp), "+A" (*__ptr) \
+ : "rJ" (~mask), "rJ" (__new << shif) \
+ : "memory"); \
+ __ret = (__ret & mask) >> shif; \
+ break; \
+ } \
case 4: \
__asm__ __volatile__ ( \
" amoswap.w %0, %2, %1\n" \
diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
index f0f36a4a0e9b..b9b234157a66 100644
--- a/arch/riscv/kernel/setup.c
+++ b/arch/riscv/kernel/setup.c
@@ -295,6 +295,10 @@ void __init setup_arch(char **cmdline_p)
setup_smp();
#endif

+#if !defined(CONFIG_NUMA) && defined(CONFIG_QUEUED_SPINLOCKS)
+ static_branch_disable(&use_qspinlock_key);
+#endif
+
riscv_fill_hwcap();
apply_boot_alternatives();
}
--
2.36.1