Re: [PATCH v3 11/11] riscv: Add qspinlock support

Alexandre Ghiti
2024-07-18T13:11:52Z

Hi Guo,

On 17/07/2024 11:30, Guo Ren wrote:
> On Wed, Jul 17, 2024 at 2:31 PM Alexandre Ghiti <alexghiti@rivosinc.com> wrote:
>> In order to produce a generic kernel, a user can select
>> CONFIG_COMBO_SPINLOCKS which will fallback at runtime to the ticket
>> spinlock implementation if Zabha or Ziccrse are not present.
>>
>> Note that we can't use alternatives here because the discovery of
>> extensions is done too late and we need to start with the qspinlock
>> implementation because the ticket spinlock implementation would pollute
>> the spinlock value, so let's use static keys.
>>
>> This is largely based on Guo's work and Leonardo reviews at [1].
>>
>> Link: https://lore.kernel.org/linux-riscv/20231225125847.2778638-1-guoren@kernel.org/ [1]
>> Signed-off-by: Guo Ren <guoren@kernel.org>
>> Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
>> ---
>> .../locking/queued-spinlocks/arch-support.txt | 2 +-
>> arch/riscv/Kconfig | 29 ++++++++++++++
>> arch/riscv/include/asm/Kbuild | 4 +-
>> arch/riscv/include/asm/spinlock.h | 39 +++++++++++++++++++
>> arch/riscv/kernel/setup.c | 33 ++++++++++++++++
>> include/asm-generic/qspinlock.h | 2 +
>> include/asm-generic/ticket_spinlock.h | 2 +
>> 7 files changed, 109 insertions(+), 2 deletions(-)
>> create mode 100644 arch/riscv/include/asm/spinlock.h
>>
>> diff --git a/Documentation/features/locking/queued-spinlocks/arch-support.txt b/Documentation/features/locking/queued-spinlocks/arch-support.txt
>> index 22f2990392ff..cf26042480e2 100644
>> --- a/Documentation/features/locking/queued-spinlocks/arch-support.txt
>> +++ b/Documentation/features/locking/queued-spinlocks/arch-support.txt
>> @@ -20,7 +20,7 @@
>> | openrisc: | ok |
>> | parisc: | TODO |
>> | powerpc: | ok |
>> - | riscv: | TODO |
>> + | riscv: | ok |
>> | s390: | TODO |
>> | sh: | TODO |
>> | sparc: | ok |
>> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
>> index 0bbaec0444d0..5040c7eac70d 100644
>> --- a/arch/riscv/Kconfig
>> +++ b/arch/riscv/Kconfig
>> @@ -72,6 +72,7 @@ config RISCV
>> select ARCH_WANT_OPTIMIZE_HUGETLB_VMEMMAP
>> select ARCH_WANTS_NO_INSTR
>> select ARCH_WANTS_THP_SWAP if HAVE_ARCH_TRANSPARENT_HUGEPAGE
>> + select ARCH_WEAK_RELEASE_ACQUIRE if ARCH_USE_QUEUED_SPINLOCKS
>> select BINFMT_FLAT_NO_DATA_START_OFFSET if !MMU
>> select BUILDTIME_TABLE_SORT if MMU
>> select CLINT_TIMER if RISCV_M_MODE
>> @@ -482,6 +483,34 @@ config NODES_SHIFT
>> Specify the maximum number of NUMA Nodes available on the target
>> system. Increases memory reserved to accommodate various tables.
>>
>> +choice
>> + prompt "RISC-V spinlock type"
>> + default RISCV_COMBO_SPINLOCKS
>> +
>> +config RISCV_TICKET_SPINLOCKS
>> + bool "Using ticket spinlock"
>> +
>> +config RISCV_QUEUED_SPINLOCKS
>> + bool "Using queued spinlock"
>> + depends on SMP && MMU
>> + select ARCH_USE_QUEUED_SPINLOCKS
>> + help
>> + The queued spinlock implementation requires the forward progress
>> + guarantee of cmpxchg()/xchg() atomic operations: CAS with Zabha or
>> + LR/SC with Ziccrse provide such guarantee.
>> +
>> + Select this if and only if Zabha or Ziccrse is available on your
>> + platform.
>> +
>> +config RISCV_COMBO_SPINLOCKS
>> + bool "Using combo spinlock"
>> + depends on SMP && MMU
>> + select ARCH_USE_QUEUED_SPINLOCKS
>> + help
>> + Embed both queued spinlock and ticket lock so that the spinlock
>> + implementation can be chosen at runtime.
>> +endchoice
>> +
>> config RISCV_ALTERNATIVE
>> bool
>> depends on !XIP_KERNEL
>> diff --git a/arch/riscv/include/asm/Kbuild b/arch/riscv/include/asm/Kbuild
>> index 504f8b7e72d4..ad72f2bd4cc9 100644
>> --- a/arch/riscv/include/asm/Kbuild
>> +++ b/arch/riscv/include/asm/Kbuild
>> @@ -2,10 +2,12 @@
>> 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 += spinlock.h
>> generic-y += spinlock_types.h
>> +generic-y += ticket_spinlock.h
>> generic-y += qrwlock.h
>> generic-y += qrwlock_types.h
>> +generic-y += qspinlock.h
>> generic-y += user.h
>> generic-y += vmlinux.lds.h
>> diff --git a/arch/riscv/include/asm/spinlock.h b/arch/riscv/include/asm/spinlock.h
>> new file mode 100644
>> index 000000000000..4856d50006f2
>> --- /dev/null
>> +++ b/arch/riscv/include/asm/spinlock.h
>> @@ -0,0 +1,39 @@
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +
>> +#ifndef __ASM_RISCV_SPINLOCK_H
>> +#define __ASM_RISCV_SPINLOCK_H
>> +
>> +#ifdef CONFIG_RISCV_COMBO_SPINLOCKS
>> +#define _Q_PENDING_LOOPS (1 << 9)
>> +
>> +#define __no_arch_spinlock_redefine
>> +#include <asm/ticket_spinlock.h>
>> +#include <asm/qspinlock.h>
>> +#include <asm/alternative.h>
>> +
>> +DECLARE_STATIC_KEY_TRUE(qspinlock_key);
>> +
>> +#define SPINLOCK_BASE_DECLARE(op, type, type_lock) \
>> +static __always_inline type arch_spin_##op(type_lock lock) \
>> +{ \
>> + if (static_branch_unlikely(&qspinlock_key)) \
>> + return queued_spin_##op(lock); \
>> + return ticket_spin_##op(lock); \
>> +}
>> +
>> +SPINLOCK_BASE_DECLARE(lock, void, arch_spinlock_t *)
>> +SPINLOCK_BASE_DECLARE(unlock, void, arch_spinlock_t *)
>> +SPINLOCK_BASE_DECLARE(is_locked, int, arch_spinlock_t *)
>> +SPINLOCK_BASE_DECLARE(is_contended, int, arch_spinlock_t *)
>> +SPINLOCK_BASE_DECLARE(trylock, bool, arch_spinlock_t *)
>> +SPINLOCK_BASE_DECLARE(value_unlocked, int, arch_spinlock_t)
>> +
>> +#else
>> +
>> +#include <asm/ticket_spinlock.h>
>> +
>> +#endif
>> +
>> +#include <asm/qrwlock.h>
>> +
>> +#endif /* __ASM_RISCV_SPINLOCK_H */
>> diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
>> index 4f73c0ae44b2..d7c31c9b8ead 100644
>> --- a/arch/riscv/kernel/setup.c
>> +++ b/arch/riscv/kernel/setup.c
>> @@ -244,6 +244,38 @@ static void __init parse_dtb(void)
>> #endif
>> }
>>
>> +DEFINE_STATIC_KEY_TRUE(qspinlock_key);
>> +EXPORT_SYMBOL(qspinlock_key);
>> +
>> +static void __init riscv_spinlock_init(void)
>> +{
>> + char *using_ext;
>> +
>> + if (IS_ENABLED(CONFIG_RISCV_ISA_ZACAS) &&
>> + IS_ENABLED(CONFIG_RISCV_ISA_ZABHA)) {
>> + using_ext = "using Zabha";
>> +
>> + asm goto(ALTERNATIVE("j %[no_zacas]", "nop", 0, RISCV_ISA_EXT_ZACAS, 1)
>> + : : : : no_zacas);
>> + asm goto(ALTERNATIVE("nop", "j %[qspinlock]", 0, RISCV_ISA_EXT_ZABHA, 1)
>> + : : : : qspinlock);
>> + }
> I'm okay with this patch.

Great, thanks!

> I suggest putting an arg such as "enable_qspinlock," which people
> could use on the non-ZABHA machines. I hope it could happen in this
> series. That's all I need, thank you very much.

Do you think that's really necessary? I added Ziccrse support just
below, to me that fits your needs to use qspinlocks on !Ziccrse.

BTW, can I add you SoB on this patch? Or a Co-developed-by or anything
to show that you greatly contributed to this patch?

Thanks,

Alex

>
>> +
>> +no_zacas:
>> + using_ext = "using Ziccrse";
>> + asm goto(ALTERNATIVE("nop", "j %[qspinlock]", 0,
>> + RISCV_ISA_EXT_ZICCRSE, 1)
>> + : : : : qspinlock);
>> +
>> + static_branch_disable(&qspinlock_key);
>> + pr_info("Ticket spinlock: enabled\n");
>> +
>> + return;
>> +
>> +qspinlock:
>> + pr_info("Queued spinlock %s: enabled\n", using_ext);
>> +}
>> +
>> extern void __init init_rt_signal_env(void);
>>
>> void __init setup_arch(char **cmdline_p)
>> @@ -295,6 +327,7 @@ void __init setup_arch(char **cmdline_p)
>> riscv_set_dma_cache_alignment();
>>
>> riscv_user_isa_enable();
>> + riscv_spinlock_init();
>> }
>>
>> bool arch_cpu_is_hotpluggable(int cpu)
>> diff --git a/include/asm-generic/qspinlock.h b/include/asm-generic/qspinlock.h
>> index 0655aa5b57b2..bf47cca2c375 100644
>> --- a/include/asm-generic/qspinlock.h
>> +++ b/include/asm-generic/qspinlock.h
>> @@ -136,6 +136,7 @@ static __always_inline bool virt_spin_lock(struct qspinlock *lock)
>> }
>> #endif
>>
>> +#ifndef __no_arch_spinlock_redefine
>> /*
>> * Remapping spinlock architecture specific functions to the corresponding
>> * queued spinlock functions.
>> @@ -146,5 +147,6 @@ static __always_inline bool virt_spin_lock(struct qspinlock *lock)
>> #define arch_spin_lock(l) queued_spin_lock(l)
>> #define arch_spin_trylock(l) queued_spin_trylock(l)
>> #define arch_spin_unlock(l) queued_spin_unlock(l)
>> +#endif
>>
>> #endif /* __ASM_GENERIC_QSPINLOCK_H */
>> diff --git a/include/asm-generic/ticket_spinlock.h b/include/asm-generic/ticket_spinlock.h
>> index cfcff22b37b3..325779970d8a 100644
>> --- a/include/asm-generic/ticket_spinlock.h
>> +++ b/include/asm-generic/ticket_spinlock.h
>> @@ -89,6 +89,7 @@ static __always_inline int ticket_spin_is_contended(arch_spinlock_t *lock)
>> return (s16)((val >> 16) - (val & 0xffff)) > 1;
>> }
>>
>> +#ifndef __no_arch_spinlock_redefine
>> /*
>> * Remapping spinlock architecture specific functions to the corresponding
>> * ticket spinlock functions.
>> @@ -99,5 +100,6 @@ static __always_inline int ticket_spin_is_contended(arch_spinlock_t *lock)
>> #define arch_spin_lock(l) ticket_spin_lock(l)
>> #define arch_spin_trylock(l) ticket_spin_trylock(l)
>> #define arch_spin_unlock(l) ticket_spin_unlock(l)
>> +#endif
>>
>> #endif /* __ASM_GENERIC_TICKET_SPINLOCK_H */
>> --
>> 2.39.2
>>