Re: [PATCH V10 07/19] riscv: qspinlock: errata: Introduce ERRATA_THEAD_QSPINLOCK

Guo Ren
2023-08-08T02:12:41Z

On Mon, Aug 07, 2023 at 01:23:34AM -0400, Stefan O'Rear wrote:
> On Wed, Aug 2, 2023, at 12:46 PM, guoren@kernel.org wrote:
> > From: Guo Ren <guoren@linux.alibaba.com>
> >
> > According to qspinlock requirements, RISC-V gives out a weak LR/SC
> > forward progress guarantee which does not satisfy qspinlock. But
> > many vendors could produce stronger forward guarantee LR/SC to
> > ensure the xchg_tail could be finished in time on any kind of
> > hart. T-HEAD is the vendor which implements strong forward
> > guarantee LR/SC instruction pairs, so enable qspinlock for T-HEAD
> > with errata help.
> >
> > T-HEAD early version of processors has the merge buffer delay
> > problem, so we need ERRATA_WRITEONCE to support qspinlock.
> >
> > Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
> > Signed-off-by: Guo Ren <guoren@kernel.org>
> > ---
> > arch/riscv/Kconfig.errata | 13 +++++++++++++
> > arch/riscv/errata/thead/errata.c | 24 ++++++++++++++++++++++++
> > arch/riscv/include/asm/errata_list.h | 20 ++++++++++++++++++++
> > arch/riscv/include/asm/vendorid_list.h | 3 ++-
> > arch/riscv/kernel/cpufeature.c | 3 ++-
> > 5 files changed, 61 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/riscv/Kconfig.errata b/arch/riscv/Kconfig.errata
> > index 4745a5c57e7c..eb43677b13cc 100644
> > --- a/arch/riscv/Kconfig.errata
> > +++ b/arch/riscv/Kconfig.errata
> > @@ -96,4 +96,17 @@ config ERRATA_THEAD_WRITE_ONCE
> >
> > If you don't know what to do here, say "Y".
> >
> > +config ERRATA_THEAD_QSPINLOCK
> > + bool "Apply T-Head queued spinlock errata"
> > + depends on ERRATA_THEAD
> > + default y
> > + help
> > + The T-HEAD C9xx processors implement strong fwd guarantee LR/SC to
> > + match the xchg_tail requirement of qspinlock.
> > +
> > + This will apply the QSPINLOCK errata to handle the non-standard
> > + behavior via using qspinlock instead of ticket_lock.
> > +
> > + If you don't know what to do here, say "Y".
>
> If this is to be applied, I would like to see a detailed explanation somewhere,
> preferably with citations, of:
>
> (a) The memory model requirements for qspinlock
These were written in commit: a8ad07e5240 ("asm-generic: qspinlock: Indicate the use of
mixed-size atomics"). For riscv, the most controversial point is xchg_tail()
implementation for native queued spinlock.

> (b) Why, with arguments, RISC-V does not architecturally meet (a)
In the spec "Eventual Success of Store-Conditional Instructions":
"By contrast, if other harts or devices continue to write to that reservation set, it is
not guaranteed that any hart will exit its LR/SC loop."

1. The arch_spinlock_t is 32-bit width, and it contains LOCK_PENDING
part and IDX_TAIL part.
- LOCK: lock holder
- PENDING: next waiter (Only once per contended situation)
- IDX: nested context (normal, hwirq, softirq, nmi)
- TAIL: last contended cpu
The xchg_tail operate on IDX_TAIL part, so there is no guarantee on "NO"
"other harts or devices continue to write to that reservation set".

2. When you do lock torture test, you may see a long contended ring queue:
xchg_tail
+-----> CPU4 (big core)
|
CPU3 (lock holder) -> CPU1 (mcs queued) -> CPU2 (mcs queued) ----+-----> CPU0 (little core)
| |
| +-----> CPU5 (big core)
| |
+--locktorture release lock (spin_unlock) and spin_lock again --+-----> CPU3 (big core)

If CPU0 doesn't have a strong fwd guarantee, xhg_tail is consistently failed.

> (c) Why, with arguments, T-HEAD C9xx meets (a)
> (d) Why at least one other architecture which defines ARCH_USE_QUEUED_SPINLOCKS
> meets (a)
I can't give the C9xx microarch implementation detail. But many
open-source riscv cores have provided strong forward progress guarantee
LR/SC implementation [1] [2]. But I would say these implementations are
too rude, which makes LR send a cacheline unique interconnect request.
It satisfies xchg_tail but not cmpxchg & cond_load. CPU vendors should
carefully consider your LR/SC fwd guarantee implementation.

[1]: https://github.com/riscv-boom/riscv-boom/blob/v3.0.0/src/main/scala/lsu/dcache.scala#L650
[2]: https://github.com/OpenXiangShan/XiangShan/blob/v1.0/src/main/scala/xiangshan/cache/MainPipe.scala#L470

>
> As far as I can tell, the RISC-V guarantees concerning constrained LR/SC loops
> (livelock freedom but no starvation freedom) are exactly the same as those in
> Armv8 (as of 0487F.c) for equivalent loops, and xchg_tail compiles to a
> constrained LR/SC loop with guaranteed eventual success (with -O1). Clearly you
> disagree; I would like to see your perspective.
For Armv8, I would use LSE for the lock-contended scenario. Ref this
commit 0ea366f5e1b6: ("arm64: atomics: prefetch the destination word for
write prior to stxr").

>
> -s
>
> > +
> > endmenu # "CPU errata selection"
> > diff --git a/arch/riscv/errata/thead/errata.c b/arch/riscv/errata/thead/errata.c
> > index 881729746d2e..d560dc45c0e7 100644
> > --- a/arch/riscv/errata/thead/errata.c
> > +++ b/arch/riscv/errata/thead/errata.c
> > @@ -86,6 +86,27 @@ static bool errata_probe_write_once(unsigned int stage,
> > return false;
> > }
> >
> > +static bool errata_probe_qspinlock(unsigned int stage,
> > + unsigned long arch_id, unsigned long impid)
> > +{
> > + if (!IS_ENABLED(CONFIG_ERRATA_THEAD_QSPINLOCK))
> > + return false;
> > +
> > + /*
> > + * The queued_spinlock torture would get in livelock without
> > + * ERRATA_THEAD_WRITE_ONCE fixup for the early versions of T-HEAD
> > + * processors.
> > + */
> > + if (arch_id == 0 && impid == 0 &&
> > + !IS_ENABLED(CONFIG_ERRATA_THEAD_WRITE_ONCE))
> > + return false;
> > +
> > + if (stage == RISCV_ALTERNATIVES_EARLY_BOOT)
> > + return true;
> > +
> > + return false;
> > +}
> > +
> > static u32 thead_errata_probe(unsigned int stage,
> > unsigned long archid, unsigned long impid)
> > {
> > @@ -103,6 +124,9 @@ static u32 thead_errata_probe(unsigned int stage,
> > if (errata_probe_write_once(stage, archid, impid))
> > cpu_req_errata |= BIT(ERRATA_THEAD_WRITE_ONCE);
> >
> > + if (errata_probe_qspinlock(stage, archid, impid))
> > + cpu_req_errata |= BIT(ERRATA_THEAD_QSPINLOCK);
> > +
> > return cpu_req_errata;
> > }
> >
> > diff --git a/arch/riscv/include/asm/errata_list.h
> > b/arch/riscv/include/asm/errata_list.h
> > index fbb2b8d39321..a696d18d1b0d 100644
> > --- a/arch/riscv/include/asm/errata_list.h
> > +++ b/arch/riscv/include/asm/errata_list.h
> > @@ -141,6 +141,26 @@ asm volatile(ALTERNATIVE( \
> > : "=r" (__ovl) : \
> > : "memory")
> >
> > +static __always_inline bool
> > +riscv_has_errata_thead_qspinlock(void)
> > +{
> > + if (IS_ENABLED(CONFIG_RISCV_ALTERNATIVE)) {
> > + asm_volatile_goto(
> > + ALTERNATIVE(
> > + "j %l[l_no]", "nop",
> > + THEAD_VENDOR_ID,
> > + ERRATA_THEAD_QSPINLOCK,
> > + CONFIG_ERRATA_THEAD_QSPINLOCK)
> > + : : : : l_no);
> > + } else {
> > + goto l_no;
> > + }
> > +
> > + return true;
> > +l_no:
> > + return false;
> > +}
> > +
> > #endif /* __ASSEMBLY__ */
> >
> > #endif
> > diff --git a/arch/riscv/include/asm/vendorid_list.h
> > b/arch/riscv/include/asm/vendorid_list.h
> > index 73078cfe4029..1f1d03877f5f 100644
> > --- a/arch/riscv/include/asm/vendorid_list.h
> > +++ b/arch/riscv/include/asm/vendorid_list.h
> > @@ -19,7 +19,8 @@
> > #define ERRATA_THEAD_CMO 1
> > #define ERRATA_THEAD_PMU 2
> > #define ERRATA_THEAD_WRITE_ONCE 3
> > -#define ERRATA_THEAD_NUMBER 4
> > +#define ERRATA_THEAD_QSPINLOCK 4
> > +#define ERRATA_THEAD_NUMBER 5
> > #endif
> >
> > #endif
> > diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> > index f8dbbe1bbd34..d9694fe40a9a 100644
> > --- a/arch/riscv/kernel/cpufeature.c
> > +++ b/arch/riscv/kernel/cpufeature.c
> > @@ -342,7 +342,8 @@ void __init riscv_fill_hwcap(void)
> > * spinlock value, the only way is to change from queued_spinlock to
> > * ticket_spinlock, but can not be vice.
> > */
> > - if (!force_qspinlock) {
> > + if (!force_qspinlock &&
> > + !riscv_has_errata_thead_qspinlock()) {
> > set_bit(RISCV_ISA_EXT_XTICKETLOCK, isainfo->isa);
> > }
> > #endif
> > --
> > 2.36.1
> >
> >
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