[RFC][PATCH] locking: Generic ticket-lock
Stafford Horne
shorne at gmail.com
Wed Apr 14 21:47:34 BST 2021
On Wed, Apr 14, 2021 at 12:16:38PM +0200, Peter Zijlstra wrote:
> On Wed, Apr 14, 2021 at 11:05:24AM +0200, Peter Zijlstra wrote:
>
> > That made me look at the qspinlock code, and queued_spin_*lock() uses
> > atomic_try_cmpxchg_acquire(), which means any arch that uses qspinlock
> > and has RCpc atomics will give us massive pain.
> >
> > Current archs using qspinlock are: x86, arm64, power, sparc64, mips and
> > openrisc (WTF?!).
> >
> > Of those, x86 and sparc are TSO archs with SC atomics, arm64 has RCsc
> > atomics, power has RCtso atomics (and is the arch we all hate for having
> > RCtso locks).
> >
> > Now MIPS has all sorts of ill specified barriers, but last time looked
> > at it it didn't actually use any of that and stuck to using smp_mb(), so
> > it will have RCsc atomics.
> >
> > /me goes look at wth openrisc is.. doesn't even appear to have
> > asm/barrier.h :-/ Looking at wikipedia it also doesn't appear to
> > actually have hardware ...
Yes, not hardware available to consumers directoy, my development is done on
FPGAs.
> FWIW this is broken, anything SMP *MUST* define mb(), at the very least.
Oh, thats right, something missed, when we developed qspinlocks we discussed
this and my point there was that l.swa/l.lwa implied a mem flush
l.msync/barrier. But mb still needs to be added.
> > I'm thinking openrisc is a prime candidate for this ticket_lock.h we're
> > all talking about.
>
> How's this then? Compile tested only on openrisc/simple_smp_defconfig.
I did my testing with this FPGA build SoC:
https://github.com/stffrdhrn/de0_nano-multicore
Note, the CPU timer sync logic uses mb() and is a bit flaky. So missing mb()
might be a reason. I thought we had defined mb() and l.msync, but it seems to
have gotten lost.
With that said I could test out this ticket-lock implementation. How would I
tell if its better than qspinlock?
> ---
> arch/openrisc/Kconfig | 1 -
> arch/openrisc/include/asm/Kbuild | 5 +-
> arch/openrisc/include/asm/spinlock.h | 3 +-
> arch/openrisc/include/asm/spinlock_types.h | 2 +-
> include/asm-generic/qspinlock.h | 30 +++++++++++
> include/asm-generic/ticket-lock-types.h | 11 ++++
> include/asm-generic/ticket-lock.h | 86 ++++++++++++++++++++++++++++++
> 7 files changed, 131 insertions(+), 7 deletions(-)
>
> diff --git a/arch/openrisc/Kconfig b/arch/openrisc/Kconfig
> index 591acc5990dc..1858cf309f1f 100644
> --- a/arch/openrisc/Kconfig
> +++ b/arch/openrisc/Kconfig
> @@ -32,7 +32,6 @@ config OPENRISC
> select HAVE_DEBUG_STACKOVERFLOW
> select OR1K_PIC
> select CPU_NO_EFFICIENT_FFS if !OPENRISC_HAVE_INST_FF1
> - select ARCH_USE_QUEUED_SPINLOCKS
> select ARCH_USE_QUEUED_RWLOCKS
> select OMPIC if SMP
> select ARCH_WANT_FRAME_POINTERS
> diff --git a/arch/openrisc/include/asm/Kbuild b/arch/openrisc/include/asm/Kbuild
> index ca5987e11053..cb260e7d73db 100644
> --- a/arch/openrisc/include/asm/Kbuild
> +++ b/arch/openrisc/include/asm/Kbuild
> @@ -1,9 +1,8 @@
> # SPDX-License-Identifier: GPL-2.0
> generic-y += extable.h
> generic-y += kvm_para.h
> -generic-y += mcs_spinlock.h
> -generic-y += qspinlock_types.h
> -generic-y += qspinlock.h
> +generic-y += ticket-lock.h
> +generic-y += ticket-lock-types.h
> generic-y += qrwlock_types.h
> generic-y += qrwlock.h
> generic-y += user.h
> diff --git a/arch/openrisc/include/asm/spinlock.h b/arch/openrisc/include/asm/spinlock.h
> index a8940bdfcb7e..0b839ed1f3a0 100644
> --- a/arch/openrisc/include/asm/spinlock.h
> +++ b/arch/openrisc/include/asm/spinlock.h
> @@ -15,8 +15,7 @@
> #ifndef __ASM_OPENRISC_SPINLOCK_H
> #define __ASM_OPENRISC_SPINLOCK_H
>
> -#include <asm/qspinlock.h>
> -
> +#include <asm/ticket-lock.h>
> #include <asm/qrwlock.h>
>
> #define arch_read_lock_flags(lock, flags) arch_read_lock(lock)
> diff --git a/arch/openrisc/include/asm/spinlock_types.h b/arch/openrisc/include/asm/spinlock_types.h
> index 7c6fb1208c88..58ea31fa65ce 100644
> --- a/arch/openrisc/include/asm/spinlock_types.h
> +++ b/arch/openrisc/include/asm/spinlock_types.h
> @@ -1,7 +1,7 @@
> #ifndef _ASM_OPENRISC_SPINLOCK_TYPES_H
> #define _ASM_OPENRISC_SPINLOCK_TYPES_H
>
> -#include <asm/qspinlock_types.h>
> +#include <asm/ticket-lock-types.h>
> #include <asm/qrwlock_types.h>
>
> #endif /* _ASM_OPENRISC_SPINLOCK_TYPES_H */
> diff --git a/include/asm-generic/qspinlock.h b/include/asm-generic/qspinlock.h
> index d74b13825501..a7a1296b0b4d 100644
> --- a/include/asm-generic/qspinlock.h
> +++ b/include/asm-generic/qspinlock.h
> @@ -2,6 +2,36 @@
> /*
> * Queued spinlock
> *
> + * A 'generic' spinlock implementation that is based on MCS locks. An
> + * architecture that's looking for a 'generic' spinlock, please first consider
> + * ticket-lock.h and only come looking here when you've considered all the
> + * constraints below and can show your hardware does actually perform better
> + * with qspinlock.
> + *
> + *
> + * It relies on atomic_*_release()/atomic_*_acquire() to be RCsc (or no weaker
> + * than RCtso if you're power), where regular code only expects atomic_t to be
> + * RCpc.
> + *
> + * It relies on a far greater (compared to ticket-lock.h) set of atomic
> + * operations to behave well together, please audit them carefully to ensure
> + * they all have forward progress. Many atomic operations may default to
> + * cmpxchg() loops which will not have good forward progress properties on
> + * LL/SC architectures.
> + *
> + * One notable example is atomic_fetch_or_acquire(), which x86 cannot (cheaply)
> + * do. Carefully read the patches that introduced queued_fetch_set_pending_acquire().
> + *
> + * It also heavily relies on mixed size atomic operations, in specific it
> + * requires architectures to have xchg16; something which many LL/SC
> + * architectures need to implement as a 32bit and+or in order to satisfy the
> + * forward progress guarantees mentioned above.
> + *
> + * Further reading on mixed size atomics that might be relevant:
> + *
> + * http://www.cl.cam.ac.uk/~pes20/popl17/mixed-size.pdf
> + *
> + *
> * (C) Copyright 2013-2015 Hewlett-Packard Development Company, L.P.
> * (C) Copyright 2015 Hewlett-Packard Enterprise Development LP
> *
> diff --git a/include/asm-generic/ticket-lock-types.h b/include/asm-generic/ticket-lock-types.h
> new file mode 100644
> index 000000000000..829759aedda8
> --- /dev/null
> +++ b/include/asm-generic/ticket-lock-types.h
> @@ -0,0 +1,11 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef __ASM_GENERIC_TICKET_LOCK_TYPES_H
> +#define __ASM_GENERIC_TICKET_LOCK_TYPES_H
> +
> +#include <linux/types.h>
> +typedef atomic_t arch_spinlock_t;
> +
> +#define __ARCH_SPIN_LOCK_UNLOCKED ATOMIC_INIT(0)
> +
> +#endif /* __ASM_GENERIC_TICKET_LOCK_TYPES_H */
> diff --git a/include/asm-generic/ticket-lock.h b/include/asm-generic/ticket-lock.h
> new file mode 100644
> index 000000000000..3f0d53e21a37
> --- /dev/null
> +++ b/include/asm-generic/ticket-lock.h
> @@ -0,0 +1,86 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +/*
> + * 'Generic' ticket-lock implementation.
> + *
> + * It relies on atomic_fetch_add() having well defined forward progress
> + * guarantees under contention. If your architecture cannot provide this, stick
> + * to a test-and-set lock.
> + *
> + * It also relies on atomic_fetch_add() being safe vs smp_store_release() on a
> + * sub-word of the value. This is generally true for anything LL/SC although
> + * you'd be hard pressed to find anything useful in architecture specifications
> + * about this. If your architecture cannot do this you might be better off with
> + * a test-and-set.
> + *
> + * It further assumes atomic_*_release() + atomic_*_acquire() is RCpc and hence
> + * uses atomic_fetch_add() which is SC to create an RCsc lock.
> + *
> + * The implementation uses smp_cond_load_acquire() to spin, so if the
> + * architecture has WFE like instructions to sleep instead of poll for word
> + * modifications be sure to implement that (see ARM64 for example).
> + *
> + */
> +
> +#ifndef __ASM_GENERIC_TICKET_LOCK_H
> +#define __ASM_GENERIC_TICKET_LOCK_H
> +
> +#include <linux/atomic.h>
> +#include <asm/ticket-lock-types.h>
> +
> +static __always_inline void ticket_lock(arch_spinlock_t *lock)
> +{
> + u32 val = atomic_fetch_add(1<<16, lock); /* SC, gives us RCsc */
> + u16 ticket = val >> 16;
> +
> + if (ticket == (u16)val)
> + return;
> +
> + atomic_cond_read_acquire(lock, ticket == (u16)VAL);
> +}
> +
> +static __always_inline bool ticket_trylock(arch_spinlock_t *lock)
> +{
> + u32 old = atomic_read(lock);
> +
> + if ((old >> 16) != (old & 0xffff))
> + return false;
> +
> + return atomic_try_cmpxchg(lock, &old, old + (1<<16)); /* SC, for RCsc */
> +}
> +
> +static __always_inline void ticket_unlock(arch_spinlock_t *lock)
> +{
> + u16 *ptr = (u16 *)lock + __is_defined(__BIG_ENDIAN);
> + u32 val = atomic_read(lock);
> +
> + smp_store_release(ptr, (u16)val + 1);
> +}
> +
> +static __always_inline int ticket_is_locked(arch_spinlock_t *lock)
> +{
> + u32 val = atomic_read(lock);
> +
> + return ((val >> 16) != (val & 0xffff));
> +}
> +
> +static __always_inline int ticket_is_contended(arch_spinlock_t *lock)
> +{
> + u32 val = atomic_read(lock);
> +
> + return (s16)((val >> 16) - (val & 0xffff)) > 1;
> +}
> +
> +static __always_inline int ticket_value_unlocked(arch_spinlock_t lock)
> +{
> + return !ticket_is_locked(&lock);
> +}
> +
> +#define arch_spin_lock(l) ticket_lock(l)
> +#define arch_spin_trylock(l) ticket_trylock(l)
> +#define arch_spin_unlock(l) ticket_unlock(l)
> +#define arch_spin_is_locked(l) ticket_is_locked(l)
> +#define arch_spin_is_contended(l) ticket_is_contended(l)
> +#define arch_spin_value_unlocked(l) ticket_value_unlocked(l)
> +
> +#endif /* __ASM_GENERIC_TICKET_LOCK_H */
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