[RFC][PATCH] locking: Generic ticket-lock

Peter Zijlstra peterz at infradead.org
Wed Apr 14 11:16:38 BST 2021 

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 ...

FWIW this is broken, anything SMP *MUST* define mb(), at the very least.

> 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.

---
 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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