The membarrier system call requires a full memory barrier after storing
to rq->curr, before going back to user-space. The barrier is only
needed when switching between processes: the barrier is implied by
mmdrop() when switching from kernel to userspace, and it's not needed
when switching from userspace to kernel.
Rely on the feature/mechanism ARCH_HAS_MEMBARRIER_CALLBACKS and on the
primitive membarrier_arch_switch_mm(), already adopted by the PowerPC
architecture, to insert the required barrier.
Fixes: fab957c11efe2f ("RISC-V: Atomic and Locking Code")
Signed-off-by: Andrea Parri <parri.andrea@xxxxxxxxx>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
---
MAINTAINERS | 2 +-
arch/riscv/Kconfig | 1 +
arch/riscv/include/asm/membarrier.h | 31 +++++++++++++++++++++++++++++
arch/riscv/mm/context.c | 2 ++
kernel/sched/core.c | 5 +++--
5 files changed, 38 insertions(+), 3 deletions(-)
create mode 100644 arch/riscv/include/asm/membarrier.h
diff --git a/MAINTAINERS b/MAINTAINERS
index 8999497011a26..d7ed83b73cbe4 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -14040,7 +14040,7 @@ M: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
M: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
L: linux-kernel@xxxxxxxxxxxxxxx
S: Supported
-F: arch/powerpc/include/asm/membarrier.h
+F: arch/*/include/asm/membarrier.h
F: include/uapi/linux/membarrier.h
F: kernel/sched/membarrier.c
diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index bffbd869a0682..087abf9e51c69 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -27,6 +27,7 @@ config RISCV
select ARCH_HAS_GCOV_PROFILE_ALL
select ARCH_HAS_GIGANTIC_PAGE
select ARCH_HAS_KCOV
+ select ARCH_HAS_MEMBARRIER_CALLBACKS
select ARCH_HAS_MMIOWB
select ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
select ARCH_HAS_PMEM_API
diff --git a/arch/riscv/include/asm/membarrier.h b/arch/riscv/include/asm/membarrier.h
new file mode 100644
index 0000000000000..6c016ebb5020a
--- /dev/null
+++ b/arch/riscv/include/asm/membarrier.h
@@ -0,0 +1,31 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef _ASM_RISCV_MEMBARRIER_H
+#define _ASM_RISCV_MEMBARRIER_H
+
+static inline void membarrier_arch_switch_mm(struct mm_struct *prev,
+ struct mm_struct *next,
+ struct task_struct *tsk)
+{
+ /*
+ * Only need the full barrier when switching between processes.
+ * Barrier when switching from kernel to userspace is not
+ * required here, given that it is implied by mmdrop(). Barrier
+ * when switching from userspace to kernel is not needed after
+ * store to rq->curr.
+ */
+ if (IS_ENABLED(CONFIG_SMP) &&
+ likely(!(atomic_read(&next->membarrier_state) &
+ (MEMBARRIER_STATE_PRIVATE_EXPEDITED |
+ MEMBARRIER_STATE_GLOBAL_EXPEDITED)) || !prev))
+ return;
+
+ /*
+ * The membarrier system call requires a full memory barrier
+ * after storing to rq->curr, before going back to user-space.
+ * Matches a full barrier in the proximity of the membarrier
+ * system call entry.
+ */
+ smp_mb();
+}
+
+#endif /* _ASM_RISCV_MEMBARRIER_H */
diff --git a/arch/riscv/mm/context.c b/arch/riscv/mm/context.c
index 217fd4de61342..ba8eb3944687c 100644
--- a/arch/riscv/mm/context.c
+++ b/arch/riscv/mm/context.c
@@ -323,6 +323,8 @@ void switch_mm(struct mm_struct *prev, struct mm_struct *next,
if (unlikely(prev == next))
return;
+ membarrier_arch_switch_mm(prev, next, task);
+
/*
* Mark the current MM context as inactive, and the next as
* active. This is at least used by the icache flushing
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9116bcc903467..c4ca8085885a3 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6709,8 +6709,9 @@ static void __sched notrace __schedule(unsigned int sched_mode)
*
* Here are the schemes providing that barrier on the
* various architectures:
- * - mm ? switch_mm() : mmdrop() for x86, s390, sparc, PowerPC.
- * switch_mm() rely on membarrier_arch_switch_mm() on PowerPC.
+ * - mm ? switch_mm() : mmdrop() for x86, s390, sparc, PowerPC,
+ * RISC-V. switch_mm() relies on membarrier_arch_switch_mm()
+ * on PowerPC and on RISC-V.
* - finish_lock_switch() for weakly-ordered
* architectures where spin_unlock is a full barrier,
* - switch_to() for arm64 (weakly-ordered, spin_unlock
--
2.34.1
