1 files changed, 50 insertions, 0 deletions
diff --git a/arch/riscv/include/asm/membarrier.h b/arch/riscv/include/asm/membarrier.h
new file mode 100644
index 00000000000000..47b240d0d596a0
--- /dev/null
+++ b/arch/riscv/include/asm/membarrier.h
@@ -0,0 +1,50 @@
+/* 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.
+	 *
+	 * This barrier is also needed for the SYNC_CORE command when
+	 * switching between processes; in particular, on a transition
+	 * from a thread belonging to another mm to a thread belonging
+	 * to the mm for which a membarrier SYNC_CORE is done on CPU0:
+	 *
+	 *   - [CPU0] sets all bits in the mm icache_stale_mask (in
+	 *     prepare_sync_core_cmd());
+	 *
+	 *   - [CPU1] stores to rq->curr (by the scheduler);
+	 *
+	 *   - [CPU0] loads rq->curr within membarrier and observes
+	 *     cpu_rq(1)->curr->mm != mm, so the IPI is skipped on
+	 *     CPU1; this means membarrier relies on switch_mm() to
+	 *     issue the sync-core;
+	 *
+	 *   - [CPU1] switch_mm() loads icache_stale_mask; if the bit
+	 *     is zero, switch_mm() may incorrectly skip the sync-core.
+	 *
+	 * Matches a full barrier in the proximity of the membarrier
+	 * system call entry.
+	 */
+	smp_mb();
+}
+
+#endif /* _ASM_RISCV_MEMBARRIER_H */