Re: [PATCH 2/8] x86/mm: Handle unlazying membarrier core sync in the arch code

On 6/16/21 10:49 AM, Mathieu Desnoyers wrote:

----- On Jun 15, 2021, at 11:21 PM, Andy Lutomirski luto@kernel.org wrote:
[...]

quoted

@@ -473,16 +474,24 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct

mm_struct *next,

[...]

quoted

@@ -510,16 +520,35 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct

mm_struct *next,
		 * If the TLB is up to date, just use it.
		 * The barrier synchronizes with the tlb_gen increment in
		 * the TLB shootdown code.
+		 *
+		 * As a future optimization opportunity, it's plausible
+		 * that the x86 memory model is strong enough that this
+		 * smp_mb() isn't needed.
		 */
		smp_mb();
		next_tlb_gen = atomic64_read(&next->context.tlb_gen);
		if (this_cpu_read(cpu_tlbstate.ctxs[prev_asid].tlb_gen) ==
-				next_tlb_gen)
+		    next_tlb_gen) {
+#ifdef CONFIG_MEMBARRIER
+			/*
+			 * We switched logical mm but we're not going to
+			 * write to CR3.  We already did smp_mb() above,
+			 * but membarrier() might require a sync_core()
+			 * as well.
+			 */
+			if (unlikely(atomic_read(&next->membarrier_state) &
+				     MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE))
+				sync_core_before_usermode();
+#endif
+
			return;
+		}

[...]

I find that mixing up preprocessor #ifdef and code logic hurts readability.
Can you lift this into a static function within the same compile unit, and
provides an empty implementation for the #else case ?

Done.

Thanks,

Mathieu

	prev->sched_class->task_dead(prev);