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Date:      Wed, 2 May 2018 08:00:57 +0000 (UTC)
From:      Konstantin Belousov <kib@FreeBSD.org>
To:        src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-stable@freebsd.org, svn-src-stable-11@freebsd.org
Subject:   svn commit: r333163 - stable/11/sys/x86/x86
Message-ID:  <201805020800.w4280vQF080590@repo.freebsd.org>
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Author: kib
Date: Wed May  2 08:00:57 2018
New Revision: 333163
URL: https://svnweb.freebsd.org/changeset/base/333163

Log:
  MFC r332934:
  Use relaxed atomics to access the monitor line.

Modified:
  stable/11/sys/x86/x86/cpu_machdep.c
Directory Properties:
  stable/11/   (props changed)

Modified: stable/11/sys/x86/x86/cpu_machdep.c
==============================================================================
--- stable/11/sys/x86/x86/cpu_machdep.c	Wed May  2 07:57:36 2018	(r333162)
+++ stable/11/sys/x86/x86/cpu_machdep.c	Wed May  2 08:00:57 2018	(r333163)
@@ -171,12 +171,12 @@ acpi_cpu_idle_mwait(uint32_t mwait_hint)
 	 */
 
 	state = (int *)PCPU_PTR(monitorbuf);
-	KASSERT(*state == STATE_SLEEPING,
-		("cpu_mwait_cx: wrong monitorbuf state"));
-	*state = STATE_MWAIT;
+	KASSERT(atomic_load_int(state) == STATE_SLEEPING,
+	    ("cpu_mwait_cx: wrong monitorbuf state"));
+	atomic_store_int(state, STATE_MWAIT);
 	handle_ibrs_entry();
 	cpu_monitor(state, 0, 0);
-	if (*state == STATE_MWAIT)
+	if (atomic_load_int(state) == STATE_MWAIT)
 		cpu_mwait(MWAIT_INTRBREAK, mwait_hint);
 	handle_ibrs_exit();
 
@@ -184,7 +184,7 @@ acpi_cpu_idle_mwait(uint32_t mwait_hint)
 	 * We should exit on any event that interrupts mwait, because
 	 * that event might be a wanted interrupt.
 	 */
-	*state = STATE_RUNNING;
+	atomic_store_int(state, STATE_RUNNING);
 }
 
 /* Get current clock frequency for the given cpu id. */
@@ -430,7 +430,7 @@ cpu_idle_acpi(sbintime_t sbt)
 	int *state;
 
 	state = (int *)PCPU_PTR(monitorbuf);
-	*state = STATE_SLEEPING;
+	atomic_store_int(state, STATE_SLEEPING);
 
 	/* See comments in cpu_idle_hlt(). */
 	disable_intr();
@@ -440,7 +440,7 @@ cpu_idle_acpi(sbintime_t sbt)
 		cpu_idle_hook(sbt);
 	else
 		acpi_cpu_c1();
-	*state = STATE_RUNNING;
+	atomic_store_int(state, STATE_RUNNING);
 }
 #endif /* !PC98 */
 
@@ -450,7 +450,7 @@ cpu_idle_hlt(sbintime_t sbt)
 	int *state;
 
 	state = (int *)PCPU_PTR(monitorbuf);
-	*state = STATE_SLEEPING;
+	atomic_store_int(state, STATE_SLEEPING);
 
 	/*
 	 * Since we may be in a critical section from cpu_idle(), if
@@ -473,7 +473,7 @@ cpu_idle_hlt(sbintime_t sbt)
 		enable_intr();
 	else
 		acpi_cpu_c1();
-	*state = STATE_RUNNING;
+	atomic_store_int(state, STATE_RUNNING);
 }
 
 static void
@@ -482,21 +482,22 @@ cpu_idle_mwait(sbintime_t sbt)
 	int *state;
 
 	state = (int *)PCPU_PTR(monitorbuf);
-	*state = STATE_MWAIT;
+	atomic_store_int(state, STATE_MWAIT);
 
 	/* See comments in cpu_idle_hlt(). */
 	disable_intr();
 	if (sched_runnable()) {
+		atomic_store_int(state, STATE_RUNNING);
 		enable_intr();
-		*state = STATE_RUNNING;
 		return;
 	}
+
 	cpu_monitor(state, 0, 0);
-	if (*state == STATE_MWAIT)
+	if (atomic_load_int(state) == STATE_MWAIT)
 		__asm __volatile("sti; mwait" : : "a" (MWAIT_C1), "c" (0));
 	else
 		enable_intr();
-	*state = STATE_RUNNING;
+	atomic_store_int(state, STATE_RUNNING);
 }
 
 static void
@@ -506,7 +507,7 @@ cpu_idle_spin(sbintime_t sbt)
 	int i;
 
 	state = (int *)PCPU_PTR(monitorbuf);
-	*state = STATE_RUNNING;
+	atomic_store_int(state, STATE_RUNNING);
 
 	/*
 	 * The sched_runnable() call is racy but as long as there is
@@ -604,20 +605,21 @@ out:
 int
 cpu_idle_wakeup(int cpu)
 {
-	struct pcpu *pcpu;
 	int *state;
 
-	pcpu = pcpu_find(cpu);
-	state = (int *)pcpu->pc_monitorbuf;
-	/*
-	 * This doesn't need to be atomic since missing the race will
-	 * simply result in unnecessary IPIs.
-	 */
-	if (*state == STATE_SLEEPING)
+	state = (int *)pcpu_find(cpu)->pc_monitorbuf;
+	switch (atomic_load_int(state)) {
+	case STATE_SLEEPING:
 		return (0);
-	if (*state == STATE_MWAIT)
-		*state = STATE_RUNNING;
-	return (1);
+	case STATE_MWAIT:
+		atomic_store_int(state, STATE_RUNNING);
+		return (1);
+	case STATE_RUNNING:
+		return (1);
+	default:
+		panic("bad monitor state");
+		return (1);
+	}
 }
 
 /*

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