Author: kib
Date: Tue Apr 24 14:02:46 2018
New Revision: 332934
URL: https://svnweb.freebsd.org/changeset/base/332934
Log:
Use relaxed atomics to access the monitor line.
We must ensure that accesses occur, they do not have any other
compiler-visible effects. Bruce found some situations where
optimization could remove an access, and provided a patch to use
volatile qualifier for the state variables. Since volatile behaviour
there is the compiler-specific interpretation of the keyword, use
relaxed atomics instead, which gives exactly the desired semantic.
Noted by and discussed with: bde
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Modified:
head/sys/x86/x86/cpu_machdep.c
Modified: head/sys/x86/x86/cpu_machdep.c
==============================================================================
--- head/sys/x86/x86/cpu_machdep.c Tue Apr 24 13:52:39 2018
(r332933)
+++ head/sys/x86/x86/cpu_machdep.c Tue Apr 24 14:02:46 2018
(r332934)
@@ -163,12 +163,12 @@ 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();
@@ -176,7 +176,7 @@ 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. */
@@ -408,7 +408,7 @@ 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();
@@ -418,7 +418,7 @@ cpu_idle_acpi(sbintime_t sbt)
cpu_idle_hook(sbt);
else
acpi_cpu_c1();
- *state = STATE_RUNNING;
+ atomic_store_int(state, STATE_RUNNING);
}
static void
@@ -427,7 +427,7 @@ 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
@@ -450,7 +450,7 @@ cpu_idle_hlt(sbintime_t sbt)
enable_intr();
else
acpi_cpu_c1();
- *state = STATE_RUNNING;
+ atomic_store_int(state, STATE_RUNNING);
}
static void
@@ -459,21 +459,22 @@ 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
@@ -483,7 +484,7 @@ 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
@@ -577,20 +578,21 @@ 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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