From 758e1b2b622d7c177dc2d95e887a11aa069b7e68 Mon Sep 17 00:00:00 2001
From: Paolo Bonzini <pbonzini@redhat.com>
Date: Fri, 2 Sep 2016 23:33:38 +0200
Subject: [PATCH] cpus-common: simplify locking for
 start_exclusive/end_exclusive
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It is not necessary to hold qemu_cpu_list_mutex throughout the
exclusive section, because no other exclusive section can run
while pending_cpus != 0.

exclusive_idle() is called in cpu_exec_start(), and that prevents
any CPUs created after start_exclusive() from entering cpu_exec()
during an exclusive section.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 cpus-common.c              | 11 ++++++++---
 docs/tcg-exclusive.promela |  4 +++-
 include/qom/cpu.h          |  4 ----
 3 files changed, 11 insertions(+), 8 deletions(-)

diff --git a/cpus-common.c b/cpus-common.c
index 80aaf9b42d..429652c7fd 100644
--- a/cpus-common.c
+++ b/cpus-common.c
@@ -171,8 +171,7 @@ static inline void exclusive_idle(void)
 }
 
 /* Start an exclusive operation.
-   Must only be called from outside cpu_exec, takes
-   qemu_cpu_list_lock.   */
+   Must only be called from outside cpu_exec.  */
 void start_exclusive(void)
 {
     CPUState *other_cpu;
@@ -191,11 +190,17 @@ void start_exclusive(void)
     while (pending_cpus > 1) {
         qemu_cond_wait(&exclusive_cond, &qemu_cpu_list_lock);
     }
+
+    /* Can release mutex, no one will enter another exclusive
+     * section until end_exclusive resets pending_cpus to 0.
+     */
+    qemu_mutex_unlock(&qemu_cpu_list_lock);
 }
 
-/* Finish an exclusive operation.  Releases qemu_cpu_list_lock.  */
+/* Finish an exclusive operation.  */
 void end_exclusive(void)
 {
+    qemu_mutex_lock(&qemu_cpu_list_lock);
     pending_cpus = 0;
     qemu_cond_broadcast(&exclusive_resume);
     qemu_mutex_unlock(&qemu_cpu_list_lock);
diff --git a/docs/tcg-exclusive.promela b/docs/tcg-exclusive.promela
index 8bb0967df6..feac679b9a 100644
--- a/docs/tcg-exclusive.promela
+++ b/docs/tcg-exclusive.promela
@@ -98,9 +98,11 @@ byte has_waiter[N_CPUS];
     do                                                            \
       :: pending_cpus > 1 -> COND_WAIT(exclusive_cond, mutex);    \
       :: else             -> break;                               \
-    od
+    od;                                                           \
+    MUTEX_UNLOCK(mutex);
 
 #define end_exclusive()                                           \
+    MUTEX_LOCK(mutex);                                            \
     pending_cpus = 0;                                             \
     COND_BROADCAST(exclusive_resume);                             \
     MUTEX_UNLOCK(mutex);
diff --git a/include/qom/cpu.h b/include/qom/cpu.h
index f8726140f6..934c07afbf 100644
--- a/include/qom/cpu.h
+++ b/include/qom/cpu.h
@@ -846,9 +846,6 @@ void cpu_exec_end(CPUState *cpu);
  * cpu_exec are exited immediately.  CPUs that call cpu_exec_start
  * during the exclusive section go to sleep until this CPU calls
  * end_exclusive.
- *
- * Returns with the CPU list lock taken (which nests outside all
- * other locks except the BQL).
  */
 void start_exclusive(void);
 
@@ -856,7 +853,6 @@ void start_exclusive(void);
  * end_exclusive:
  *
  * Concludes an exclusive execution section started by start_exclusive.
- * Releases the CPU list lock.
  */
 void end_exclusive(void);