Re: [PATCH 3/8] qemu-thread: Avoid futex abstraction for non-Linux

[Phil Dennis-Jordan]

This fix makes sense. I've also successfully tested with this new "pure" pthread-based QemuEvent in the apple-gfx code where we initially considered using QemuEvent and ran into the destruction issue on macOS.

Tested-by: Phil Dennis-Jordan <phil@philjordan.eu>

Reviewed-by: Phil Dennis-Jordan <phil@philjordan.eu>

(Unrelated: email to the devel@daynix.com address in the cc list seems to bounce.)

On Wed, 25 Dec 2024 at 06:44, Akihiko Odaki <akihiko.odaki@daynix.com> wrote:

qemu-thread used to abstract pthread primitives into futex for the
QemuEvent implementation of POSIX systems other than Linux. However,
this abstraction has one key difference: unlike futex, pthread
primitives require an explicit destruction, and it must be ordered after
wait and wake operations.

It would be easier to perform destruction if a wait operation ensures
the corresponding wake operation finishes as POSIX semaphore does, but
that requires to protect state accesses in qemu_event_set() and
qemu_event_wait() with a mutex. On the other hand, real futex does not
need such a protection but needs complex barrier and atomic operations
before wait and wake operations instead.

Add special implementations of qemu_event_set() and qemu_event_wait()
using pthread primitives. qemu_event_wait() will ensure qemu_event_set()
finishes, and these functions will avoid complex barrier and atomic
operations.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
---
 util/qemu-thread-posix.c | 45 ++++++++++++++++++---------------------------
 1 file changed, 18 insertions(+), 27 deletions(-)

diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
index 13459e44c768..805cac444f15 100644
--- a/util/qemu-thread-posix.c
+++ b/util/qemu-thread-posix.c
@@ -319,28 +319,6 @@ void qemu_sem_wait(QemuSemaphore *sem)

 #ifdef __linux__
 #include "qemu/futex.h"
-#else
-static inline void qemu_futex_wake(QemuEvent *ev, int n)
-{
-    assert(ev->initialized);
-    pthread_mutex_lock(&ev->lock);
-    if (n == 1) {
-        pthread_cond_signal(&ev->cond);
-    } else {
-        pthread_cond_broadcast(&ev->cond);
-    }
-    pthread_mutex_unlock(&ev->lock);
-}
-
-static inline void qemu_futex_wait(QemuEvent *ev, unsigned val)
-{
-    assert(ev->initialized);
-    pthread_mutex_lock(&ev->lock);
-    if (ev->value == val) {
-        pthread_cond_wait(&ev->cond, &ev->lock);
-    }
-    pthread_mutex_unlock(&ev->lock);
-}
 #endif

 /* Valid transitions:
@@ -363,7 +341,7 @@ static inline void qemu_futex_wait(QemuEvent *ev, unsigned val)

 void qemu_event_init(QemuEvent *ev, bool init)
 {
-#ifndef __linux__
+#ifndef CONFIG_LINUX
     pthread_mutex_init(&ev->lock, NULL);
     pthread_cond_init(&ev->cond, NULL);
 #endif
@@ -376,7 +354,7 @@ void qemu_event_destroy(QemuEvent *ev)
 {
     assert(ev->initialized);
     ev->initialized = false;
-#ifndef __linux__
+#ifndef CONFIG_LINUX
     pthread_mutex_destroy(&ev->lock);
     pthread_cond_destroy(&ev->cond);
 #endif
@@ -386,6 +364,7 @@ void qemu_event_set(QemuEvent *ev)
 {
     assert(ev->initialized);

+#ifdef CONFIG_LINUX
     /*
      * Pairs with both qemu_event_reset() and qemu_event_wait().
      *
@@ -403,6 +382,12 @@ void qemu_event_set(QemuEvent *ev)
             qemu_futex_wake_all(ev);
         }
     }
+#else
+    pthread_mutex_lock(&ev->lock);
+    qatomic_set(&ev->value, EV_SET);
+    pthread_cond_broadcast(&ev->cond);
+    pthread_mutex_unlock(&ev->lock);
+#endif
 }

 void qemu_event_reset(QemuEvent *ev)
@@ -424,10 +409,9 @@ void qemu_event_reset(QemuEvent *ev)

 void qemu_event_wait(QemuEvent *ev)
 {
-    unsigned value;
-
     assert(ev->initialized);

+#ifdef CONFIG_LINUX
     while (true) {
         /*
          * qemu_event_wait must synchronize with qemu_event_set even if it does
@@ -438,7 +422,7 @@ void qemu_event_wait(QemuEvent *ev)
          * might miss a qemu_event_set() here but ultimately the memory barrier
          * in qemu_futex_wait() will ensure the check is done correctly.
          */
-        value = qatomic_load_acquire(&ev->value);
+        unsigned value = qatomic_load_acquire(&ev->value);
         if (value == EV_SET) {
             break;
         }
@@ -467,6 +451,13 @@ void qemu_event_wait(QemuEvent *ev)
          */
         qemu_futex_wait(ev, EV_BUSY);
     }
+#else
+    pthread_mutex_lock(&ev->lock);
+    if (qatomic_read(&ev->value) != EV_SET) {
+        pthread_cond_wait(&ev->cond, &ev->lock);
+    }
+    pthread_mutex_unlock(&ev->lock);
+#endif
 }

 static __thread NotifierList thread_exit;

--
2.47.1