Set the iothread's eventfd/pipe descriptors to non-blocking

From ab5ae4bb69f8ab6c9a476f7823cb8d6729d31594 Mon Sep 17 00:00:00 2001

It replaces the previous fix of using select.

Quoted 1 line

From ab5ae4bb69f8ab6c9a476f7823cb8d6729d31594 Mon Sep 17 00:00:00 2001

From: Dor Laor <[email protected]>
Date: Wed, 15 Jul 2009 17:53:16 +0300
Subject: [PATCH] Set the iothread's eventfd/pipe descriptors to non-blocking.

It fixes migration issue when the destination is loaded.

If the migration socket is full, we get EAGAIN for the write. The set_fd_handler2 defers the write for later on. The function tries to wake up the iothread by qemu_kvm_notify_work.
Since this happens in a loop, multiple times, the pipe that emulates eventfd becomes full and we get a deadlock.

Mark McLoughlin suggested to remove spurious wake-up of the migration code when we get EAGAIN and wait for the socket to become writeable. (+1)

Nevertheless, the pipe descriptors shouldn't be blocking and the reader can also read several chunks in a time.

Signed-off-by Dor Laor <[email protected]>

---
 qemu-kvm.c |   24 ++++++++++++++----------
 1 files changed, 14 insertions(+), 10 deletions(-)

qemu-kvm.c +11 -10

index ed7e466..68a453c 100644
@@ -2107,14 +2107,17 @@ void qemu_kvm_notify_work(void)
 	if (len == -1 && errno == EINTR)
 	    continue;
 
-	if (len <= 0)
+        /* In case we have a pipe, there is not reason to insist writing
+         * 8 bytes
+         */
+	if (len == -1 && errno == EAGAIN)
 	    break;
 
+        if (len <= 0)
+            break;
+
 	offset += len;
     }
-
-    if (offset != 8)
-	fprintf(stderr, "failed to notify io thread\n");
 }
 
 /* If we have signalfd, we mask out the signals we want to handle and then

@@ -2153,20 +2156,18 @@ static void sigfd_handler(void *opaque)
 static void io_thread_wakeup(void *opaque)
 {
     int fd = (unsigned long)opaque;
-    char buffer[8];
-    size_t offset = 0;
+    char buffer[4096];
 
-    while (offset < 8) {
+    /* Drain the pipe/(eventfd) */
+    while (1) {
 	ssize_t len;
 
-	len = read(fd, buffer + offset, 8 - offset);
+	len = read(fd, buffer, sizeof(buffer));
 	if (len == -1 && errno == EINTR)
 	    continue;
 
 	if (len <= 0)
 	    break;
-
-	offset += len;
     }
 }
 

It replaces the previous fix of using select.

Quoted 1 line

From ab5ae4bb69f8ab6c9a476f7823cb8d6729d31594 Mon Sep 17 00:00:00 2001

From: Dor Laor <[email protected]>
Date: Wed, 15 Jul 2009 17:53:16 +0300
Subject: [PATCH] Set the iothread's eventfd/pipe descriptors to non-blocking.

It fixes migration issue when the destination is loaded.

If the migration socket is full, we get EAGAIN for the write. The set_fd_handler2 defers the write for later on. The function tries to wake up the iothread by qemu_kvm_notify_work.
Since this happens in a loop, multiple times, the pipe that emulates eventfd becomes full and we get a deadlock.

Mark McLoughlin suggested to remove spurious wake-up of the migration code when we get EAGAIN and wait for the socket to become writeable. (+1)

Nevertheless, the pipe descriptors shouldn't be blocking and the reader can also read several chunks in a time.

Signed-off-by Dor Laor <[email protected]>

---
 qemu-kvm.c |   24 ++++++++++++++----------
 1 files changed, 14 insertions(+), 10 deletions(-)

qemu-kvm.c +11 -10

index ed7e466..68a453c 100644
@@ -2107,14 +2107,17 @@ void qemu_kvm_notify_work(void)
 	if (len == -1 && errno == EINTR)
 	    continue;
 
-	if (len <= 0)
+        /* In case we have a pipe, there is not reason to insist writing
+         * 8 bytes
+         */
+	if (len == -1 && errno == EAGAIN)
 	    break;
 
+        if (len <= 0)
+            break;
+
 	offset += len;
     }
-
-    if (offset != 8)
-	fprintf(stderr, "failed to notify io thread\n");
 }
 
 /* If we have signalfd, we mask out the signals we want to handle and then

@@ -2153,20 +2156,18 @@ static void sigfd_handler(void *opaque)
 static void io_thread_wakeup(void *opaque)
 {
     int fd = (unsigned long)opaque;
-    char buffer[8];
-    size_t offset = 0;
+    char buffer[4096];
 
-    while (offset < 8) {
+    /* Drain the pipe/(eventfd) */
+    while (1) {
 	ssize_t len;
 
-	len = read(fd, buffer + offset, 8 - offset);
+	len = read(fd, buffer, sizeof(buffer));
 	if (len == -1 && errno == EINTR)
 	    continue;
 
 	if (len <= 0)
 	    break;
-
-	offset += len;
     }
 }