Re: [Qemu-devel] [PATCH] migrate.c: migrate_fd_put_buffer: Do not busyloop: stop writing if EWOULDBLOCK

[Dor Laor]

Uri Lublin wrote:
> The migration code is non-blocking, designed for live migration.
>
> Practically migrate_fd_put_buffer busy-loops trying to write, as
> on many machines EWOULDBLOCK==EAGAIN (look in include/asm-generic/errno.h).
>
> Signed-off-by: Uri Lublin <address@hidden>
> ---
>  migration.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/migration.c b/migration.c
> index 859d945..ca397fa 100644
> --- a/migration.c
> +++ b/migration.c
> @@ -176,7 +176,7 @@ ssize_t migrate_fd_put_buffer(void *opaque, const void 
> *data, size_t size)
>  
>      do {
>          ret = s->write(s, data, size);
> -    } while (ret == -1 && ((s->get_error(s)) == EINTR || (s->get_error(s)) == 
> EWOULDBLOCK));
> +    } while (ret == -1 && ((s->get_error(s)) == EINTR));
>  
>      if (ret == -1)
>          ret = -(s->get_error(s));
>   
There is additional stuff that needs to test EWOULDBLOCK in further 
locations in the code.
Otherwise migration will fail.

commit 2d875410cb168fa728dcde96885a3e4ddfd0bb58
Author: Dor Laor <address@hidden>
Date:   Thu May 21 12:29:39 2009 +0300

   The migration code is non-blocking, designed for live migration.
  
   Practically migrate_fd_put_buffer busy-loops trying to write, as
   on many machines EWOULDBLOCK==EAGAIN (look in 
include/asm-generic/errno.h).
  
   Based on Uri Lublin's patch.
   Signed-off-by: Dor Laor <address@hidden>

diff --git a/qemu/buffered_file.c b/qemu/buffered_file.c
index be5baea..34cde6e 100644
--- a/qemu/buffered_file.c
+++ b/qemu/buffered_file.c
@@ -86,7 +86,7 @@ static void buffered_flush(QEMUFileBuffered *s)

        ret = s->put_buffer(s->opaque, s->buffer + offset,
                            s->buffer_size - offset);
-        if (ret == -EAGAIN) {
+        if (ret == -EAGAIN || ret == -EWOULDBLOCK) {
            dprintf("backend not ready, freezing\n");
            s->freeze_output = 1;
            break;
@@ -132,7 +132,7 @@ static int buffered_put_buffer(void *opaque, const 
uint8_t *buf, int64_t pos, in
        }

        ret = s->put_buffer(s->opaque, buf + offset, size - offset);
-        if (ret == -EAGAIN) {
+        if (ret == -EAGAIN || ret ==-EWOULDBLOCK) {
            dprintf("backend not ready, freezing\n");
            s->freeze_output = 1;
            break;
diff --git a/qemu/migration.c b/qemu/migration.c
index 85757c5..8846eee 100644
--- a/qemu/migration.c
+++ b/qemu/migration.c
@@ -174,7 +174,7 @@ ssize_t migrate_fd_put_buffer(void *opaque, const 
void *data, size_t size)
    if (ret == -1)
        ret = -(s->get_error(s));

-    if (ret == -EAGAIN)
+    if (ret == -EAGAIN || ret == -EWOULDBLOCK)
        qemu_set_fd_handler2(s->fd, NULL, NULL, migrate_fd_put_notify, s);

    return ret;