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

[Dor Laor <dlaor@redhat.com>]

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 <uril@redhat.com>
> ---
>  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 <dor@redhat.com>
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 <dor@redhat.com>

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;