Re: [Qemu-devel] [PATCH 2/3] qmp: dump-guest-memory: don't spin if non-blocking fd would block

[Wen Congyang]

At 09/22/2012 01:07 AM, Luiz Capitulino Wrote:
> fd_write_vmcore() will indefinitely spin for a non-blocking
> file-descriptor that would block. However, if the fd is non-blocking,
> how does it make sense to spin?
> 
> Change this behavior to return an error instead.
> 
> Note that this can only happen with an fd provided by a management
> application. The fd opened internally by dump-guest-memory is blocking.
> 
> Signed-off-by: Luiz Capitulino <address@hidden>
> ---
>  dump.c | 13 +++----------
>  1 file changed, 3 insertions(+), 10 deletions(-)
> 
> diff --git a/dump.c b/dump.c
> index 2bf8d8d..5eea015 100644
> --- a/dump.c
> +++ b/dump.c
> @@ -100,18 +100,11 @@ static void dump_error(DumpState *s, const char *reason)
>  static int fd_write_vmcore(void *buf, size_t size, void *opaque)
>  {
>      DumpState *s = opaque;
> -    int fd = s->fd;
>      size_t writen_size;
>  
> -    /* The fd may be passed from user, and it can be non-blocked */
> -    while (size) {
> -        writen_size = qemu_write_full(fd, buf, size);
> -        if (writen_size != size && errno != EAGAIN) {

Hmm, if the fd is a blocking fd, errno can't be EAGAIN. So the
function doesn't spin. What problems do you meet?

Thanks
Wen Congyang

> -            return -1;
> -        }
> -
> -        buf += writen_size;
> -        size -= writen_size;
> +    writen_size = qemu_write_full(s->fd, buf, size);
> +    if (writen_size != size) {
> +        return -1;
>      }
>  
>      return 0;