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

[Wen Congyang <wency@cn.fujitsu.com>]

At 09/24/2012 09:34 PM, Luiz Capitulino Wrote:
> On Mon, 24 Sep 2012 14:27:17 +0800
> Wen Congyang <wency@cn.fujitsu.com> wrote:
> 
>> 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 <lcapitulino@redhat.com>
>>> ---
>>>  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?
> 
> The problem is with non-blocking fds, where spinning isn't correct, for
> two reasons:
> 
>  1. If the fd is non-blocking, that means you don't want to block
>     and spinning for a long time will have the same effects
> 
>  2. Spinning consumes host resources
> 


If so, I agree with it, and the patch looks fine to me

Thanks
Wen Congyang