Re: [Qemu-devel] [PATCH] block: Fix compiler warning (-Werror=uninitialized)

[Paolo Bonzini <pbonzini@redhat.com>]

Il 19/09/2013 18:59, Andreas Färber ha scritto:
> Am 17.09.2013 18:43, schrieb Stefan Weil:
>> The patch fixes a warning from gcc (Debian 4.6.3-14+rpi1) 4.6.3:
>>
>> block/stream.c:141:22: error:
>> ‘copy’ may be used uninitialized in this function [-Werror=uninitialized]
>>
>> This is not a real bug - a better compiler would not complain.
>>
>> Now 'copy' has always a defined value, so the check for ret >= 0
>> can be removed.
>>
>> Signed-off-by: Stefan Weil <sw@weilnetz.de>
>> ---
>>  block/stream.c |    5 ++---
>>  1 file changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/block/stream.c b/block/stream.c
>> index 078ce4a..fc19194 100644
>> --- a/block/stream.c
>> +++ b/block/stream.c
>> @@ -108,7 +108,7 @@ static void coroutine_fn stream_run(void *opaque)
>>  
>>      for (sector_num = 0; sector_num < end; sector_num += n) {
>>          uint64_t delay_ns = 0;
>> -        bool copy;
>> +        bool copy = false;
>>  
>>  wait:
>>          /* Note that even when no rate limit is applied we need to yield
>> @@ -123,7 +123,6 @@ wait:
>>                                  STREAM_BUFFER_SIZE / BDRV_SECTOR_SIZE, &n);
>>          if (ret == 1) {
>>              /* Allocated in the top, no need to copy.  */
>> -            copy = false;
>>          } else if (ret >= 0) {
>>              /* Copy if allocated in the intermediate images.  Limit to the
>>               * known-unallocated area [sector_num, sector_num+n).  */
> 
> Sorry for not spotting this patch earlier. This hunk looks wrong and
> needs to be dropped, I believe. In the ret >= 0 && copy case, there is a
> "goto wait" which would now no longer be able to go from copy == true ->
> copy == false. Not sure if that can happen in practice.

Yes, if the guest writes to the area that the job is copying.  It
wouldn't cause any data corruption, only do useless work on those
sectors.  A better patch would thus remove the initializer and move the
"copy = false" before "if (ret == 1)".

Paolo

> Andreas
> 
>> @@ -138,7 +137,7 @@ wait:
>>              copy = (ret == 1);
>>          }
>>          trace_stream_one_iteration(s, sector_num, n, ret);
>> -        if (ret >= 0 && copy) {
>> +        if (copy) {
>>              if (s->common.speed) {
>>                  delay_ns = ratelimit_calculate_delay(&s->limit, n);
>>                  if (delay_ns > 0) {
>>
> 
>