Re: [Qemu-devel] [PATCH] block: Don't lock /dev/null and /dev/zero automatically

[Max Reitz]

On 2018-07-19 05:41, Fam Zheng wrote:
> On my Fedora 28, /dev/null is locked by some other process (couldn't
> inspect it due to the current lslocks limitation), so iotests 226 fails
> with some unexpected image locking errors because it uses qemu-io to
> open it.
> 
> Actually it's safe to not use any lock on /dev/null or /dev/zero.
> 
> Signed-off-by: Fam Zheng <address@hidden>
> ---
>  block/file-posix.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/block/file-posix.c b/block/file-posix.c
> index 60af4b3d51..8bf034108a 100644
> --- a/block/file-posix.c
> +++ b/block/file-posix.c
> @@ -503,7 +503,12 @@ static int raw_open_common(BlockDriverState *bs, QDict 
> *options,
>          s->use_lock = false;
>          break;
>      case ON_OFF_AUTO_AUTO:
> -        s->use_lock = qemu_has_ofd_lock();
> +        if (!strcmp(filename, "/dev/null") ||
> +                   !strcmp(filename, "/dev/zero")) {

I’m not sure I like a strcmp() based on filename (though it should work
for all of the cases where someone would want to specify either of those
for a qemu block device).  Isn’t there some specific major/minor number
we can use to check for these two files?  Or are those Linux-specific?

I could also imagine just not locking any host_device, but since people
do use qcow2 immediately on block devices, maybe we do want to lock them.

Finally, if really all you are trying to do is to make test code easier,
then I don’t know exactly why.  Just disabling locking in 226 shouldn’t
be too hard.

Max

> +            s->use_lock = false;
> +        } else {
> +            s->use_lock = qemu_has_ofd_lock();
> +        }
>          break;
>      default:
>          abort();
>

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