Re: [Qemu-devel] [PATCH v8 03/36] block: Introduce image file locking

[Fam Zheng <famz@redhat.com>]

On Fri, 10/21 23:04, Max Reitz wrote:
> > +ImageLockMode bdrv_lock_mode_from_flags(int flags)
> > +{
> > +    if (flags & BDRV_O_NO_LOCK) {
> > +        return IMAGE_LOCK_MODE_NOLOCK;
> > +    } else if (flags & BDRV_O_SHARED_LOCK) {
> > +        return IMAGE_LOCK_MODE_SHARED;
> > +    } else if (flags & BDRV_O_EXCLUSIVE_LOCK) {
> > +        return IMAGE_LOCK_MODE_EXCLUSIVE;
> > +    } else {
> > +        return IMAGE_LOCK_MODE_AUTO;
> > +    }
> > +}
> 
> I don't know if there's been any discussion about the order of the flags
> here, but I personally would order them exactly the other way around:
> Asking for exclusive locking should override nolock, in my opinion.

The idea was to assert no two bits are set at the same time. But I seem to have
forgotten to actually add the assertion.

> 
> > +
> > +ImageLockMode bdrv_get_lock_mode(BlockDriverState *bs)
> > +{
> > +    return bs->cur_lock;
> > +}
> > +
> > +int bdrv_set_lock_mode(BlockDriverState *bs, ImageLockMode mode)
> > +{
> > +    int ret;
> > +
> > +    if (bs->cur_lock == mode) {
> > +        return 0;
> > +    } else if (!bs->drv) {
> > +        return -ENOMEDIUM;
> > +    } else if (!bs->drv->bdrv_lockf) {
> > +        if (bs->file) {
> > +            return bdrv_set_lock_mode(bs->file->bs, mode);
> > +        }
> > +        return 0;
> > +    }
> > +    ret = bs->drv->bdrv_lockf(bs, mode);
> > +    if (ret == -ENOTSUP) {
> > +        /* Handle it the same way as !bs->drv->bdrv_lockf */
> > +        ret = 0;
> 
> Yes, well, why do you handle both as success? Wouldn't returning
> -ENOTSUP make more sense?
> 
> I guess the caller can find out itself by checking whether bs->cur_lock
> has changed, but...

I can't think of a reason for any caller to do something different for -ENOTSUP
from success, hence the check here.

> 
> > +    } else if (ret == 0) {
> > +        bs->cur_lock = mode;
> > +    }
> > +    return ret;
> > +}
> > +
> >  static QemuOptsList bdrv_runtime_opts = {
> >      .name = "bdrv_common",
> >      .head = QTAILQ_HEAD_INITIALIZER(bdrv_runtime_opts.head),
> > @@ -1076,6 +1119,10 @@ static int bdrv_open_common(BlockDriverState *bs, BdrvChild *file,
> >          goto free_and_fail;
> >      }
> >  
> > +    if (open_flags & BDRV_O_INACTIVE) {
> > +        open_flags = (open_flags & ~BDRV_O_LOCK_MASK) & BDRV_O_NO_LOCK;
> 
> I suppose the second & is supposed to be a |?

Yes. Thanks for catching it.

> 
> > +    }
> > +
> >      ret = refresh_total_sectors(bs, bs->total_sectors);
> >      if (ret < 0) {
> >          error_setg_errno(errp, -ret, "Could not refresh total sector count");
> > @@ -2273,6 +2320,7 @@ static void bdrv_close(BlockDriverState *bs)
> >      if (bs->drv) {
> >          BdrvChild *child, *next;
> >  
> > +        bdrv_set_lock_mode(bs, IMAGE_LOCK_MODE_NOLOCK);
> >          bs->drv->bdrv_close(bs);
> >          bs->drv = NULL;
> >  
> > @@ -3188,6 +3236,9 @@ void bdrv_invalidate_cache(BlockDriverState *bs, Error **errp)
> 
> This function's name is pretty weird... Maybe it would be better to
> rename it to "bdrv_complete_incoming" or something. (Unrelated to this
> series, of course.)
> 
> >          error_setg_errno(errp, -ret, "Could not refresh total sector count");
> >          return;
> >      }
> > +    if (bs->cur_lock != IMAGE_LOCK_MODE__MAX) {
> > +        bdrv_set_lock_mode(bs, bs->cur_lock);
> > +    }
> >  }
> >  
> >  void bdrv_invalidate_cache_all(Error **errp)
> > @@ -3230,6 +3281,7 @@ static int bdrv_inactivate_recurse(BlockDriverState *bs,
> >      }
> >  
> >      if (setting_flag) {
> > +        ret = bdrv_set_lock_mode(bs, IMAGE_LOCK_MODE_NOLOCK);
> 
> Maybe it would make sense to do something with the return value...? :-)

Yes, sounds good.

Fam