Re: [Qemu-devel] [PATCH v19 3/7] xbitmap: add more operations

[Wei Wang]

On 12/16/2017 02:42 AM, Matthew Wilcox wrote:
> On Tue, Dec 12, 2017 at 07:55:55PM +0800, Wei Wang wrote:
> > +int xb_preload_and_set_bit(struct xb *xb, unsigned long bit, gfp_t gfp);
> I'm struggling to understand when one would use this.  The xb_ API
> requires you to handle your own locking.  But specifying GFP flags
> here implies you can sleep.  So ... um ... there's no locking?

In the regular use cases, people would do xb_preload() before taking the 
lock, and the xb_set/clear within the lock.

In the virtio-balloon usage, we have a large number of bits to set with 
the balloon_lock being held (we're not unlocking for each bit), so we 
used the above wrapper to do preload and set within the balloon_lock, 
and passed in GFP_NOWAIT to avoid sleeping. Probably we can change to 
put this wrapper implementation to virtio-balloon, since it would not be 
useful for the regular cases.


Best,
Wei