callout_stop() return value

[Mark Johnston]

On Wed, May 02, 2018 at 09:34:57AM -0600, Ian Lepore wrote:
> On Wed, 2018-05-02 at 11:20 -0400, Mark Johnston wrote:
> > Hi,
> > 
> > We have a few pieces of code that follow a pattern: a thread
> > allocates a
> > resource and schedules a callout. The callout handler releases the
> > resource and never reschedules itself. In the common case, a thread
> > will cancel the callout before it executes. To know whether it must
> > release the resource associated with the callout, this thread must
> > know
> > whether the pending callout was cancelled.
> > 
> 
> It seems to me a better solution would be to track the state / lifetime
> of the resource separately rather than trying to infer the state of the
> resource from the state of the callout as viewed through a semi-opaque
> interface.
> 
> If the original thread that schedules the callout keeps enough
> information about the resource to free it after cancelling, then it is
> already maintaining some kind of sideband info about the resource, even
> if it's just a pointer to be freed. Couldn't the callout routine
> manipulate this resource tracking info (null out the pointer or set a
> bool or whatever), then after cancelling you don't really care whether
> the callout ran or not, you just examine the pointer/bool/whatever and
> free or not based on that.

I'd considered that. It's not quite as elegant a solution as you
suggest, since in my case the resource is embedded in an opaque
structure, so I need to add an extra field beside the callout to track
state that's already tracked by the callout subsystem. That plus the
fact that we have multiple instances of this bug make me want to fix it
in a general way, though I recognize that the callout API is already
overly complicated.