callout_stop() return value

[Mark Johnston]

On Wed, May 02, 2018 at 07:24:12PM +0300, Konstantin Belousov wrote:
> On Wed, May 02, 2018 at 11:42:37AM -0400, Mark Johnston wrote:
> > 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.

I forgot to add that in some cases, extra synchronization would be
needed to maintain this hypothetical flag. Specifically, some of these
callouts do not have an associated lock, so the callout handler doesn't
have an easy way to mark the resource as consumed.

> I gave up on trying to get this fixed.  r302350 was not the first time
> the return value was broken.
>
> I had to do r303426 exactly for this reason.

What kind of fix did you envision? The problem seems to be that
callout_stop()'s return value simply does not contain enough information
for certain use cases.