On Wednesday 25 June 2003 15:56, Angelo Dell'Aera wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> On Tue, 24 Jun 2003 20:07:57 +0200
>
> Frank.Uepping@t-online.de (Frank A. Uepping) wrote:
> >Imagine we have a service which we can use by registering a callback
> > function. The registered callback is then called whenever the service
> > decides so. When we are no more interested in the service we release the
> > callback.
> >
> >E.g.:
> >
> >struct X {
> > void (*callback)(void *data);
> > ...
> >};
> >
> >void register_x(struct X *x, void *data); // data is passed to
> > x.callback() void release_x(struct X* x);
> >
> >I think we could get in trouble on SMP machines with that.
> >It could be that one thread is about to call callback() while
> >another thread is about to call release_x().
> >If release_x() returns before callback() is finished and
> >the subsequent code following release_x() invalidates data
> >(e.g. kfree(data)) callback() works on with an invalid data ptr
> >-- havoc is the result.
> >
> >How to make this SMP safe?
> >Yes we can use spinlocks to make release_x() wait while callback()
> >is in execution. But this has the consequence that callback() must be
> >fast/non-blocking (holding a spinlock a longer time is not permitted).
> >
> >Are there any other solutions?
>
> A simple solution you can use is a mutex.
>
> Simply do this
>
> struct semaphore sem;
>
> void release_x(struct foo *foo)
> {
> down_interruptible(&sem);
>
> CRITICAL CODE SECTION
>
> up(&sem);
> }
>
> void callback(void *data)
> {
> down_interruptible(&sem);
>
> CRITICAL CODE SECTION
>
> up(&sem);
> }
>
This didn't work well!!!
Data could be invalidated while callback() is waiting for sem.
/FAU
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