powerpc/spufs: Fix possible scheduling of a context to multiple SPEs

We currently have a race when scheduling a context to a SPE -
after we have found a runnable context in spusched_tick, the same
context may have been scheduled by spu_activate().

This may result in a panic if we try to unschedule a context that has
been freed in the meantime.

This change exits spu_schedule() if the context has already been
scheduled, so we don't end up scheduling it twice.
Signed-off-by: Andre Detsch <adetsch@br.ibm.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>

arch/powerpc/platforms/cell/spufs/sched.c

@@ -728,6 +728,7 @@ static void spu_schedule(struct spu *spu, struct spu_context *ctx)
 	/* not a candidate for interruptible because it's called either
 	   from the scheduler thread or from spu_deactivate */
 	mutex_lock(&ctx->state_mutex);
+	if (ctx->state == SPU_STATE_SAVED)
 		__spu_schedule(spu, ctx);
 	spu_release(ctx);
 }