powerpc/spufs: Fix race for a free SPU

We currently have a race for a free SPE. With one thread doing a
spu_yield(), and another doing a spu_activate():

thread 1				thread 2
spu_yield(oldctx)			spu_activate(ctx)
  __spu_deactivate(oldctx)
  spu_unschedule(oldctx, spu)
  spu->alloc_state = SPU_FREE
					spu = spu_get_idle(ctx)
					    - searches for a SPE in
					      state SPU_FREE, gets
					      the context just
					      freed by thread 1
					spu_schedule(ctx, spu)
					  spu->alloc_state = SPU_USED
spu_schedule(newctx, spu)
  - assumes spu is still free
  - tries to schedule context on
    already-used spu

This change introduces a 'free_spu' flag to spu_unschedule, to indicate
whether or not the function should free the spu after descheduling the
context. We only set this flag if we're not going to re-schedule
another context on this SPU.

Add a comment to document this behaviour.
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>

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

@@ -732,13 +732,28 @@ static void spu_schedule(struct spu *spu, struct spu_context *ctx)
 	spu_release(ctx);
 }
 
-static void spu_unschedule(struct spu *spu, struct spu_context *ctx)
+/**
+ * spu_unschedule - remove a context from a spu, and possibly release it.
+ * @spu:	The SPU to unschedule from
+ * @ctx:	The context currently scheduled on the SPU
+ * @free_spu	Whether to free the SPU for other contexts
+ *
+ * Unbinds the context @ctx from the SPU @spu. If @free_spu is non-zero, the
+ * SPU is made available for other contexts (ie, may be returned by
+ * spu_get_idle). If this is zero, the caller is expected to schedule another
+ * context to this spu.
+ *
+ * Should be called with ctx->state_mutex held.
+ */
+static void spu_unschedule(struct spu *spu, struct spu_context *ctx,
+		int free_spu)
 {
 	int node = spu->node;
 
 	mutex_lock(&cbe_spu_info[node].list_mutex);
 	cbe_spu_info[node].nr_active--;
-	spu->alloc_state = SPU_FREE;
+	if (free_spu)
+		spu->alloc_state = SPU_FREE;
 	spu_unbind_context(spu, ctx);
 	ctx->stats.invol_ctx_switch++;
 	spu->stats.invol_ctx_switch++;
@@ -838,7 +853,7 @@ static int __spu_deactivate(struct spu_context *ctx, int force, int max_prio)
 	if (spu) {
 		new = grab_runnable_context(max_prio, spu->node);
 		if (new || force) {
-			spu_unschedule(spu, ctx);
+			spu_unschedule(spu, ctx, new == NULL);
 			if (new) {
 				if (new->flags & SPU_CREATE_NOSCHED)
 					wake_up(&new->stop_wq);
@@ -911,7 +926,7 @@ static noinline void spusched_tick(struct spu_context *ctx)
 
 	new = grab_runnable_context(ctx->prio + 1, spu->node);
 	if (new) {
-		spu_unschedule(spu, ctx);
+		spu_unschedule(spu, ctx, 0);
 		if (test_bit(SPU_SCHED_SPU_RUN, &ctx->sched_flags))
 			spu_add_to_rq(ctx);
 	} else {