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>

diff --git a/arch/powerpc/platforms/cell/spufs/sched.c b/arch/powerpc/platforms/cell/spufs/sched.c
index 9bb45c6..897c740 100644 (file)
--- a/arch/powerpc/platforms/cell/spufs/sched.c
+++ b/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 {