Re: 4.5.0+ panic when setup loop device

[Jens Axboe]

On 03/17/2016 09:42 AM, Jens Axboe wrote:
> On 03/17/2016 05:01 AM, Thomas Gleixner wrote:
> > On Thu, 17 Mar 2016, Peter Zijlstra wrote:
> > > On Thu, Mar 17, 2016 at 12:39:46PM +0100, Thomas Gleixner wrote:
> > > > But we have to clarify and document whether holes in
> > > > cpu_possible_mask are not
> > > > allowed at all or if code like the above is simply broken.
> > >
> > > So the general rule is that cpumasks can have holes, and exempting one
> > > just muddles the water.
> > >
> > > Therefore I'd call the code just plain broken.
> >
> > Agreed.
> >
> > That macro is not really helping the readability of the code at all. So a
> > simple for_each_possible_cpu() loop would have avoided that wreckage.
>
> Does the attached work? The rest of blk-mq should deal with holes just
> fine, we found some of those issues on sparc. Not sure why this one
> slipped through the cracks.

This might be better, we need to start at -1 to not miss the first 
one... Still untested.

--
Jens Axboe

diff --git a/block/blk-mq.h b/block/blk-mq.h
index eaede8e45c9c..ce68ceda652c 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -118,4 +118,22 @@ static inline bool blk_mq_hw_queue_mapped(struct blk_mq_hw_ctx *hctx)
 	return hctx->nr_ctx && hctx->tags;
 }
 
+static inline struct blk_mq_ctx *next_ctx(struct request_queue *q, int *i)
+{
+	do {
+		(*i)++;
+		if (*i < q->nr_queues) {
+			if (cpu_possible(*i))
+				return per_cpu_ptr(q->queue_ctx, *i);
+			continue;
+		}
+		break;
+	} while (1);
+
+	return NULL;
+}
+
+#define queue_for_each_ctx(q, ctx, i)					\
+	for ((i) = -1; (ctx = next_ctx((q), &(i))) != NULL;)
+
 #endif
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 7fc9296b5742..a8756cd00767 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -261,22 +261,8 @@ static inline void *blk_mq_rq_to_pdu(struct request *rq)
 	for ((i) = 0; (i) < (q)->nr_hw_queues &&			\
 	     ({ hctx = (q)->queue_hw_ctx[i]; 1; }); (i)++)
 
-#define queue_for_each_ctx(q, ctx, i)					\
-	for ((i) = 0; (i) < (q)->nr_queues &&				\
-	     ({ ctx = per_cpu_ptr((q)->queue_ctx, (i)); 1; }); (i)++)
-
 #define hctx_for_each_ctx(hctx, ctx, i)					\
 	for ((i) = 0; (i) < (hctx)->nr_ctx &&				\
 	     ({ ctx = (hctx)->ctxs[(i)]; 1; }); (i)++)
 
-#define blk_ctx_sum(q, sum)						\
-({									\
-	struct blk_mq_ctx *__x;						\
-	unsigned int __ret = 0, __i;					\
-									\
-	queue_for_each_ctx((q), __x, __i)				\
-		__ret += sum;						\
-	__ret;								\
-})
-
 #endif