relay: allow the use of const callback structs

None of the relay users require the use of mutable structs for callbacks,
however the relay code does.  Instead of assigning the default callback
for subbuf_start, add a wrapper to conditionally call the client callback
if available, and fall back to default behaviour otherwise.

This lets all relay users make their struct rchan_callbacks const data.

[jani.nikula@intel.com: cleanups, per Christoph]
  Link: https://lkml.kernel.org/r/20201124115412.32402-1-jani.nikula@intel.com

Link: https://lkml.kernel.org/r/cc3ff292e4eb4fdc56bee3d690c7b8e39209cd37.1606153547.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/relay.h

@@ -62,7 +62,7 @@ struct rchan
 	size_t subbuf_size;		/* sub-buffer size */
 	size_t n_subbufs;		/* number of sub-buffers per buffer */
 	size_t alloc_size;		/* total buffer size allocated */
-	struct rchan_callbacks *cb;	/* client callbacks */
+	const struct rchan_callbacks *cb; /* client callbacks */
 	struct kref kref;		/* channel refcount */
 	void *private_data;		/* for user-defined data */
 	size_t last_toobig;		/* tried to log event > subbuf size */
@@ -157,7 +157,7 @@ struct rchan *relay_open(const char *base_filename,
 			 struct dentry *parent,
 			 size_t subbuf_size,
 			 size_t n_subbufs,
-			 struct rchan_callbacks *cb,
+			 const struct rchan_callbacks *cb,
 			 void *private_data);
 extern int relay_late_setup_files(struct rchan *chan,
 				  const char *base_filename,

kernel/relay.c

@@ -252,23 +252,14 @@ EXPORT_SYMBOL_GPL(relay_buf_full);
  * High-level relay kernel API and associated functions.
  */
 
-/*
- * rchan_callback implementations defining default channel behavior.  Used
- * in place of corresponding NULL values in client callback struct.
- */
-
-/*
- * subbuf_start() default callback.  Does nothing.
- */
-static int subbuf_start_default_callback (struct rchan_buf *buf,
-					  void *subbuf,
-					  void *prev_subbuf,
-					  size_t prev_padding)
+static int relay_subbuf_start(struct rchan_buf *buf, void *subbuf,
+			      void *prev_subbuf, size_t prev_padding)
 {
-	if (relay_buf_full(buf))
-		return 0;
+	if (!buf->chan->cb->subbuf_start)
+		return !relay_buf_full(buf);
 
-	return 1;
+	return buf->chan->cb->subbuf_start(buf, subbuf,
+					   prev_subbuf, prev_padding);
 }
 
 /**
@@ -314,7 +305,7 @@ static void __relay_reset(struct rchan_buf *buf, unsigned int init)
 	for (i = 0; i < buf->chan->n_subbufs; i++)
 		buf->padding[i] = 0;
 
-	buf->chan->cb->subbuf_start(buf, buf->data, NULL, 0);
+	relay_subbuf_start(buf, buf->data, NULL, 0);
 }
 
 /**
@@ -442,14 +433,6 @@ static void relay_close_buf(struct rchan_buf *buf)
 	kref_put(&buf->kref, relay_remove_buf);
 }
 
-static void setup_callbacks(struct rchan *chan,
-				   struct rchan_callbacks *cb)
-{
-	if (!cb->subbuf_start)
-		cb->subbuf_start = subbuf_start_default_callback;
-	chan->cb = cb;
-}
-
 int relay_prepare_cpu(unsigned int cpu)
 {
 	struct rchan *chan;
@@ -495,7 +478,7 @@ struct rchan *relay_open(const char *base_filename,
 			 struct dentry *parent,
 			 size_t subbuf_size,
 			 size_t n_subbufs,
-			 struct rchan_callbacks *cb,
+			 const struct rchan_callbacks *cb,
 			 void *private_data)
 {
 	unsigned int i;
@@ -529,7 +512,7 @@ struct rchan *relay_open(const char *base_filename,
 		chan->has_base_filename = 1;
 		strlcpy(chan->base_filename, base_filename, NAME_MAX);
 	}
-	setup_callbacks(chan, cb);
+	chan->cb = cb;
 	kref_init(&chan->kref);
 
 	mutex_lock(&relay_channels_mutex);
@@ -712,7 +695,7 @@ size_t relay_switch_subbuf(struct rchan_buf *buf, size_t length)
 	new_subbuf = buf->subbufs_produced % buf->chan->n_subbufs;
 	new = buf->start + new_subbuf * buf->chan->subbuf_size;
 	buf->offset = 0;
-	if (!buf->chan->cb->subbuf_start(buf, new, old, buf->prev_padding)) {
+	if (!relay_subbuf_start(buf, new, old, buf->prev_padding)) {
 		buf->offset = buf->chan->subbuf_size + 1;
 		return 0;
 	}