io_channel: add helper to get an I/O channel's thread

This is faster than spdk_get_thread(), which iterates over all of the
threads under a lock until it finds the current one.

Change-Id: I957a68438efb5b7d632d2b9b1b567fed7714e02a
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>
Reviewed-on: https://review.gerrithub.io/365079
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>

include/spdk/io_channel.h

@@ -78,6 +78,8 @@ void spdk_free_thread(void);
 /**
  * \brief Get a handle to the current thread. This handle may be passed
  * to other threads and used as the target of spdk_thread_send_msg().
+ *
+ * \sa spdk_io_channel_get_thread()
  */
 struct spdk_thread *spdk_get_thread(void);
 
@@ -140,6 +142,11 @@ void spdk_put_io_channel(struct spdk_io_channel *ch);
  */
 void *spdk_io_channel_get_ctx(struct spdk_io_channel *ch);
 
+/**
+ * \brief Returns the spdk_thread associated with an I/O channel.
+ */
+struct spdk_thread *spdk_io_channel_get_thread(struct spdk_io_channel *ch);
+
 /**
  * \brief Call 'fn' on each channel associated with io_device. This happens
  * asynchronously, so fn may be called after spdk_for_each_channel returns.

lib/util/io_channel.c

@@ -296,6 +296,12 @@ spdk_io_channel_get_ctx(struct spdk_io_channel *ch)
 	return (uint8_t *)ch + sizeof(*ch);
 }
 
+struct spdk_thread *
+spdk_io_channel_get_thread(struct spdk_io_channel *ch)
+{
+	return ch->thread;
+}
+
 struct call_channel {
 	void *io_device;
 	spdk_channel_msg fn;