vhost: add spdk_vhost_trylock()

We'll make use of it inside the vhost device backend
code. The function itself is generic enough to be put
in the public vhost.h header rather than vhost_internal.h.

Change-Id: I60602c61d8bba665dcf9c6d27af2e910c208a7be
Signed-off-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/448226
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>

include/spdk/vhost.h

@@ -109,10 +109,17 @@ void spdk_vhost_shutdown_cb(void);
 struct spdk_vhost_dev;
 
 /**
- * Lock the global vhost mutex, which synchronizes all the vhost device accesses.
+ * Lock the global vhost mutex synchronizing all the vhost device accesses.
  */
 void spdk_vhost_lock(void);
 
+/**
+ * Lock the global vhost mutex synchronizing all the vhost device accesses.
+ *
+ * \return 0 if the mutex could be locked immediately, negative errno otherwise.
+ */
+int spdk_vhost_trylock(void);
+
 /**
  * Unlock the global vhost mutex.
  */

lib/vhost/vhost.c

@@ -1403,6 +1403,12 @@ spdk_vhost_lock(void)
 	pthread_mutex_lock(&g_spdk_vhost_mutex);
 }
 
+int
+spdk_vhost_trylock(void)
+{
+	return -pthread_mutex_trylock(&g_spdk_vhost_mutex);
+}
+
 void
 spdk_vhost_unlock(void)
 {