io context sharing: preliminary support

Detach task state from ioc, instead keep track of how many processes
are accessing the ioc.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

2 files changed, 19 insertions, 5 deletions

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 510a18ba1ec..2483a05231c 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -34,7 +34,7 @@ struct sg_io_hdr;
 #define BLKDEV_MIN_RQ   4
 #define BLKDEV_MAX_RQ   128     /* Default maximum */
 
-void put_io_context(struct io_context *ioc);
+int put_io_context(struct io_context *ioc);
 void exit_io_context(void);
 struct io_context *get_io_context(gfp_t gfp_flags, int node);
 struct io_context *alloc_io_context(gfp_t gfp_flags, int node);
diff --git a/include/linux/iocontext.h b/include/linux/iocontext.h
index 186807ea62e..cd44d458124 100644
--- a/include/linux/iocontext.h
+++ b/include/linux/iocontext.h
@@ -54,13 +54,15 @@ struct cfq_io_context {
 };
 
 /*
- * This is the per-process I/O subsystem state.  It is refcounted and
- * kmalloc'ed. Currently all fields are modified in process io context
- * (apart from the atomic refcount), so require no locking.
+ * I/O subsystem state of the associated processes.  It is refcounted
+ * and kmalloc'ed. These could be shared between processes.
  */
 struct io_context {
         atomic_t refcount;
-        struct task_struct *task;
+        atomic_t nr_tasks;
+
+        /* all the fields below are protected by this lock */
+        spinlock_t lock;
 
         unsigned short ioprio;
         unsigned short ioprio_changed;
@@ -76,4 +78,16 @@ struct io_context {
         void *ioc_data;
 };
 
+static inline struct io_context *ioc_task_link(struct io_context *ioc)
+{
+        /*
+         * if ref count is zero, don't allow sharing (ioc is going away, it's
+         * a race).
+         */
+        if (ioc && atomic_inc_not_zero(&ioc->refcount))
+                return ioc;
+
+        return NULL;
+}
+
 #endif