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>

diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
index b9bb02e..d4550ec 100644 (file)
--- a/block/ll_rw_blk.c
+++ b/block/ll_rw_blk.c
@@ -3854,12 +3854,13 @@ int __init blk_dev_init(void)
 }
 
 /*
- * IO Context helper functions
+ * IO Context helper functions. put_io_context() returns 1 if there are no
+ * more users of this io context, 0 otherwise.
  */
-void put_io_context(struct io_context *ioc)
+int put_io_context(struct io_context *ioc)
 {
        if (ioc == NULL)
-               return;
+               return 1;
 
        BUG_ON(atomic_read(&ioc->refcount) == 0);
 
@@ -3878,7 +3879,9 @@ void put_io_context(struct io_context *ioc)
                rcu_read_unlock();
 
                kmem_cache_free(iocontext_cachep, ioc);
+               return 1;
        }
+       return 0;
 }
 EXPORT_SYMBOL(put_io_context);
 
@@ -3893,15 +3896,17 @@ void exit_io_context(void)
        current->io_context = NULL;
        task_unlock(current);
 
-       ioc->task = NULL;
-       if (ioc->aic && ioc->aic->exit)
-               ioc->aic->exit(ioc->aic);
-       if (ioc->cic_root.rb_node != NULL) {
-               cic = rb_entry(rb_first(&ioc->cic_root), struct cfq_io_context, rb_node);
-               cic->exit(ioc);
-       }
+       if (atomic_dec_and_test(&ioc->nr_tasks)) {
+               if (ioc->aic && ioc->aic->exit)
+                       ioc->aic->exit(ioc->aic);
+               if (ioc->cic_root.rb_node != NULL) {
+                       cic = rb_entry(rb_first(&ioc->cic_root),
+                               struct cfq_io_context, rb_node);
+                       cic->exit(ioc);
+               }
 
-       put_io_context(ioc);
+               put_io_context(ioc);
+       }
 }
 
 struct io_context *alloc_io_context(gfp_t gfp_flags, int node)
@@ -3911,7 +3916,8 @@ struct io_context *alloc_io_context(gfp_t gfp_flags, int node)
        ret = kmem_cache_alloc_node(iocontext_cachep, gfp_flags, node);
        if (ret) {
                atomic_set(&ret->refcount, 1);
-               ret->task = current;
+               atomic_set(&ret->nr_tasks, 1);
+               spin_lock_init(&ret->lock);
                ret->ioprio_changed = 0;
                ret->ioprio = 0;
                ret->last_waited = jiffies; /* doesn't matter... */
@@ -3959,10 +3965,18 @@ static struct io_context *current_io_context(gfp_t gfp_flags, int node)
  */
 struct io_context *get_io_context(gfp_t gfp_flags, int node)
 {
-       struct io_context *ret;
-       ret = current_io_context(gfp_flags, node);
-       if (likely(ret))
-               atomic_inc(&ret->refcount);
+       struct io_context *ret = NULL;
+
+       /*
+        * Check for unlikely race with exiting task. ioc ref count is
+        * zero when ioc is being detached.
+        */
+       do {
+               ret = current_io_context(gfp_flags, node);
+               if (unlikely(!ret))
+                       break;
+       } while (!atomic_inc_not_zero(&ret->refcount));
+
        return ret;
 }
 EXPORT_SYMBOL(get_io_context);

diff --git a/fs/ioprio.c b/fs/ioprio.c
index a760040..06b5d97 100644 (file)
--- a/fs/ioprio.c
+++ b/fs/ioprio.c
@@ -54,7 +54,6 @@ static int set_task_ioprio(struct task_struct *task, int ioprio)
                        break;
                }
                task->io_context = ioc;
-               ioc->task = task;
        } while (1);
 
        if (!err) {

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 510a18b..2483a05 100644 (file)
--- 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 186807e..cd44d45 100644 (file)
--- 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

diff --git a/kernel/fork.c b/kernel/fork.c
index 2a86c9d..1987c57 100644 (file)
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -805,7 +805,6 @@ static int copy_io(struct task_struct *tsk)
                if (unlikely(!tsk->io_context))
                        return -ENOMEM;
 
-               tsk->io_context->task = tsk;
                tsk->io_context->ioprio = ioc->ioprio;
        }
 #endif