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>

5 files changed, 49 insertions, 23 deletions

diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
index b9bb02e845cd..d4550ecae443 100644
--- 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 a7600401ecf7..06b5d97c5fdd 100644
--- 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 510a18ba1ec5..2483a05231c7 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 186807ea62e2..cd44d458124a 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
diff --git a/kernel/fork.c b/kernel/fork.c
index 2a86c9dff744..1987c57abb08 100644
--- 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