aio: make aio_put_req() lockless

Freeing a kiocb needed to touch the kioctx for three things:

 * Pull it off the reqs_active list
 * Decrementing reqs_active
 * Issuing a wakeup, if the kioctx was in the process of being freed.

This patch moves these to aio_complete(), for a couple reasons:

 * aio_complete() already has to issue the wakeup, so if we drop the
   kioctx refcount before aio_complete does its wakeup we don't have to
   do it twice.
 * aio_complete currently has to take the kioctx lock, so it makes sense
   for it to pull the kiocb off the reqs_active list too.
 * A later patch is going to change reqs_active to include unreaped
   completions - this will mean allocating a kiocb doesn't have to look
   at the ringbuffer. So taking the decrement of reqs_active out of
   kiocb_free() is useful prep work for that patch.

This doesn't really affect cancellation, since existing (usb) code that
implements a cancel function still calls aio_complete() - we just have
to make sure that aio_complete does the necessary teardown for cancelled
kiocbs.

It does affect code paths where we free kiocbs that were never
submitted; they need to decrement reqs_active and pull the kiocb off the
reqs_active list.  This occurs in two places: kiocb_batch_free(), which
is going away in a later patch, and the error path in io_submit_one.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Kent Overstreet <koverstreet@google.com>
Cc: Zach Brown <zab@redhat.com>
Cc: Felipe Balbi <balbi@ti.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Asai Thambi S P <asamymuthupa@micron.com>
Cc: Selvan Mani <smani@micron.com>
Cc: Sam Bradshaw <sbradshaw@micron.com>
Acked-by: Jeff Moyer <jmoyer@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Benjamin LaHaise <bcrl@kvack.org>
Reviewed-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 34 insertions, 52 deletions

diff --git a/fs/aio.c b/fs/aio.c
index ffa8b8d2cb8e..f877417f3c42 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -89,7 +89,7 @@ struct kioctx {
 
         spinlock_t              ctx_lock;
 
-        int                     reqs_active;
+        atomic_t                reqs_active;
         struct list_head        active_reqs;    /* used for cancellation */
 
         /* sys_io_setup currently limits this to an unsigned int */
@@ -250,7 +250,7 @@ static void ctx_rcu_free(struct rcu_head *head)
 static void __put_ioctx(struct kioctx *ctx)
 {
         unsigned nr_events = ctx->max_reqs;
-        BUG_ON(ctx->reqs_active);
+        BUG_ON(atomic_read(&ctx->reqs_active));
 
         aio_free_ring(ctx);
         if (nr_events) {
@@ -284,7 +284,7 @@ static int kiocb_cancel(struct kioctx *ctx, struct kiocb *kiocb,
         cancel = kiocb->ki_cancel;
         kiocbSetCancelled(kiocb);
         if (cancel) {
-                kiocb->ki_users++;
+                atomic_inc(&kiocb->ki_users);
                 spin_unlock_irq(&ctx->ctx_lock);
 
                 memset(res, 0, sizeof(*res));
@@ -383,12 +383,12 @@ static void kill_ctx(struct kioctx *ctx)
                 kiocb_cancel(ctx, req, &res);
         }
 
-        if (!ctx->reqs_active)
+        if (!atomic_read(&ctx->reqs_active))
                 goto out;
 
         add_wait_queue(&ctx->wait, &wait);
         set_task_state(tsk, TASK_UNINTERRUPTIBLE);
-        while (ctx->reqs_active) {
+        while (atomic_read(&ctx->reqs_active)) {
                 spin_unlock_irq(&ctx->ctx_lock);
                 io_schedule();
                 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
@@ -406,9 +406,9 @@ out:
  */
 ssize_t wait_on_sync_kiocb(struct kiocb *iocb)
 {
-        while (iocb->ki_users) {
+        while (atomic_read(&iocb->ki_users)) {
                 set_current_state(TASK_UNINTERRUPTIBLE);
-                if (!iocb->ki_users)
+                if (!atomic_read(&iocb->ki_users))
                         break;
                 io_schedule();
         }
@@ -438,7 +438,7 @@ void exit_aio(struct mm_struct *mm)
                         printk(KERN_DEBUG
                                 "exit_aio:ioctx still alive: %d %d %d\n",
                                 atomic_read(&ctx->users), ctx->dead,
-                                ctx->reqs_active);
+                                atomic_read(&ctx->reqs_active));
                 /*
                  * We don't need to bother with munmap() here -
                  * exit_mmap(mm) is coming and it'll unmap everything.
@@ -453,11 +453,11 @@ void exit_aio(struct mm_struct *mm)
 }
 
 /* aio_get_req
- *      Allocate a slot for an aio request.  Increments the users count
+ *      Allocate a slot for an aio request.  Increments the ki_users count
  * of the kioctx so that the kioctx stays around until all requests are
  * complete.  Returns NULL if no requests are free.
  *
- * Returns with kiocb->users set to 2.  The io submit code path holds
+ * Returns with kiocb->ki_users set to 2.  The io submit code path holds
  * an extra reference while submitting the i/o.
  * This prevents races between the aio code path referencing the
  * req (after submitting it) and aio_complete() freeing the req.
@@ -471,7 +471,7 @@ static struct kiocb *__aio_get_req(struct kioctx *ctx)
                 return NULL;
 
         req->ki_flags = 0;
-        req->ki_users = 2;
+        atomic_set(&req->ki_users, 2);
         req->ki_key = 0;
         req->ki_ctx = ctx;
         req->ki_cancel = NULL;
@@ -512,9 +512,9 @@ static void kiocb_batch_free(struct kioctx *ctx, struct kiocb_batch *batch)
                 list_del(&req->ki_batch);
                 list_del(&req->ki_list);
                 kmem_cache_free(kiocb_cachep, req);
-                ctx->reqs_active--;
+                atomic_dec(&ctx->reqs_active);
         }
-        if (unlikely(!ctx->reqs_active && ctx->dead))
+        if (unlikely(!atomic_read(&ctx->reqs_active) && ctx->dead))
                 wake_up_all(&ctx->wait);
         spin_unlock_irq(&ctx->ctx_lock);
 }
@@ -545,7 +545,8 @@ static int kiocb_batch_refill(struct kioctx *ctx, struct kiocb_batch *batch)
         spin_lock_irq(&ctx->ctx_lock);
         ring = kmap_atomic(ctx->ring_info.ring_pages[0]);
 
-        avail = aio_ring_avail(&ctx->ring_info, ring) - ctx->reqs_active;
+        avail = aio_ring_avail(&ctx->ring_info, ring) -
+                                atomic_read(&ctx->reqs_active);
         BUG_ON(avail < 0);
         if (avail < allocated) {
                 /* Trim back the number of requests. */
@@ -560,7 +561,7 @@ static int kiocb_batch_refill(struct kioctx *ctx, struct kiocb_batch *batch)
         batch->count -= allocated;
         list_for_each_entry(req, &batch->head, ki_batch) {
                 list_add(&req->ki_list, &ctx->active_reqs);
-                ctx->reqs_active++;
+                atomic_inc(&ctx->reqs_active);
         }
 
         kunmap_atomic(ring);
@@ -583,10 +584,8 @@ static inline struct kiocb *aio_get_req(struct kioctx *ctx,
         return req;
 }
 
-static inline void really_put_req(struct kioctx *ctx, struct kiocb *req)
+static void kiocb_free(struct kiocb *req)
 {
-        assert_spin_locked(&ctx->ctx_lock);
-
         if (req->ki_filp)
                 fput(req->ki_filp);
         if (req->ki_eventfd != NULL)
@@ -596,40 +595,12 @@ static inline void really_put_req(struct kioctx *ctx, struct kiocb *req)
         if (req->ki_iovec != &req->ki_inline_vec)
                 kfree(req->ki_iovec);
         kmem_cache_free(kiocb_cachep, req);
-        ctx->reqs_active--;
-
-        if (unlikely(!ctx->reqs_active && ctx->dead))
-                wake_up_all(&ctx->wait);
 }
 
-/* __aio_put_req
- *      Returns true if this put was the last user of the request.
- */
-static void __aio_put_req(struct kioctx *ctx, struct kiocb *req)
-{
-        assert_spin_locked(&ctx->ctx_lock);
-
-        req->ki_users--;
-        BUG_ON(req->ki_users < 0);
-        if (likely(req->ki_users))
-                return;
-        list_del(&req->ki_list);                /* remove from active_reqs */
-        req->ki_cancel = NULL;
-        req->ki_retry = NULL;
-
-        really_put_req(ctx, req);
-}
-
-/* aio_put_req
- *      Returns true if this put was the last user of the kiocb,
- *      false if the request is still in use.
- */
 void aio_put_req(struct kiocb *req)
 {
-        struct kioctx *ctx = req->ki_ctx;
-        spin_lock_irq(&ctx->ctx_lock);
-        __aio_put_req(ctx, req);
-        spin_unlock_irq(&ctx->ctx_lock);
+        if (atomic_dec_and_test(&req->ki_users))
+                kiocb_free(req);
 }
 EXPORT_SYMBOL(aio_put_req);
 
@@ -677,9 +648,9 @@ void aio_complete(struct kiocb *iocb, long res, long res2)
          *  - the sync task helpfully left a reference to itself in the iocb
          */
         if (is_sync_kiocb(iocb)) {
-                BUG_ON(iocb->ki_users != 1);
+                BUG_ON(atomic_read(&iocb->ki_users) != 1);
                 iocb->ki_user_data = res;
-                iocb->ki_users = 0;
+                atomic_set(&iocb->ki_users, 0);
                 wake_up_process(iocb->ki_obj.tsk);
                 return;
         }
@@ -694,6 +665,8 @@ void aio_complete(struct kiocb *iocb, long res, long res2)
          */
         spin_lock_irqsave(&ctx->ctx_lock, flags);
 
+        list_del(&iocb->ki_list); /* remove from active_reqs */
+
         /*
          * cancelled requests don't get events, userland was given one
          * when the event got cancelled.
@@ -740,7 +713,8 @@ void aio_complete(struct kiocb *iocb, long res, long res2)
 
 put_rq:
         /* everything turned out well, dispose of the aiocb. */
-        __aio_put_req(ctx, iocb);
+        aio_put_req(iocb);
+        atomic_dec(&ctx->reqs_active);
 
         /*
          * We have to order our ring_info tail store above and test
@@ -905,7 +879,7 @@ static int read_events(struct kioctx *ctx,
                                 break;
                         /* Try to only show up in io wait if there are ops
                          *  in flight */
-                        if (ctx->reqs_active)
+                        if (atomic_read(&ctx->reqs_active))
                                 io_schedule();
                         else
                                 schedule();
@@ -1369,6 +1343,14 @@ static int io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
         return 0;
 
 out_put_req:
+        spin_lock_irq(&ctx->ctx_lock);
+        list_del(&req->ki_list);
+        spin_unlock_irq(&ctx->ctx_lock);
+
+        atomic_dec(&ctx->reqs_active);
+        if (unlikely(!atomic_read(&ctx->reqs_active) && ctx->dead))
+                wake_up_all(&ctx->wait);
+
         aio_put_req(req);       /* drop extra ref to req */
         aio_put_req(req);       /* drop i/o ref to req */
         return ret;