Re: serial percpu_ref draining in exit_aio()

[Jens Axboe]

On 03/19/2015 03:27 PM, Jens Axboe wrote:
> On 03/19/2015 03:00 PM, Tejun Heo wrote:
> > On Thu, Mar 19, 2015 at 04:58:33PM -0400, Jeff Moyer wrote:
> > > Do you really want to do memory allocation in the exit path?  That
> > > sounds like a bad idea to me.  (Of course, now you're going to point out
> > > all the places that currently happens, right?  ;-)
> >
> > I think we just need a counter there - let everyone count down as they
> > exit and the last one trigger the completion, no?
>
> Yeah that's a good point, that'd be cleaner too. I'll change it and test
> that.

Here's a cleaner variant that pairs the completion even with an atomic 
count. Works for me as well.

--
Jens Axboe

diff --git a/fs/aio.c b/fs/aio.c
index f8e52a1854c1..cabb5edd9bc1 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -77,6 +77,11 @@ struct kioctx_cpu {
 	unsigned		reqs_available;
 };
 
+struct ctx_rq_wait {
+	struct completion comp;
+	atomic_t count;
+};
+
 struct kioctx {
 	struct percpu_ref	users;
 	atomic_t		dead;
@@ -115,7 +120,7 @@ struct kioctx {
 	/*
 	 * signals when all in-flight requests are done
 	 */
-	struct completion *requests_done;
+	struct ctx_rq_wait	*rq_wait;
 
 	struct {
 		/*
@@ -535,8 +540,8 @@ static void free_ioctx_reqs(struct percpu_ref *ref)
 	struct kioctx *ctx = container_of(ref, struct kioctx, reqs);
 
 	/* At this point we know that there are no any in-flight requests */
-	if (ctx->requests_done)
-		complete(ctx->requests_done);
+	if (ctx->rq_wait && atomic_dec_and_test(&ctx->rq_wait->count))
+		complete(&ctx->rq_wait->comp);
 
 	INIT_WORK(&ctx->free_work, free_ioctx);
 	schedule_work(&ctx->free_work);
@@ -744,7 +749,7 @@ err:
  *	the rapid destruction of the kioctx.
  */
 static int kill_ioctx(struct mm_struct *mm, struct kioctx *ctx,
-		struct completion *requests_done)
+		      struct ctx_rq_wait *wait)
 {
 	struct kioctx_table *table;
 
@@ -773,7 +778,7 @@ static int kill_ioctx(struct mm_struct *mm, struct kioctx *ctx,
 	if (ctx->mmap_size)
 		vm_munmap(ctx->mmap_base, ctx->mmap_size);
 
-	ctx->requests_done = requests_done;
+	ctx->rq_wait = wait;
 	percpu_ref_kill(&ctx->users);
 	return 0;
 }
@@ -805,18 +810,24 @@ EXPORT_SYMBOL(wait_on_sync_kiocb);
 void exit_aio(struct mm_struct *mm)
 {
 	struct kioctx_table *table = rcu_dereference_raw(mm->ioctx_table);
-	int i;
+	struct ctx_rq_wait wait;
+	int i, skipped;
 
 	if (!table)
 		return;
 
+	atomic_set(&wait.count, table->nr);
+	init_completion(&wait.comp);
+
+	skipped = 0;
 	for (i = 0; i < table->nr; ++i) {
 		struct kioctx *ctx = table->table[i];
-		struct completion requests_done =
-			COMPLETION_INITIALIZER_ONSTACK(requests_done);
 
-		if (!ctx)
+		if (!ctx) {
+			skipped++;
 			continue;
+		}
+
 		/*
 		 * We don't need to bother with munmap() here - exit_mmap(mm)
 		 * is coming and it'll unmap everything. And we simply can't,
@@ -825,10 +836,12 @@ void exit_aio(struct mm_struct *mm)
 		 * that it needs to unmap the area, just set it to 0.
 		 */
 		ctx->mmap_size = 0;
-		kill_ioctx(mm, ctx, &requests_done);
+		kill_ioctx(mm, ctx, &wait);
+	}
 
+	if (!atomic_sub_and_test(skipped, &wait.count)) {
 		/* Wait until all IO for the context are done. */
-		wait_for_completion(&requests_done);
+		wait_for_completion(&wait.comp);
 	}
 
 	RCU_INIT_POINTER(mm->ioctx_table, NULL);
@@ -1313,15 +1326,17 @@ SYSCALL_DEFINE1(io_destroy, aio_context_t, ctx)
 {
 	struct kioctx *ioctx = lookup_ioctx(ctx);
 	if (likely(NULL != ioctx)) {
-		struct completion requests_done =
-			COMPLETION_INITIALIZER_ONSTACK(requests_done);
+		struct ctx_rq_wait wait;
 		int ret;
 
+		init_completion(&wait.comp);
+		atomic_set(&wait.count, 1);
+
 		/* Pass requests_done to kill_ioctx() where it can be set
 		 * in a thread-safe way. If we try to set it here then we have
 		 * a race condition if two io_destroy() called simultaneously.
 		 */
-		ret = kill_ioctx(current->mm, ioctx, &requests_done);
+		ret = kill_ioctx(current->mm, ioctx, &wait);
 		percpu_ref_put(&ioctx->users);
 
 		/* Wait until all IO for the context are done. Otherwise kernel
@@ -1329,7 +1344,7 @@ SYSCALL_DEFINE1(io_destroy, aio_context_t, ctx)
 		 * is destroyed.
 		 */
 		if (!ret)
-			wait_for_completion(&requests_done);
+			wait_for_completion(&wait.comp);
 
 		return ret;
 	}