Re: [PATCH 1/1] io_uring: use proper references for fallback_req locking

[Pavel Begunkov <asml.silence@gmail.com>]

On 30/04/2020 17:52, Jens Axboe wrote:
> On 4/29/20 6:47 PM, Bijan Mottahedeh wrote:
>> Use ctx->fallback_req address for test_and_set_bit_lock() and
>> clear_bit_unlock().
> 
> Thanks, applied.
> 

How about getting rid of it? As once was fairly noticed, we're screwed in many
other ways in case of OOM. Otherwise we at least need to make async context
allocation more resilient.


Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index b699e9a8d247..62f9709565d0 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -282,9 +282,6 @@ struct io_ring_ctx {
 	/* 0 is for ctx quiesce/reinit/free, 1 is for sqo_thread started */
 	struct completion	*completions;

-	/* if all else fails... */
-	struct io_kiocb		*fallback_req;
-
 #if defined(CONFIG_UNIX)
 	struct socket		*ring_sock;
 #endif
@@ -911,10 +908,6 @@ static struct io_ring_ctx *io_ring_ctx_alloc(struct
io_uring_params *p)
 	if (!ctx)
 		return NULL;

-	ctx->fallback_req = kmem_cache_alloc(req_cachep, GFP_KERNEL);
-	if (!ctx->fallback_req)
-		goto err;
-
 	ctx->completions = kmalloc(2 * sizeof(struct completion), GFP_KERNEL);
 	if (!ctx->completions)
 		goto err;
@@ -956,8 +949,6 @@ static struct io_ring_ctx *io_ring_ctx_alloc(struct
io_uring_params *p)
 	INIT_LIST_HEAD(&ctx->inflight_list);
 	return ctx;
 err:
-	if (ctx->fallback_req)
-		kmem_cache_free(req_cachep, ctx->fallback_req);
 	kfree(ctx->completions);
 	kfree(ctx->cancel_hash);
 	kfree(ctx);
@@ -1308,34 +1299,16 @@ static void io_cqring_add_event(struct io_kiocb *req,
long res)
 	__io_cqring_add_event(req, res, 0);
 }

-static inline bool io_is_fallback_req(struct io_kiocb *req)
-{
-	return req == (struct io_kiocb *)
-			((unsigned long) req->ctx->fallback_req & ~1UL);
-}
-
-static struct io_kiocb *io_get_fallback_req(struct io_ring_ctx *ctx)
-{
-	struct io_kiocb *req;
-
-	req = ctx->fallback_req;
-	if (!test_and_set_bit_lock(0, (unsigned long *) &ctx->fallback_req))
-		return req;
-
-	return NULL;
-}
-
 static struct io_kiocb *io_alloc_req(struct io_ring_ctx *ctx,
 				     struct io_submit_state *state)
 {
 	gfp_t gfp = GFP_KERNEL | __GFP_NOWARN;
 	struct io_kiocb *req;

-	if (!state) {
-		req = kmem_cache_alloc(req_cachep, gfp);
-		if (unlikely(!req))
-			goto fallback;
-	} else if (!state->free_reqs) {
+	if (!state)
+		return kmem_cache_alloc(req_cachep, gfp);
+
+	if (!state->free_reqs) {
 		size_t sz;
 		int ret;

@@ -1349,7 +1322,7 @@ static struct io_kiocb *io_alloc_req(struct io_ring_ctx *ctx,
 		if (unlikely(ret <= 0)) {
 			state->reqs[0] = kmem_cache_alloc(req_cachep, gfp);
 			if (!state->reqs[0])
-				goto fallback;
+				return NULL;
 			ret = 1;
 		}
 		state->free_reqs = ret - 1;
@@ -1360,8 +1333,6 @@ static struct io_kiocb *io_alloc_req(struct io_ring_ctx *ctx,
 	}

 	return req;
-fallback:
-	return io_get_fallback_req(ctx);
 }

 static inline void io_put_file(struct io_kiocb *req, struct file *file,
@@ -1403,10 +1374,7 @@ static void __io_free_req(struct io_kiocb *req)
 	}

 	percpu_ref_put(&req->ctx->refs);
-	if (likely(!io_is_fallback_req(req)))
-		kmem_cache_free(req_cachep, req);
-	else
-		clear_bit_unlock(0, (unsigned long *) &req->ctx->fallback_req);
+	kmem_cache_free(req_cachep, req);
 }

 struct req_batch {
@@ -1699,7 +1667,7 @@ static inline unsigned int io_sqring_entries(struct
io_ring_ctx *ctx)

 static inline bool io_req_multi_free(struct req_batch *rb, struct io_kiocb *req)
 {
-	if ((req->flags & REQ_F_LINK_HEAD) || io_is_fallback_req(req))
+	if (req->flags & REQ_F_LINK_HEAD)
 		return false;

 	if (!(req->flags & REQ_F_FIXED_FILE) || req->io)
@@ -7296,7 +7264,6 @@ static void io_ring_ctx_free(struct io_ring_ctx *ctx)
 	put_cred(ctx->creds);
 	kfree(ctx->completions);
 	kfree(ctx->cancel_hash);
-	kmem_cache_free(req_cachep, ctx->fallback_req);
 	kfree(ctx);
 }