io_uring: fix iowq_limits data race in tctx node addition

__io_uring_add_tctx_node() reads ctx->int_flags and
ctx->iowq_limits[0..1] without holding ctx->uring_lock, while
io_register_iowq_max_workers() writes these same fields under the lock.

Mostly an application problem if you try and make these race, but let's
silence KCSAN by just grabbing the ->uring_lock around the operation.
This is a slow path operation anyway, and ->uring_lock will be grabbed
by submission right after anyway.

Fixes: 2e48005 ("io-wq: provide a way to limit max number of workers")
Signed-off-by: Jens Axboe <[email protected]>

Author: axboe

io_uring/tctx.c

@@ -146,9 +146,13 @@ int __io_uring_add_tctx_node(struct io_ring_ctx *ctx)
 		if (IS_ERR(tctx))
 			return PTR_ERR(tctx);
 
-		if (ctx->int_flags & IO_RING_F_IOWQ_LIMITS_SET) {
-			unsigned int limits[2] = { ctx->iowq_limits[0],
-						   ctx->iowq_limits[1], };
+		if (data_race(ctx->int_flags) & IO_RING_F_IOWQ_LIMITS_SET) {
+			unsigned int limits[2];
+
+			mutex_lock(&ctx->uring_lock);
+			limits[0] = ctx->iowq_limits[0];
+			limits[1] = ctx->iowq_limits[1];
+			mutex_unlock(&ctx->uring_lock);
 
 			ret = io_wq_max_workers(tctx->io_wq, limits);
 			if (ret)