linux: always use io_uring for epoll batching (libuv#4638)

io_uring support was default-disabled because of numerous kernel bugs
but those are all in the sqpoll (file i/o) parts of io_uring.

Batching of epoll_ctl calls through io_uring works fine, is a nice
optimization, and is therefore unconditionally enabled again.

The UV_USE_IO_URING environment variable now only affects sqpoll, and
only when the UV_LOOP_ENABLE_IO_URING_SQPOLL event loop flag is set.

Fixes: libuv#4616

src/unix/linux.c

@@ -455,7 +455,7 @@ int uv__io_uring_register(int fd, unsigned opcode, void* arg, unsigned nargs) {
 }
 
 
-static int uv__use_io_uring(void) {
+static int uv__use_io_uring(uint32_t flags) {
 #if defined(__ANDROID_API__)
   return 0;  /* Possibly available but blocked by seccomp. */
 #elif defined(__arm__) && __SIZEOF_POINTER__ == 4
@@ -470,25 +470,27 @@ static int uv__use_io_uring(void) {
   char* val;
   int use;
 
-  use = atomic_load_explicit(&use_io_uring, memory_order_relaxed);
-
-  if (use == 0) {
-    use = uv__kernel_version() >=
 #if defined(__hppa__)
-    /* io_uring first supported on parisc in 6.1, functional in .51 */
-    /* https://lore.kernel.org/all/cb912694-b1fe-dbb0-4d8c-d608f3526905@gmx.de/ */
-    /* 6.1.51 */ 0x060133
-#else
-    /* Older kernels have a bug where the sqpoll thread uses 100% CPU. */
-    /* 5.10.186 */ 0x050ABA
+  /* io_uring first supported on parisc in 6.1, functional in .51
+   * https://lore.kernel.org/all/cb912694-b1fe-dbb0-4d8c-d608f3526905@gmx.de/
+   */
+  if (uv__kernel_version() < /*6.1.51*/0x060133)
+    return 0;
 #endif
-    ? 1 : -1;
 
-    /* But users can still enable it if they so desire. */
-    val = getenv("UV_USE_IO_URING");
-    if (val != NULL)
-      use = atoi(val) ? 1 : -1;
+  /* SQPOLL is all kinds of buggy but epoll batching should work fine. */
+  if (0 == (flags & UV__IORING_SETUP_SQPOLL))
+    return 1;
 
+  /* Older kernels have a bug where the sqpoll thread uses 100% CPU. */
+  if (uv__kernel_version() < /*5.10.186*/0x050ABA)
+    return 0;
+
+  use = atomic_load_explicit(&use_io_uring, memory_order_relaxed);
+
+  if (use == 0) {
+    val = getenv("UV_USE_IO_URING");
+    use = val != NULL && atoi(val) > 0 ? 1 : -1;
     atomic_store_explicit(&use_io_uring, use, memory_order_relaxed);
   }
 
@@ -518,7 +520,7 @@ static void uv__iou_init(int epollfd,
   sq = MAP_FAILED;
   sqe = MAP_FAILED;
 
-  if (!uv__use_io_uring())
+  if (!uv__use_io_uring(flags))
     return;
 
   kernel_version = uv__kernel_version();
@@ -766,14 +768,13 @@ static struct uv__io_uring_sqe* uv__iou_get_sqe(struct uv__iou* iou,
    */
   if (iou->ringfd == -2) {
     /* By default, the SQPOLL is not created. Enable only if the loop is
-     * configured with UV_LOOP_USE_IO_URING_SQPOLL.
+     * configured with UV_LOOP_USE_IO_URING_SQPOLL and the UV_USE_IO_URING
+     * environment variable is unset or a positive number.
      */
-    if ((loop->flags & UV_LOOP_ENABLE_IO_URING_SQPOLL) == 0) {
-      iou->ringfd = -1;
-      return NULL;
-    }
+    if (loop->flags & UV_LOOP_ENABLE_IO_URING_SQPOLL)
+      if (uv__use_io_uring(UV__IORING_SETUP_SQPOLL))
+        uv__iou_init(loop->backend_fd, iou, 64, UV__IORING_SETUP_SQPOLL);
 
-    uv__iou_init(loop->backend_fd, iou, 64, UV__IORING_SETUP_SQPOLL);
     if (iou->ringfd == -2)
       iou->ringfd = -1;  /* "failed" */
   }