sched_ext: Maintain an online cid mask in the scheduler arenagrafted

Schedulers on the default cid mapping treat [0, nr_online_cids) as the
online set and restart on hotplug. Schedulers that install their own mapping
with scx_bpf_cid_override() have no way to learn which cids are online: the
count no longer identifies members and the CPU-form cpumask is unusable from
cid programs. This is an obvious hole in the cid API.

Add scx_bpf_online_cmask(), a kernel-maintained cmask in the scheduler's
arena, allocated alongside the per-CPU scratch masks and populated after the
cid mapping is finalized and before ops.init(), for child schedulers too.
The pointer stays valid through ops.exit() with no reference to take. It is
the arena offset as a void pointer, the same form struct_ops arena arguments
arrive in. The verifier types the void return as a scalar for the program's
arena cast.

The mask follows the SCX hotplug notifications: seeded from cpu_active_mask
and updated before ops.cid_online/offline() runs, so it lags cpu_online_mask
only inside a hotplug transition. Updates walk the scheduler list under the
lock that also serializes unlinking. Reads are live, not atomic snapshots.
Root initialization excludes hotplug.

v2: Reworded the getter kerneldoc (Andrea Righi).

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>

3 files changed, 87 insertions, 0 deletions

diff --git a/tools/testing/selftests/tmpfs/.gitignore b/tools/testing/selftests/tmpfs/.gitignore
new file mode 100644
index 000000000..b1afaa925
--- /dev/null
+++ b/tools/testing/selftests/tmpfs/.gitignore
@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0-only
+/bug-link-o-tmpfile
diff --git a/tools/testing/selftests/tmpfs/Makefile b/tools/testing/selftests/tmpfs/Makefile
new file mode 100644
index 000000000..3be931e11
--- /dev/null
+++ b/tools/testing/selftests/tmpfs/Makefile
@@ -0,0 +1,7 @@
+# SPDX-License-Identifier: GPL-2.0-only
+CFLAGS += -Wall -O2
+
+TEST_GEN_PROGS :=
+TEST_GEN_PROGS += bug-link-o-tmpfile
+
+include ../lib.mk
diff --git a/tools/testing/selftests/tmpfs/bug-link-o-tmpfile.c b/tools/testing/selftests/tmpfs/bug-link-o-tmpfile.c
new file mode 100644
index 000000000..5cb4e404a
--- /dev/null
+++ b/tools/testing/selftests/tmpfs/bug-link-o-tmpfile.c
@@ -0,0 +1,78 @@
+/*
+ * Copyright (c) 2019 Alexey Dobriyan <adobriyan@gmail.com>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+/* Test that open(O_TMPFILE), linkat() doesn't screw accounting. */
+#include <errno.h>
+#include <sched.h>
+#include <stdio.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <sys/mount.h>
+#include <unistd.h>
+
+#include "kselftest.h"
+
+int main(void)
+{
+	int fd;
+
+	// Setting up kselftest framework
+	ksft_print_header();
+	ksft_set_plan(1);
+
+	// Check if test is run as root
+	if (geteuid()) {
+		ksft_exit_skip("This test needs root to run!\n");
+		return 1;
+	}
+
+	if (unshare(CLONE_NEWNS) == -1) {
+		if (errno == ENOSYS || errno == EPERM) {
+			ksft_exit_skip("unshare() error: unshare, errno %d\n", errno);
+		} else {
+			ksft_exit_fail_msg("unshare() error: unshare, errno %d\n", errno);
+		}
+	}
+
+	if (mount(NULL, "/", NULL, MS_PRIVATE|MS_REC, NULL) == -1) {
+		ksft_exit_fail_msg("mount() error: Root filesystem private mount: Fail %d\n", errno);
+	}
+
+	/* Our heroes: 1 root inode, 1 O_TMPFILE inode, 1 permanent inode. */
+	if (mount(NULL, "/tmp", "tmpfs", 0, "nr_inodes=3") == -1) {
+		ksft_exit_fail_msg("mount() error: Mounting tmpfs on /tmp: Fail %d\n", errno);
+	}
+
+	fd = openat(AT_FDCWD, "/tmp", O_WRONLY|O_TMPFILE, 0600);
+	if (fd == -1) {
+		ksft_exit_fail_msg("openat() error: Open first temporary file: Fail %d\n", errno);
+	}
+
+	if (linkat(fd, "", AT_FDCWD, "/tmp/1", AT_EMPTY_PATH) == -1) {
+		ksft_exit_fail_msg("linkat() error: Linking the temporary file: Fail %d\n", errno);
+		/* Ensure fd is closed on failure */
+		close(fd);
+	}
+	close(fd);
+
+	fd = openat(AT_FDCWD, "/tmp", O_WRONLY|O_TMPFILE, 0600);
+	if (fd == -1) {
+		ksft_exit_fail_msg("openat() error: Opening the second temporary file: Fail %d\n", errno);
+	}
+	ksft_test_result_pass(" ");
+	ksft_exit_pass();
+	return 0;
+}