Merge branch 'Use this_cpu_xxx for preemption-safety'

Hou Tao says:

====================

From: Hou Tao <houtao1@huawei.com>

Hi,

The patchset aims to make the update of per-cpu prog->active and per-cpu
bpf_task_storage_busy being preemption-safe. The problem is on same
architectures (e.g. arm64), __this_cpu_{inc|dec|inc_return} are neither
preemption-safe nor IRQ-safe, so under fully preemptible kernel the
concurrent updates on these per-cpu variables may be interleaved and the
final values of these variables may be not zero.

Patch 1 & 2 use the preemption-safe per-cpu helpers to manipulate
prog->active and bpf_task_storage_busy. Patch 3 & 4 add a test case in
map_tests to show the concurrent updates on the per-cpu
bpf_task_storage_busy by using __this_cpu_{inc|dec} are not atomic.

Comments are always welcome.

Regards,
Tao

Change Log:
v2:
* Patch 1: update commit message to indicate the problem is only
  possible for fully preemptible kernel
* Patch 2: a new patch which fixes the problem for prog->active
* Patch 3 & 4: move it to test_maps and make it depend on CONFIG_PREEMPT

v1: https://lore.kernel.org/bpf/20220829142752.330094-1-houtao@huaweicloud.com/
====================

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

kernel/bpf/bpf_local_storage.c

@@ -555,11 +555,11 @@ void bpf_local_storage_map_free(struct bpf_local_storage_map *smap,
 				struct bpf_local_storage_elem, map_node))) {
 			if (busy_counter) {
 				migrate_disable();
-				__this_cpu_inc(*busy_counter);
+				this_cpu_inc(*busy_counter);
 			}
 			bpf_selem_unlink(selem, false);
 			if (busy_counter) {
-				__this_cpu_dec(*busy_counter);
+				this_cpu_dec(*busy_counter);
 				migrate_enable();
 			}
 			cond_resched_rcu();

kernel/bpf/bpf_task_storage.c

@@ -26,20 +26,20 @@ static DEFINE_PER_CPU(int, bpf_task_storage_busy);
 static void bpf_task_storage_lock(void)
 {
 	migrate_disable();
-	__this_cpu_inc(bpf_task_storage_busy);
+	this_cpu_inc(bpf_task_storage_busy);
 }
 
 static void bpf_task_storage_unlock(void)
 {
-	__this_cpu_dec(bpf_task_storage_busy);
+	this_cpu_dec(bpf_task_storage_busy);
 	migrate_enable();
 }
 
 static bool bpf_task_storage_trylock(void)
 {
 	migrate_disable();
-	if (unlikely(__this_cpu_inc_return(bpf_task_storage_busy) != 1)) {
-		__this_cpu_dec(bpf_task_storage_busy);
+	if (unlikely(this_cpu_inc_return(bpf_task_storage_busy) != 1)) {
+		this_cpu_dec(bpf_task_storage_busy);
 		migrate_enable();
 		return false;
 	}

kernel/bpf/trampoline.c

@@ -895,7 +895,7 @@ u64 notrace __bpf_prog_enter(struct bpf_prog *prog, struct bpf_tramp_run_ctx *ru
 
 	run_ctx->saved_run_ctx = bpf_set_run_ctx(&run_ctx->run_ctx);
 
-	if (unlikely(__this_cpu_inc_return(*(prog->active)) != 1)) {
+	if (unlikely(this_cpu_inc_return(*(prog->active)) != 1)) {
 		inc_misses_counter(prog);
 		return 0;
 	}
@@ -930,7 +930,7 @@ void notrace __bpf_prog_exit(struct bpf_prog *prog, u64 start, struct bpf_tramp_
 	bpf_reset_run_ctx(run_ctx->saved_run_ctx);
 
 	update_prog_stats(prog, start);
-	__this_cpu_dec(*(prog->active));
+	this_cpu_dec(*(prog->active));
 	migrate_enable();
 	rcu_read_unlock();
 }
@@ -966,7 +966,7 @@ u64 notrace __bpf_prog_enter_sleepable(struct bpf_prog *prog, struct bpf_tramp_r
 	migrate_disable();
 	might_fault();
 
-	if (unlikely(__this_cpu_inc_return(*(prog->active)) != 1)) {
+	if (unlikely(this_cpu_inc_return(*(prog->active)) != 1)) {
 		inc_misses_counter(prog);
 		return 0;
 	}
@@ -982,7 +982,7 @@ void notrace __bpf_prog_exit_sleepable(struct bpf_prog *prog, u64 start,
 	bpf_reset_run_ctx(run_ctx->saved_run_ctx);
 
 	update_prog_stats(prog, start);
-	__this_cpu_dec(*(prog->active));
+	this_cpu_dec(*(prog->active));
 	migrate_enable();
 	rcu_read_unlock_trace();
 }

tools/testing/selftests/bpf/map_tests/task_storage_map.c

@@ -0,0 +1,122 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (C) 2022. Huawei Technologies Co., Ltd */
+#define _GNU_SOURCE
+#include <sched.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <stdbool.h>
+#include <errno.h>
+#include <string.h>
+#include <pthread.h>
+
+#include <bpf/bpf.h>
+#include <bpf/libbpf.h>
+
+#include "test_maps.h"
+#include "task_local_storage_helpers.h"
+#include "read_bpf_task_storage_busy.skel.h"
+
+struct lookup_ctx {
+	bool start;
+	bool stop;
+	int pid_fd;
+	int map_fd;
+	int loop;
+};
+
+static void *lookup_fn(void *arg)
+{
+	struct lookup_ctx *ctx = arg;
+	long value;
+	int i = 0;
+
+	while (!ctx->start)
+		usleep(1);
+
+	while (!ctx->stop && i++ < ctx->loop)
+		bpf_map_lookup_elem(ctx->map_fd, &ctx->pid_fd, &value);
+	return NULL;
+}
+
+static void abort_lookup(struct lookup_ctx *ctx, pthread_t *tids, unsigned int nr)
+{
+	unsigned int i;
+
+	ctx->stop = true;
+	ctx->start = true;
+	for (i = 0; i < nr; i++)
+		pthread_join(tids[i], NULL);
+}
+
+void test_task_storage_map_stress_lookup(void)
+{
+#define MAX_NR_THREAD 4096
+	unsigned int i, nr = 256, loop = 8192, cpu = 0;
+	struct read_bpf_task_storage_busy *skel;
+	pthread_t tids[MAX_NR_THREAD];
+	struct lookup_ctx ctx;
+	cpu_set_t old, new;
+	const char *cfg;
+	int err;
+
+	cfg = getenv("TASK_STORAGE_MAP_NR_THREAD");
+	if (cfg) {
+		nr = atoi(cfg);
+		if (nr > MAX_NR_THREAD)
+			nr = MAX_NR_THREAD;
+	}
+	cfg = getenv("TASK_STORAGE_MAP_NR_LOOP");
+	if (cfg)
+		loop = atoi(cfg);
+	cfg = getenv("TASK_STORAGE_MAP_PIN_CPU");
+	if (cfg)
+		cpu = atoi(cfg);
+
+	skel = read_bpf_task_storage_busy__open_and_load();
+	err = libbpf_get_error(skel);
+	CHECK(err, "open_and_load", "error %d\n", err);
+
+	/* Only for a fully preemptible kernel */
+	if (!skel->kconfig->CONFIG_PREEMPT)
+		return;
+
+	/* Save the old affinity setting */
+	sched_getaffinity(getpid(), sizeof(old), &old);
+
+	/* Pinned on a specific CPU */
+	CPU_ZERO(&new);
+	CPU_SET(cpu, &new);
+	sched_setaffinity(getpid(), sizeof(new), &new);
+
+	ctx.start = false;
+	ctx.stop = false;
+	ctx.pid_fd = sys_pidfd_open(getpid(), 0);
+	ctx.map_fd = bpf_map__fd(skel->maps.task);
+	ctx.loop = loop;
+	for (i = 0; i < nr; i++) {
+		err = pthread_create(&tids[i], NULL, lookup_fn, &ctx);
+		if (err) {
+			abort_lookup(&ctx, tids, i);
+			CHECK(err, "pthread_create", "error %d\n", err);
+			goto out;
+		}
+	}
+
+	ctx.start = true;
+	for (i = 0; i < nr; i++)
+		pthread_join(tids[i], NULL);
+
+	skel->bss->pid = getpid();
+	err = read_bpf_task_storage_busy__attach(skel);
+	CHECK(err, "attach", "error %d\n", err);
+
+	/* Trigger program */
+	syscall(SYS_gettid);
+	skel->bss->pid = 0;
+
+	CHECK(skel->bss->busy != 0, "bad bpf_task_storage_busy", "got %d\n", skel->bss->busy);
+out:
+	read_bpf_task_storage_busy__destroy(skel);
+	/* Restore affinity setting */
+	sched_setaffinity(getpid(), sizeof(old), &old);
+}

tools/testing/selftests/bpf/prog_tests/test_bprm_opts.c

@@ -9,18 +9,10 @@
 
 #include "bprm_opts.skel.h"
 #include "network_helpers.h"
-
-#ifndef __NR_pidfd_open
-#define __NR_pidfd_open 434
-#endif
+#include "task_local_storage_helpers.h"
 
 static const char * const bash_envp[] = { "TMPDIR=shouldnotbeset", NULL };
 
-static inline int sys_pidfd_open(pid_t pid, unsigned int flags)
-{
-	return syscall(__NR_pidfd_open, pid, flags);
-}
-
 static int update_storage(int map_fd, int secureexec)
 {
 	int task_fd, ret = 0;

tools/testing/selftests/bpf/prog_tests/test_local_storage.c

@@ -11,15 +11,7 @@
 
 #include "local_storage.skel.h"
 #include "network_helpers.h"
-
-#ifndef __NR_pidfd_open
-#define __NR_pidfd_open 434
-#endif
-
-static inline int sys_pidfd_open(pid_t pid, unsigned int flags)
-{
-	return syscall(__NR_pidfd_open, pid, flags);
-}
+#include "task_local_storage_helpers.h"
 
 static unsigned int duration;
 

tools/testing/selftests/bpf/progs/read_bpf_task_storage_busy.c

@@ -0,0 +1,39 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (C) 2022. Huawei Technologies Co., Ltd */
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+extern bool CONFIG_PREEMPT __kconfig __weak;
+extern const int bpf_task_storage_busy __ksym;
+
+char _license[] SEC("license") = "GPL";
+
+int pid = 0;
+int busy = 0;
+
+struct {
+	__uint(type, BPF_MAP_TYPE_TASK_STORAGE);
+	__uint(map_flags, BPF_F_NO_PREALLOC);
+	__type(key, int);
+	__type(value, long);
+} task SEC(".maps");
+
+SEC("raw_tp/sys_enter")
+int BPF_PROG(read_bpf_task_storage_busy)
+{
+	int *value;
+	int key;
+
+	if (!CONFIG_PREEMPT)
+		return 0;
+
+	if (bpf_get_current_pid_tgid() >> 32 != pid)
+		return 0;
+
+	value = bpf_this_cpu_ptr(&bpf_task_storage_busy);
+	if (value)
+		busy = *value;
+
+	return 0;
+}

tools/testing/selftests/bpf/task_local_storage_helpers.h

@@ -0,0 +1,18 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __TASK_LOCAL_STORAGE_HELPER_H
+#define __TASK_LOCAL_STORAGE_HELPER_H
+
+#include <unistd.h>
+#include <sys/syscall.h>
+#include <sys/types.h>
+
+#ifndef __NR_pidfd_open
+#define __NR_pidfd_open 434
+#endif
+
+static inline int sys_pidfd_open(pid_t pid, unsigned int flags)
+{
+	return syscall(__NR_pidfd_open, pid, flags);
+}
+
+#endif