Messages in this thread | ![/](/images/icornerl.gif) | | Subject | Re: [PATCH bpf-next v4 10/11] bpf: Add tests for new BPF atomic operations | From | Yonghong Song <> | Date | Tue, 8 Dec 2020 08:38:04 -0800 |
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On 12/8/20 4:41 AM, Brendan Jackman wrote: > On Mon, Dec 07, 2020 at 07:18:57PM -0800, Yonghong Song wrote: >> >> >> On 12/7/20 8:07 AM, Brendan Jackman wrote: >>> The prog_test that's added depends on Clang/LLVM features added by >>> Yonghong in commit 286daafd6512 (was https://reviews.llvm.org/D72184 ). >>> >>> Note the use of a define called ENABLE_ATOMICS_TESTS: this is used >>> to: >>> >>> - Avoid breaking the build for people on old versions of Clang >>> - Avoid needing separate lists of test objects for no_alu32, where >>> atomics are not supported even if Clang has the feature. >>> >>> The atomics_test.o BPF object is built unconditionally both for >>> test_progs and test_progs-no_alu32. For test_progs, if Clang supports >>> atomics, ENABLE_ATOMICS_TESTS is defined, so it includes the proper >>> test code. Otherwise, progs and global vars are defined anyway, as >>> stubs; this means that the skeleton user code still builds. >>> >>> The atomics_test.o userspace object is built once and used for both >>> test_progs and test_progs-no_alu32. A variable called skip_tests is >>> defined in the BPF object's data section, which tells the userspace >>> object whether to skip the atomics test. >>> >>> Signed-off-by: Brendan Jackman <jackmanb@google.com> >> >> Ack with minor comments below. >> >> Acked-by: Yonghong Song <yhs@fb.com> >> >>> --- >>> tools/testing/selftests/bpf/Makefile | 10 + >>> .../selftests/bpf/prog_tests/atomics.c | 246 ++++++++++++++++++ >>> tools/testing/selftests/bpf/progs/atomics.c | 154 +++++++++++ >>> .../selftests/bpf/verifier/atomic_and.c | 77 ++++++ >>> .../selftests/bpf/verifier/atomic_cmpxchg.c | 96 +++++++ >>> .../selftests/bpf/verifier/atomic_fetch_add.c | 106 ++++++++ >>> .../selftests/bpf/verifier/atomic_or.c | 77 ++++++ >>> .../selftests/bpf/verifier/atomic_xchg.c | 46 ++++ >>> .../selftests/bpf/verifier/atomic_xor.c | 77 ++++++ >>> 9 files changed, 889 insertions(+) >>> create mode 100644 tools/testing/selftests/bpf/prog_tests/atomics.c >>> create mode 100644 tools/testing/selftests/bpf/progs/atomics.c >>> create mode 100644 tools/testing/selftests/bpf/verifier/atomic_and.c >>> create mode 100644 tools/testing/selftests/bpf/verifier/atomic_cmpxchg.c >>> create mode 100644 tools/testing/selftests/bpf/verifier/atomic_fetch_add.c >>> create mode 100644 tools/testing/selftests/bpf/verifier/atomic_or.c >>> create mode 100644 tools/testing/selftests/bpf/verifier/atomic_xchg.c >>> create mode 100644 tools/testing/selftests/bpf/verifier/atomic_xor.c >>> >>> diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile >>> index ac25ba5d0d6c..13bc1d736164 100644 >>> --- a/tools/testing/selftests/bpf/Makefile >>> +++ b/tools/testing/selftests/bpf/Makefile >>> @@ -239,6 +239,12 @@ BPF_CFLAGS = -g -D__TARGET_ARCH_$(SRCARCH) $(MENDIAN) \ >>> -I$(INCLUDE_DIR) -I$(CURDIR) -I$(APIDIR) \ >>> -I$(abspath $(OUTPUT)/../usr/include) >>> +# BPF atomics support was added to Clang in llvm-project commit 286daafd6512 >>> +# (release 12.0.0). >>> +BPF_ATOMICS_SUPPORTED = $(shell \ >>> + echo "int x = 0; int foo(void) { return __sync_val_compare_and_swap(&x, 1, 2); }" \ >>> + | $(CLANG) -x cpp-output -S -target bpf -mcpu=v3 - -o /dev/null && echo 1 || echo 0) >> >> '-x c' here more intuitive? >> >>> + >>> CLANG_CFLAGS = $(CLANG_SYS_INCLUDES) \ >>> -Wno-compare-distinct-pointer-types >>> @@ -399,11 +405,15 @@ TRUNNER_EXTRA_FILES := $(OUTPUT)/urandom_read $(OUTPUT)/bpf_testmod.ko \ >>> $(wildcard progs/btf_dump_test_case_*.c) >>> TRUNNER_BPF_BUILD_RULE := CLANG_BPF_BUILD_RULE >>> TRUNNER_BPF_CFLAGS := $(BPF_CFLAGS) $(CLANG_CFLAGS) >>> +ifeq ($(BPF_ATOMICS_SUPPORTED),1) >>> + TRUNNER_BPF_CFLAGS += -DENABLE_ATOMICS_TESTS >>> +endif >>> TRUNNER_BPF_LDFLAGS := -mattr=+alu32 >>> $(eval $(call DEFINE_TEST_RUNNER,test_progs)) >>> # Define test_progs-no_alu32 test runner. >>> TRUNNER_BPF_BUILD_RULE := CLANG_NOALU32_BPF_BUILD_RULE >>> +TRUNNER_BPF_CFLAGS := $(BPF_CFLAGS) $(CLANG_CFLAGS) >>> TRUNNER_BPF_LDFLAGS := >>> $(eval $(call DEFINE_TEST_RUNNER,test_progs,no_alu32)) >>> diff --git a/tools/testing/selftests/bpf/prog_tests/atomics.c b/tools/testing/selftests/bpf/prog_tests/atomics.c >>> new file mode 100644 >>> index 000000000000..c841a3abc2f7 >>> --- /dev/null >>> +++ b/tools/testing/selftests/bpf/prog_tests/atomics.c >>> @@ -0,0 +1,246 @@ >>> +// SPDX-License-Identifier: GPL-2.0 >>> + >>> +#include <test_progs.h> >>> + >>> +#include "atomics.skel.h" >>> + >>> +static void test_add(struct atomics *skel) >>> +{ >>> + int err, prog_fd; >>> + __u32 duration = 0, retval; >>> + struct bpf_link *link; >>> + >>> + link = bpf_program__attach(skel->progs.add); >>> + if (CHECK(IS_ERR(link), "attach(add)", "err: %ld\n", PTR_ERR(link))) >>> + return; >>> + >>> + prog_fd = bpf_program__fd(skel->progs.add); >>> + err = bpf_prog_test_run(prog_fd, 1, NULL, 0, >>> + NULL, NULL, &retval, &duration); >>> + if (CHECK(err || retval, "test_run add", >>> + "err %d errno %d retval %d duration %d\n", err, errno, retval, duration)) >>> + goto cleanup; >>> + >>> + ASSERT_EQ(skel->data->add64_value, 3, "add64_value"); >>> + ASSERT_EQ(skel->bss->add64_result, 1, "add64_result"); >>> + >>> + ASSERT_EQ(skel->data->add32_value, 3, "add32_value"); >>> + ASSERT_EQ(skel->bss->add32_result, 1, "add32_result"); >>> + >>> + ASSERT_EQ(skel->bss->add_stack_value_copy, 3, "add_stack_value"); >>> + ASSERT_EQ(skel->bss->add_stack_result, 1, "add_stack_result"); >>> + >>> + ASSERT_EQ(skel->data->add_noreturn_value, 3, "add_noreturn_value"); >>> + >>> +cleanup: >>> + bpf_link__destroy(link); >>> +} >>> + >> [...] >>> + >>> +__u64 xchg64_value = 1; >>> +__u64 xchg64_result = 0; >>> +__u32 xchg32_value = 1; >>> +__u32 xchg32_result = 0; >>> + >>> +SEC("fentry/bpf_fentry_test1") >>> +int BPF_PROG(xchg, int a) >>> +{ >>> +#ifdef ENABLE_ATOMICS_TESTS >>> + __u64 val64 = 2; >>> + __u32 val32 = 2; >>> + >>> + __atomic_exchange(&xchg64_value, &val64, &xchg64_result, __ATOMIC_RELAXED); >>> + __atomic_exchange(&xchg32_value, &val32, &xchg32_result, __ATOMIC_RELAXED); >> >> Interesting to see this also works. I guess we probably won't advertise >> this, right? Currently for LLVM, the memory ordering parameter is ignored. > > Well IIUC this specific case is fine: the ordering that you get with > BPF_[CMP]XCHG (via kernel atomic_[cmpxchg]) is sequential consistency, > and its' fine to provide a stronger ordering than the one requested. I > should change it to say __ATOMIC_SEQ_CST to avoid confusing readers, > though. > > (I wrote it this way because I didn't see a __sync* function for > unconditional atomic exchange, and I didn't see an __atomic* function > where you don't need to specify the ordering).
For the above code, __atomic_exchange(&xchg64_value, &val64, &xchg64_result, __ATOMIC_RELAXED); It tries to do an atomic exchange between &xchg64_value and &val64, and store the old &xchg64_value to &xchg64_result. So it is equivalent to xchg64_result = __sync_lock_test_and_set(&xchg64_value, val64);
So I think this test case can be dropped.
> > However... this led me to double-check the semantics and realise that we > do have a problem with ordering: The kernel's atomic_{add,and,or,xor} do > not imply memory barriers and therefore neither do the corresponding BPF > instructions. That means Clang can compile this: > > (void)__atomic_fetch_add(&val, 1, __ATOMIC_SEQ_CST) > > to a {.code = (BPF_STX | BPF_DW | BPF_ATOMIC), .imm = BPF_ADD}, > which is implemented with atomic_add, which doesn't actually satisfy > __ATOMIC_SEQ_CST.
This is the main reason in all my llvm selftests I did not use __atomic_* intrinsics because we cannot handle *different* memory ordering properly.
> > In fact... I think this is a pre-existing issue with BPF_XADD. > > If all I've written here is correct, the fix is to use > (void)atomic_fetch_add etc (these imply barriers) even when BPF_FETCH is > not set. And that change ought to be backported to fix BPF_XADD.
We cannot change BPF_XADD behavior. If we change BPF_XADD to use atomic_fetch_add, then suddenly old code compiled with llvm12 will suddenly requires latest kernel, which will break userland very badly.
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