Message-ID: <67ee3925-9388-c9d4-8ad8-9c28cff35d55@fb.com>
Date:   Tue, 8 Dec 2020 10:15:35 -0800
From:   Yonghong Song <yhs@...com>
To:     Brendan Jackman <jackmanb@...gle.com>
CC:     <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        KP Singh <kpsingh@...omium.org>,
        Florent Revest <revest@...omium.org>,
        <linux-kernel@...r.kernel.org>, Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH bpf-next v4 10/11] bpf: Add tests for new BPF atomic
 operations



On 12/8/20 8:59 AM, Brendan Jackman wrote:
> On Tue, Dec 08, 2020 at 08:38:04AM -0800, Yonghong Song wrote:
>>
>>
>> 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@...gle.com>
>>>>
>>>> Ack with minor comments below.
>>>>
>>>> Acked-by: Yonghong Song <yhs@...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.
> 
> Ah nice, I didn't know about __sync_lock_test_and_set, let's switch to
> that I think.
> 
>>> 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.
> 
> Sorry I should have been more explicit: I meant that the fix would be to
> call atomic_fetch_add but discard the return value, purely for the
> implied barrier. The x86 JIT would stay the same. It would not break any
> existing code, only add ordering guarantees that the user probably
> already expected (since these builtins come from GCC originally and the
> GCC docs say "these builtins are considered a full barrier" [1])
> 
> [1] https://gcc.gnu.org/onlinedocs/gcc-4.1.1/gcc/Atomic-Builtins.html

This is indeed the issue. In the past, people already use gcc
__sync_fetch_and_add() for xadd instruction for which git generated
code does not implying barrier.

The new atomics support has the following logic:
   . if return value is used, it is atomic_fetch_add
   . if return value is not used, it is xadd
The reason to do this is to preserve backward compabiility
and this way, we can get rid of -mcpu=v4.

barrier issue is tricky and as we discussed earlier let us
delay this after basic atomics support landed. We may not
100% conform to gcc __sync_fetch_and_add() or __atomic_*()
semantics. We do need to clearly document what is expected
in llvm and kernel.

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