Re: Some -serious- BPF-related litmus tests

[Akira Yokosawa <akiyks@gmail.com>]

On Mon, 25 May 2020 11:31:27 -0700, Andrii Nakryiko wrote:
> On Sun, May 24, 2020 at 5:09 AM Akira Yokosawa <akiyks@gmail.com> wrote:
>>
>> On Fri, 22 May 2020 12:38:21 -0700, Andrii Nakryiko wrote:
>>> On 5/22/20 10:43 AM, Paul E. McKenney wrote:
>>>> On Fri, May 22, 2020 at 10:32:01AM -0400, Alan Stern wrote:
>>>>> On Fri, May 22, 2020 at 11:44:07AM +0200, Peter Zijlstra wrote:
>>>>>> On Thu, May 21, 2020 at 05:38:50PM -0700, Paul E. McKenney wrote:
>>>>>>> Hello!
>>>>>>>
>>>>>>> Just wanted to call your attention to some pretty cool and pretty serious
>>>>>>> litmus tests that Andrii did as part of his BPF ring-buffer work:
>>>>>>>
>>>>>>> https://lore.kernel.org/bpf/20200517195727.279322-3-andriin@fb.com/
>>>>>>>
>>>>>>> Thoughts?
>>>>>>
>>>>>> I find:
>>>>>>
>>>>>>     smp_wmb()
>>>>>>     smp_store_release()
>>>>>>
>>>>>> a _very_ weird construct. What is that supposed to even do?
>>>>>
>>>>> Indeed, it looks like one or the other of those is redundant (depending
>>>>> on the context).
>>>>
>>>> Probably.  Peter instead asked what it was supposed to even do.  ;-)
>>>
>>> I agree, I think smp_wmb() is redundant here. Can't remember why I thought that it's necessary, this algorithm went through a bunch of iterations, starting as completely lockless, also using READ_ONCE/WRITE_ONCE at some point, and settling on smp_read_acquire/smp_store_release, eventually. Maybe there was some reason, but might be that I was just over-cautious. See reply on patch thread as well ([0]).
>>>
>>>   [0] https://lore.kernel.org/bpf/CAEf4Bza26AbRMtWcoD5+TFhnmnU6p5YJ8zO+SoAJCDtp1jVhcQ@mail.gmail.com/
>>>
>>>
>>>>
>>>>> Also, what use is a spinlock that is accessed in only one thread?
>>>>
>>>> Multiple writers synchronize via the spinlock in this case.  I am
>>>> guessing that his larger 16-hour test contended this spinlock.
>>>
>>> Yes, spinlock is for coordinating multiple producers. 2p1c cases (bounded and unbounded) rely on this already. 1p1c cases are sort of subsets (but very fast to verify) checking only consumer/producer interaction.
>>>
>>>>
>>>>> Finally, I doubt that these tests belong under tools/memory-model.
>>>>> Shouldn't they go under the new Documentation/ directory for litmus
>>>>> tests?  And shouldn't the patch update a README file?
>>>>
>>>> Agreed, and I responded to that effect to his original patch:
>>>>
>>>> https://lore.kernel.org/bpf/20200522003433.GG2869@paulmck-ThinkPad-P72/
>>>
>>> Yep, makes sense, I'll will move.
>>
>> Hi Andrii,
>>
>> Andrea reported off-the-list that your litmus tests are incompatible
>> with the to-be-released version 7.56 of herd7 and currently available
>> versions of klitmus7.
>>
>> This is due to a couple of C-language level issues.
>>
>> herd7 used to be fairly generous in parsing C litmus tests.
>> On the other hand, klitmus7 converts a litmus test into a
>> kernel module.  The converted code is built by an actual C compiler
>> with kernel headers included, and can fail to build due to syntax errors
>> or serious warnings.
>> herd7 HEAD is getting slightly stricter on uninitialized variable and
>> it emits an error to mpsc-rb+1p1c+bounded.litmus:
>>
>> Warning: File "mpsc-rb+1p1c+bounded.litmus": read on location 0 does not match any write
>>
>> Converted code by klitmus7 fails to build with the following warning messages:
>>
>> $ make
>> make -C /lib/modules/5.3.0-53-generic/build/ M=/home/akira/bpf-rb/klitmus modules
>> make[1]: Entering directory '/usr/src/linux-headers-5.3.0-53-generic'
>>   CC [M]  /home/akira/bpf-rb/klitmus/litmus000.o
>> /home/akira/bpf-rb/klitmus/litmus000.c: In function ‘code1’:
>> /home/akira/bpf-rb/klitmus/litmus000.c:426:14: error: passing argument 1 of ‘atomic_inc’
>>   from incompatible pointer type [-Werror=incompatible-pointer-types]
>>    atomic_inc(dropped);
>>               ^~~~~~~
>> In file included from ./arch/x86/include/asm/atomic.h:265:0,
>>                  from ./arch/x86/include/asm/msr.h:67,
>>                  from ./arch/x86/include/asm/processor.h:21,
>>                  from ./arch/x86/include/asm/cpufeature.h:5,
>>                  from ./arch/x86/include/asm/thread_info.h:53,
>>                  from ./include/linux/thread_info.h:38,
>>                  from ./arch/x86/include/asm/preempt.h:7,
>>                  from ./include/linux/preempt.h:78,
>>                  from ./include/linux/spinlock.h:51,
>>                  from ./include/linux/seqlock.h:36,
>>                  from ./include/linux/time.h:6,
>>                  from ./include/linux/stat.h:19,
>>                  from ./include/linux/module.h:10,
>>                  from /home/akira/bpf-rb/klitmus/litmus000.c:11:
>> ./include/asm-generic/atomic-instrumented.h:237:1: note: expected ‘atomic_t * {aka struct <anonymous> *}’ but argument is of type ‘int *’
>>  atomic_inc(atomic_t *v)
>>  ^~~~~~~~~~
>> In file included from ./include/linux/export.h:45:0,
>>                  from ./include/linux/linkage.h:7,
>>                  from ./include/linux/kernel.h:8,
>>                  from ./include/linux/list.h:9,
>>                  from ./include/linux/module.h:9,
>>                  from /home/akira/bpf-rb/klitmus/litmus000.c:11:
>> /home/akira/bpf-rb/klitmus/litmus000.c: In function ‘thread0’:
>> ./include/linux/compiler.h:187:26: warning: ‘rLenPtr’ may be used uninitialized in this function [-Wmaybe-uninitialized]
>>   case 4: *(__u32 *)res = *(volatile __u32 *)p; break;  \
>>                           ^
>> /home/akira/bpf-rb/klitmus/litmus000.c:365:7: note: ‘rLenPtr’ was declared here
>>   int *rLenPtr;
>>        ^~~~~~~
>> In file included from ./include/linux/export.h:45:0,
>>                  from ./include/linux/linkage.h:7,
>>                  from ./include/linux/kernel.h:8,
>>                  from ./include/linux/list.h:9,
>>                  from ./include/linux/module.h:9,
>>                  from /home/akira/bpf-rb/klitmus/litmus000.c:11:
>> /home/akira/bpf-rb/klitmus/litmus000.c: In function ‘thread1’:
>> ./include/linux/compiler.h:225:31: warning: ‘rLenPtr’ may be used uninitialized in this function [-Wmaybe-uninitialized]
>>   case 4: *(volatile __u32 *)p = *(__u32 *)res; break;
>>           ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~
>> /home/akira/bpf-rb/klitmus/litmus000.c:417:7: note: ‘rLenPtr’ was declared here
>>   int *rLenPtr;
>>        ^~~~~~~
>> cc1: some warnings being treated as errors
>> scripts/Makefile.build:288: recipe for target '/home/akira/bpf-rb/klitmus/litmus000.o' failed
>> make[2]: *** [/home/akira/bpf-rb/klitmus/litmus000.o] Error 1
>> Makefile:1656: recipe for target '_module_/home/akira/bpf-rb/klitmus' failed
>> make[1]: *** [_module_/home/akira/bpf-rb/klitmus] Error 2
>> make[1]: Leaving directory '/usr/src/linux-headers-5.3.0-53-generic'
>> Makefile:8: recipe for target 'all' failed
>> make: *** [all] Error 2
>>
>> Appended below is a patch I applied to mpsc-rb+1p1c+bounded.litmus to make
>> herd7 HEAD and klitmus7 happy. (Give or take the redundant memory barrier.)
>>
>> The other variants need similar changes.
> 
> Ok, cool, thanks for letting me know. I'll see if I can upgrade
> everything and test on my side (if you have a pointer to instructions
> how to use klitmus7, that would be greatly appreaciated!)
> 
>>
>> What I did here are:
>>
>>     - Remove unnecessary initialization (shared variables are 0 by default)
>>     - Declare "dropped" as atomic_t
> 
> These two look good.
> 
>>     - Promote rLenPtr to a shared variable LenPtr
> 
> This one might work for single producer litmus tests, but it's wrong
> for 2- and 3-producer cases, because it has to be local to producer.

Ah, I knew I had missed something...

> But I think I can work around unitialized read warning by assigning it
> to 0 in failure case.

Yes, that should work.

You can find a basic introduction of klitmus7 in tools/memory-model/README.

        Thanks, Akira

> 
>>
>> Please note that if you are on Linux 5.6 (or later), you need an up-to-date
>> klitmus7 due to a change in kernel API.
>>
>> Any question is welcome!
>>
>>         Thanks, Akira
>>
>> -----------------------
>> diff --git a/mpsc-rb+1p1c+bounded.litmus b/mpsc-rb+1p1c+bounded.litmus
>> index cafd17a..5af43c1 100644
>> --- a/mpsc-rb+1p1c+bounded.litmus
>> +++ b/mpsc-rb+1p1c+bounded.litmus
>> @@ -17,15 +17,11 @@ C mpsc-rb+1p1c+bounded
>>
>>  {
>>         max_len = 1;
>> -       len1 = 0;
>> -       px = 0;
>> -       cx = 0;
>> -       dropped = 0;
>> +       atomic_t dropped;
>>  }
>>
>> -P0(int *len1, int *cx, int *px)
>> +P0(int *len1, int *cx, int *px, int *LenPtr)
>>  {
>> -       int *rLenPtr;
>>         int rLen;
>>         int rPx;
>>         int rCx;
>> @@ -37,11 +33,11 @@ P0(int *len1, int *cx, int *px)
>>         rPx = smp_load_acquire(px);
>>         if (rCx < rPx) {
>>                 if (rCx == 0)
>> -                       rLenPtr = len1;
>> +                       LenPtr = len1;
>>                 else
>>                         rFail = 1;
>>
>> -               rLen = smp_load_acquire(rLenPtr);
>> +               rLen = smp_load_acquire(LenPtr);
>>                 if (rLen == 0) {
>>                         rFail = 1;
>>                 } else if (rLen == 1) {
>> @@ -51,12 +47,11 @@ P0(int *len1, int *cx, int *px)
>>         }
>>  }
>>
>> -P1(int *len1, spinlock_t *rb_lock, int *px, int *cx, int *dropped, int *max_len)
>> +P1(int *len1, spinlock_t *rb_lock, int *px, int *cx, atomic_t *dropped, int *max_len, int *LenPtr)
>>  {
>>         int rPx;
>>         int rCx;
>>         int rFail;
>> -       int *rLenPtr;
>>
>>         rFail = 0;
>>         rCx = smp_load_acquire(cx);
>> @@ -69,17 +64,17 @@ P1(int *len1, spinlock_t *rb_lock, int *px, int *cx, int *dropped, int *max_len)
>>                 spin_unlock(rb_lock);
>>         } else {
>>                 if (rPx == 0)
>> -                       rLenPtr = len1;
>> +                       LenPtr = len1;
>>                 else
>>                         rFail = 1;
>>
>> -               *rLenPtr = -1;
>> +               *LenPtr = -1;
>>                 smp_wmb();
>>                 smp_store_release(px, rPx + 1);
>>
>>                 spin_unlock(rb_lock);
>>
>> -               smp_store_release(rLenPtr, 1);
>> +               smp_store_release(LenPtr, 1);
>>         }
>>  }
>>
>> ----------------
>>