On Sun, May 24, 2020 at 5:09 AM Akira Yokosawa <akiyks@xxxxxxxxx> 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@xxxxxx/ > >>>>> > >>>>> 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@xxxxxxxxxxxxxx/ > > > > > >> > >>> 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. But I think I can work around unitialized read warning by assigning it to 0 in failure case. > > 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); > } > } > > ---------------- >
