Messages in this thread | | | Date | Wed, 27 Oct 2021 14:17:47 +0200 | From | Peter Zijlstra <> | Subject | Re: Potentially Broken Address Dependency via test_bit() When Compiling With Clang |
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On Wed, Oct 27, 2021 at 12:19:48PM +0200, Paul Heidekrüger wrote: > Hi all, > > For my bachelor thesis, I have been working on the infamous problem of > potentially broken dependency orderings in the Linux kernel. I'm being > advised by Marco Elver, Charalampos Mainas, Pramod Bhatotia (Cc'd).
Nice! Great to see someone working on this!
> For context, see: > https://linuxplumbersconf.org/event/7/contributions/821/attachments/598/1075/LPC_2020_--_Dependency_ordering.pdf > > Our approach consists of two LLVM compiler passes which annotate > dependencies in unoptimised intermediate representation (IR) and verify > the annotated dependencies in optimised IR. ATM, the passes only > recognise a subset of address dependencies - everything is still WIP ;-) > > We have been cross-compiling with a slightly modified version of > allyesconfig for arm64, and the passes have now found a case that we > would like to share with LKML for feedback: an address dependency being > broken (?) through compiler optimisations in > fs/afs/addr_list.c::afs_iterate_addresses(). > > Address dependency in source code, lines 373 - 375 in fs/afs/addr_list.c: > > > [...] > > index = READ_ONCE(ac->alist->preferred); > > if (test_bit(index, &set)) > > goto selected; > > [...] > > where test_bit() expands to the following in > include/asm-generic/bitops/non-atomic.h, lines 115 - 122: > > > static __always_inline int > > arch_test_bit(unsigned int nr, const volatile unsigned long *addr) > > { > > return 1UL & (addr[BIT_WORD(nr)] >> (nr & (BITS_PER_LONG-1))); > > } > > #define test_bit arch_test_bit > > The address dependency gets preserved in unoptimised IR since the virtual register %33 transitively depends on %28: > > > %28 = load volatile i8, i8* %preferred, align 2, !annotation !15 > > store i8 %28, i8* %tmp21, align 1 > > %29 = load i8, i8* %tmp21, align 1 > > %conv23 = zext i8 %29 to i32 > > store i32 %conv23, i32* %index, align 4 > > %30 = load i32, i32* %index, align 4 > > store i32 %30, i32* %nr.addr.i, align 4 > > store i64* %set, i64** %addr.addr.i, align 8 > > %31 = load i64*, i64** %addr.addr.i, align 8 > > %32 = load i32, i32* %nr.addr.i, align 4 > > %div.i = udiv i32 %32, 64 > > %idxprom.i = zext i32 %div.i to i64 > > %arrayidx.i = getelementptr i64, i64* %31, i64 %idxprom.i > > %33 = load volatile i64, i64* %arrayidx.i, align 8, !annotation !16 > > In optimised IR, there is no dependency between the two volatile loads > anymore: > > > %11 = load volatile i8, i8* %preferred, align 2, !annotation !19 > > %conv25 = zext i8 %11 to i32 > > %set.0. = load volatile i64, i64* %set, align 8 > > Now, since @nr traces back to the READ_ONCE() to @index, does this make > the load from @addr in test_bit() address-dependent on that READ_ONCE()? > Should the load from @addr therefore be ordered against the READ_ONCE()?
I would personally not consider this a dependend load. The result depends on two loads, but there is no actual ordering between them.
r1 = *x r2 = *y b = 1 & (r1 >> r2);
(more or less)
A dependent load would be something where the address of the second load depends on the value of the first load, eg:
r1 = *x; r2 = *(y + r1);
typically derefencing or array accesses have this pattern. The canonical example being rcu_dereference(), and is the reason Paul Mckenney is arguing that pointers should carry dependecies; I'll let him refer to the many C language papers on this.
Other examples, ones we're actually worried about the compiler breaking, are, for example, the array access as found in __ktime_get_fast_ns():
seq = READ_ONCE(tkf->seq); tkr = tkf->base + (seq & 1); now = tkr->...
Here the dependency is on an integer (seq), and worse, only a single bit of it. If the compiler were this to transform into something like:
seq = READ_ONCE(tkf->seq) if (seq & 1) { // use tkf->base[1] } else { // use tkf->base[0] }
Then it would be broken, since the condition doesn't order the two loads and they can be re-ordered. Which in turn breaks the premise of the seqcount_latch construct -- see the comment that goes with raw_write_seqcount_latch() in seqlock.h.
hth,
~Peter
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