Messages in this thread | | | From | David Laight <> | Subject | RE: Potentially Broken Address Dependency via test_bit() When Compiling With Clang | Date | Wed, 27 Oct 2021 11:56:14 +0000 |
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From: Paul Heidekrüger > Sent: 27 October 2021 11:20 > > 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). > > 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
I don't think there is expected to be an address dependency. The READ_ONCE() is needed to ensure the generated code doesn't use two different values for 'index' - eg for 'nr' inside arch_test_bit().
David
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