On Thu, 21 May 2020 at 15:18, Will Deacon <[email protected]> wrote: > > On Thu, May 21, 2020 at 01:08:46PM +0200, Marco Elver wrote: > > In the kernel, volatile is used in various concurrent context, whether > > in low-level synchronization primitives or for legacy reasons. If > > supported by the compiler, we will assume that aligned volatile accesses > > up to sizeof(long long) (matching compiletime_assert_rwonce_type()) are > > atomic. > > > > Recent versions Clang [1] (GCC tentative [2]) can instrument volatile > > accesses differently. Add the option (required) to enable the > > instrumentation, and provide the necessary runtime functions. None of > > the updated compilers are widely available yet (Clang 11 will be the > > first release to support the feature). > > > > [1] > > https://github.com/llvm/llvm-project/commit/5a2c31116f412c3b6888be361137efd705e05814 > > [2] https://gcc.gnu.org/pipermail/gcc-patches/2020-April/544452.html > > > > This patch allows removing any explicit checks in primitives such as > > READ_ONCE() and WRITE_ONCE(). > > > > Signed-off-by: Marco Elver <[email protected]> > > --- > > v2: > > * Reword Makefile comment. > > --- > > kernel/kcsan/core.c | 43 ++++++++++++++++++++++++++++++++++++++++++ > > scripts/Makefile.kcsan | 5 ++++- > > 2 files changed, 47 insertions(+), 1 deletion(-) > > > > diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c > > index a73a66cf79df..15f67949d11e 100644 > > --- a/kernel/kcsan/core.c > > +++ b/kernel/kcsan/core.c > > @@ -789,6 +789,49 @@ void __tsan_write_range(void *ptr, size_t size) > > } > > EXPORT_SYMBOL(__tsan_write_range); > > > > +/* > > + * Use of explicit volatile is generally disallowed [1], however, volatile > > is > > + * still used in various concurrent context, whether in low-level > > + * synchronization primitives or for legacy reasons. > > + * [1] https://lwn.net/Articles/233479/ > > + * > > + * We only consider volatile accesses atomic if they are aligned and would > > pass > > + * the size-check of compiletime_assert_rwonce_type(). > > + */ > > +#define DEFINE_TSAN_VOLATILE_READ_WRITE(size) > > \ > > + void __tsan_volatile_read##size(void *ptr) > > \ > > + { > > \ > > + const bool is_atomic = size <= sizeof(long long) && > > \ > > + IS_ALIGNED((unsigned long)ptr, size); > > \ > > + if (IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS) && is_atomic) > > \ > > + return; > > \ > > + check_access(ptr, size, is_atomic ? KCSAN_ACCESS_ATOMIC : 0); > > \ > > + } > > \ > > + EXPORT_SYMBOL(__tsan_volatile_read##size); > > \ > > + void __tsan_unaligned_volatile_read##size(void *ptr) > > \ > > + __alias(__tsan_volatile_read##size); > > \ > > + EXPORT_SYMBOL(__tsan_unaligned_volatile_read##size); > > \ > > + void __tsan_volatile_write##size(void *ptr) > > \ > > + { > > \ > > + const bool is_atomic = size <= sizeof(long long) && > > \ > > + IS_ALIGNED((unsigned long)ptr, size); > > \ > > + if (IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS) && is_atomic) > > \ > > + return; > > \ > > + check_access(ptr, size, > > \ > > + KCSAN_ACCESS_WRITE | > > \ > > + (is_atomic ? KCSAN_ACCESS_ATOMIC : 0)); > > \ > > + } > > \ > > + EXPORT_SYMBOL(__tsan_volatile_write##size); > > \ > > + void __tsan_unaligned_volatile_write##size(void *ptr) > > \ > > + __alias(__tsan_volatile_write##size); > > \ > > + EXPORT_SYMBOL(__tsan_unaligned_volatile_write##size) > > + > > +DEFINE_TSAN_VOLATILE_READ_WRITE(1); > > +DEFINE_TSAN_VOLATILE_READ_WRITE(2); > > +DEFINE_TSAN_VOLATILE_READ_WRITE(4); > > +DEFINE_TSAN_VOLATILE_READ_WRITE(8); > > +DEFINE_TSAN_VOLATILE_READ_WRITE(16); > > Having a 16-byte case seems a bit weird to me, but I guess clang needs this > for some reason?
Yes, the emitted fixed-size instrumentation is up to 16 bytes, so we'll need it (for both volatile and non-volatile -- otherwise we'll get linker errors). It doesn't mean we'll consider 16 byte volatile accesses as atomic, because of the size check to compute is_atomic above. Thanks, -- Marco
