Hi Peter,
[+KCSAN folks]
On Wed, Apr 22, 2020 at 01:26:27PM +0100, Will Deacon wrote:
> On Wed, Apr 22, 2020 at 01:37:21PM +0200, Peter Zijlstra wrote:
> > On Wed, Apr 22, 2020 at 09:18:39AM +0100, Will Deacon wrote:
> > > On Tue, Apr 21, 2020 at 11:42:56AM -0700, Linus Torvalds wrote:
> > > > On Tue, Apr 21, 2020 at 8:15 AM Will Deacon <will@xxxxxxxxxx> wrote:
> > > > >
> > > > > It's me again. This is version four of the READ_ONCE() codegen improvement
> > > > > patches [...]
> > > >
> > > > Let's just plan on biting the bullet and do this for 5.8. I'm assuming
> > > > that I'll juet get a pull request from you?
> > >
> > > Sure thing, thanks. I'll get it into -next along with the arm64 bits for
> > > 5.8, but I'll send it as a separate pull when the time comes. I'll also
> > > include the sparc32 changes because otherwise the build falls apart and
> > > we'll get an army of angry robots yelling at us (they seem to form the
> > > majority of the active sparc32 user base afaict).
> >
> > So I'm obviously all for these patches; do note however that it collides
> > most mighty with the KCSAN stuff, which I believe is still pending.
>
> That stuff has been pending for the last two releases afaict :/
>
> Anyway, I'm happy to either provide a branch with this series on, or do
> the merge myself, or send this again based on something else. What works
> best for you? The only thing I'd obviously like to avoid is tightly
> coupling this to KCSAN if there's a chance of it missing the merge window
> again.
FWIW, I had a go at rebasing onto linux-next, just to get an idea for how
bad it is. It's fairly bad, and I don't think it's fair to inflict it on
sfr. I've included the interesting part of the resulting compiler.h below
for you and the KCSAN crowd to take a look at (yes, there's room for
subsequent cleanup, but I was focussing on the conflict resolution for now).
So, I think the best bet is either for my changes to go into -tip on top
of the KCSAN stuff, or for the KCSAN stuff to be dropped from -next (it's
been there since at least January). Do you know if they are definitely
supposed to be going in for 5.8?
Any other ideas?
Cheers,
Will
--->8
/*
* Prevent the compiler from merging or refetching reads or writes. The
* compiler is also forbidden from reordering successive instances of
* READ_ONCE and WRITE_ONCE, but only when the compiler is aware of some
* particular ordering. One way to make the compiler aware of ordering is to
* put the two invocations of READ_ONCE or WRITE_ONCE in different C
* statements.
*
* These two macros will also work on aggregate data types like structs or
* unions.
*
* Their two major use cases are: (1) Mediating communication between
* process-level code and irq/NMI handlers, all running on the same CPU,
* and (2) Ensuring that the compiler does not fold, spindle, or otherwise
* mutilate accesses that either do not require ordering or that interact
* with an explicit memory barrier or atomic instruction that provides the
* required ordering.
*/
#include <asm/barrier.h>
#include <linux/kasan-checks.h>
#include <linux/kcsan-checks.h>
/*
* Use __READ_ONCE() instead of READ_ONCE() if you do not require any
* atomicity or dependency ordering guarantees. Note that this may result
* in tears!
*/
#define __READ_ONCE(x) (*(const volatile __unqual_scalar_typeof(x) *)&(x))
#define __READ_ONCE_SCALAR(x) \
({ \
typeof(x) *__xp = &(x); \
kcsan_check_atomic_read(__xp, sizeof(*__xp)); \
kcsan_disable_current(); \
({ \
__unqual_scalar_typeof(x) __x = __READ_ONCE(*__xp); \
kcsan_enable_current(); \
smp_read_barrier_depends(); \
(typeof(x))__x; \
}); \
})
#define READ_ONCE(x) \
({ \
compiletime_assert_rwonce_type(x); \
__READ_ONCE_SCALAR(x); \
})
#define __WRITE_ONCE(x, val) \
do { \
*(volatile typeof(x) *)&(x) = (val); \
} while (0)
#define __WRITE_ONCE_SCALAR(x, val) \
do { \
typeof(x) *__xp = &(x); \
kcsan_check_atomic_write(__xp, sizeof(*__xp)); \
kcsan_disable_current(); \
__WRITE_ONCE(*__xp, val); \
kcsan_enable_current(); \
} while (0)
#define WRITE_ONCE(x, val) \
do { \
compiletime_assert_rwonce_type(x); \
__WRITE_ONCE_SCALAR(x, val); \
} while (0)
#ifdef CONFIG_KASAN
/*
* We can't declare function 'inline' because __no_sanitize_address conflicts
* with inlining. Attempt to inline it may cause a build failure.
* https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67368
* '__maybe_unused' allows us to avoid defined-but-not-used warnings.
*/
# define __no_kasan_or_inline __no_sanitize_address notrace __maybe_unused
# define __no_sanitize_or_inline __no_kasan_or_inline
#else
# define __no_kasan_or_inline __always_inline
#endif
#define __no_kcsan __no_sanitize_thread
#ifdef __SANITIZE_THREAD__
/*
* Rely on __SANITIZE_THREAD__ instead of CONFIG_KCSAN, to avoid not inlining in
* compilation units where instrumentation is disabled. The attribute 'noinline'
* is required for older compilers, where implicit inlining of very small
* functions renders __no_sanitize_thread ineffective.
*/
# define __no_kcsan_or_inline __no_kcsan noinline notrace __maybe_unused
# define __no_sanitize_or_inline __no_kcsan_or_inline
#else
# define __no_kcsan_or_inline __always_inline
#endif
#ifndef __no_sanitize_or_inline
#define __no_sanitize_or_inline __always_inline
#endif
static __no_sanitize_or_inline
unsigned long __read_once_word_nocheck(const void *addr)
{
return __READ_ONCE(*(unsigned long *)addr);
}
/*
* Use READ_ONCE_NOCHECK() instead of READ_ONCE() if you need to load a
* word from memory atomically but without telling KASAN/KCSAN. This is
* usually used by unwinding code when walking the stack of a running process.
*/
#define READ_ONCE_NOCHECK(x) \
({ \
unsigned long __x = __read_once_word_nocheck(&(x)); \
smp_read_barrier_depends(); \
__x; \
})
static __no_kasan_or_inline
unsigned long read_word_at_a_time(const void *addr)
{
kasan_check_read(addr, 1);
return *(unsigned long *)addr;
}
/**
* data_race - mark an expression as containing intentional data races
*
* This data_race() macro is useful for situations in which data races
* should be forgiven. One example is diagnostic code that accesses
* shared variables but is not a part of the core synchronization design.
*
* This macro *does not* affect normal code generation, but is a hint
* to tooling that data races here are to be ignored.
*/
#define data_race(expr) \
({ \
typeof(({ expr; })) __val; \
kcsan_disable_current(); \
__val = ({ expr; }); \
kcsan_enable_current(); \
__val; \
})
