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However, in the case of READ_ONCE(some_bool) is used, this > UB seems dangerous and unnecessary. I can easily imagine some Rust code > that looks like this: > > if READ_ONCE(&raw const (*my_c_struct).my_bool_field) { > ... > } > > And by making an analogy to what the equivalent C code is, anyone > writing this probably just meant to treat any non-zero value as true. In C, bool can only hold value `false` and `true`, too, and putting any other value there is going to be UB. The C language provides automatic cast so when you write an integer to it, non-zero values will cause `true` to be written. However, you're not allowed to cast it into a char ptr and write other values into it. So I think there shouldn't be any special treatment to boolean type in this regard. Best, Gary > > For WRITE_ONCE no special logic is required. > > Signed-off-by: Alice Ryhl > --- > rust/kernel/sync/rwonce.rs | 19 +++++++++++++++++++ > 1 file changed, 19 insertions(+) > > diff --git a/rust/kernel/sync/rwonce.rs b/rust/kernel/sync/rwonce.rs > index a1660e43c9ef94011812d1816713cf031a73de1d..73477f53131926996614df573b2d50fff98e624f 100644 > --- a/rust/kernel/sync/rwonce.rs > +++ b/rust/kernel/sync/rwonce.rs > @@ -163,6 +163,7 @@ unsafe fn write_once(ptr: *mut Self, val: Self) { > // sizes, so picking the wrong helper should lead to a build error. > > impl_rw_once_type! { > + bool, read_once_bool, write_once_1; > u8, read_once_1, write_once_1; > i8, read_once_1, write_once_1; > u16, read_once_2, write_once_2; > @@ -186,3 +187,21 @@ unsafe fn write_once(ptr: *mut Self, val: Self) { > usize, read_once_8, write_once_8; > isize, read_once_8, write_once_8; > } > + > +/// Read an integer as a boolean once. > +/// > +/// Returns `true` if the value behind the pointer is non-zero. Otherwise returns `false`. > +/// > +/// # Safety > +/// > +/// It must be safe to `READ_ONCE` the `ptr` with type `u8`. > +#[inline(always)] > +#[track_caller] > +unsafe fn read_once_bool(ptr: *const bool) -> bool { > + // Implement `read_once_bool` in terms of `read_once_1`. The arch-specific logic is inside > + // of `read_once_1`. > + // > + // SAFETY: It is safe to `READ_ONCE` the `ptr` with type `u8`. > + let byte = unsafe { read_once_1(ptr.cast::()) }; > + byte != 0u8 > +} >