Re: musl without atomic instructions?

[Masanori Ogino <masanori.ogino@gmail.com>]

2016-03-13 9:21 GMT+09:00 Rich Felker <dalias@libc.org>:
> On Sun, Mar 13, 2016 at 08:47:36AM +0900, Masanori Ogino wrote:
>> Hello,
>>
>> While I work on my GSoC proposal, I doubt whether musl can be built
>> without hardware atomic operation supports.
>>
>> Could we build musl without such instructions?
>> If we could, what will happen with the features of musl?
>
> Atomic compare and swap (usually provided by either a direct cas
> instruction or ll/sc pair type) is a hard requirement for musl.

OK, I understood.

> The normal profiles of riscv have at least ll/sc style and possibly cas
> too.

Yes, ll/sc style primitives are provided in A standard extension,
according to the ISA spec v2.0, section 5.3.

> Minimal profiles for microcontroller use lack it (this was a
> mistake in the riscv ISA specification, IMO), so if supporting these
> ISA levels is interesting, there are at least three options:
>
> 1. Have the kernel trap the unimplemented instructions and emulate
>    them.
>
> 2. Have userspace issue a system call to have the kernel mediate
>    atomic accesses.
>
> 3. Integrate atomic sequence restart with the scheduler: at scheduling
>    time, the kernel determines if the task being resumed was
>    interrupted in the middle of a sequence of instructions that's
>    supposed to be atomic, and if so, resets the program counter to the
>    beginning of the sequence. (This is how pre-v6 ARM and most SH
>    models work.)
>
> Option 3 offers by far the best performance but inherently only works
> on uniprocessor. Options 1 and 2 could theoretically support SMP as
> long as the kernel has some other way of ensuring mutual exclusion and
> memory synchronization between the processors.
>
> Of course the best of all worlds is to have the kernel provide a vdso
> function for atomic cas which it can then provide an optimal
> implementation of for the particular processor being used. Then
> baseline-ISA-level riscv binaries would use the vdso, and ones
> targeting an ISA level that's known to have native atomic instructions
> would use the inline instructions.

OK, I will ask about the current status on the RISC-V sw-dev ML.

Thank you for clarification.

-- 
Masanori Ogino