Re: musl without atomic instructions?

[Rich Felker <dalias@libc.org>]

On Mon, Mar 14, 2016 at 10:34:22AM +0900, Masanori Ogino wrote:
> 2016-03-13 9:54 GMT+09:00 Masanori Ogino <masanori.ogino@gmail.com>:
> > 2016-03-13 9:21 GMT+09:00 Rich Felker <dalias@libc.org>:
> >> 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.
> 
> On sw-dev, Darius Rad taught me that there is a syscall to perform CAS
> on RISC-V without the A standard extension. CONFIG_RV_SYSRISCV_ATOMIC
> enables it (with RISC-V patches.)
> 
> For reference, the source code is here:
> https://github.com/riscv/riscv-linux/blob/master/arch/riscv/kernel/sys_riscv.c

IMO a vdso function should be added that makes the syscall, rather
than having libc call the syscall directly; this would allow the
kernel to automatically provide a better implementation in the future
without the need to rebuild applications. Using a syscall for this is
very slow. Working with kernel people to propose such a thing (or even
implementing it and submitting kernel patches) is certainly one option
for something to add to a GSoC project proposal to make it more
substantial.

Rich