Re: musl without atomic instructions?

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

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

-- 
Masanori Ogino