Re: musl without atomic instructions?

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

2016-03-14 12:43 GMT+09:00 Rich Felker <dalias@libc.org>:
> On Mon, Mar 14, 2016 at 11:55:15AM +0900, Masanori Ogino wrote:
>> Well, it seems that I don't really understand vDSO.
>
> The way vdso works is that the kernel contains an image of a small ELF
> shared library file, and maps it into the virtual address space of
> each user process, and exposes its address as part of the "aux vector"
> that the dynamic linker or main program entry point receives and can
> process.
>
> While anything could be included in the vdso, normally what the kernel
> puts there are functions that allow userspace to bypass actually
> making a system call for some things that _can_ be done without a
> system call (no need for kernel privs) but where the _way_ to do them
> is only known by the kernel (e.g. hardware model specific, or
> dependent on memory structures the kernel writes and exposes to
> userspace but does not guarantee stability for). Some examples are
> time/gettimeofday/clock_gettime, getcpu, etc.
>
> If userspace chooses to use the vdso, it does symbol lookups in it
> using the same mechanisms used for dynamic library symbol lookup, then
> calls the resulting function instead of making a syscall.

OK, it is getting clear to me now. Thank you.

>> My current understanding is, vDSO make it possible that:
>>
>> 1. programs targeting without-A processors use syscalls on without-A
>> processors, and
>> 2. the programs use atomic instructions on with-A processors. (no
>> interruption, no context switching!)
>> (3. programs targeting with-A processors runs normally, without
>> calling such vDSO function)
>>
>> Is it correct? If so, it would be really nice.
>
> Even better.
>
> Indeed, a baseline vdso-based compare-and-swap for riscv would look
> like your above items 1 and 2, and item 3 if you build binaries that
> depend on a processor with the "A" option.
>
> But in the future, for non-SMP setups, case 1 could be replaced with a
> scheduler-based restart approach like pre-v6 ARM and SH3/SH4 use,
> yielding a huge performance boost (maybe around 100x speedup in
> locking/atomics). The way this works is that, when resuming a task
> that was preempted, the scheduler just has to check if the program
> counter is in the cas function in the vdso. If so, it resets the
> program counter to the start of that function before resuming
> userspace. At one point there was a good article on how the ARM
> implementation of this works, but I can't find it right now.

Fantastic! I will append this to the work list. It is really
worthwhile to work on.

-- 
Masanori Ogino