Re: [PATCH 0/3] sys_membarrier (x86, generic)

[Rusty Russell]

Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> writes:
> ----- On Oct 5, 2015, at 7:21 PM, Rusty Russell rusty@xxxxxxxxxx wrote:
>
>> Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> writes:
>>> Hi Andrew,
>>>
>>> Here is a repost of sys_membarrier, rebased on top of Linus commit
>>> c4b5fd3fb2058b650447372472ad24e2a989f9f6 without any change since the
>>> last v19 post other that proceeding to further testing. When merging
>>> with other system calls, system call number conflicts should be quite
>>> straightforward to handle, there is nothing special there.
>> 
>> Hi Mathieu,
>> 
>>        Great to see this go in!  One small note: it talks about
>> threads, but membarrier as currently implemented would cover any shared
>> memory.  If you plan to optimize in future, that might not be the case:
>> we'd want an address argument for those cases?
>
> Hi Rusty,
>
> Indeed, the current membarrier implementation only supports
> the MEMBARRIER_CMD_SHARED flag, which works even with shared
> memory across processes. If we ever want to optimize that for
> single-process, multi-threaded cases, we would have to add
> a new flag (e.g. MEMBARRIER_CMD_PRIVATE). This is quite
> similar to what already exists in the futex system call.
>
> I'm not sure I fully understand where the address argument
> you are describing would be useful. So far, I see two
> main use-cases: we either interact with memory that is
> local to a single process, or with memory shared across
> processes.
>
> We could indeed think about sending a membarrier to all
> processes using a specific shared memory area (hence the
> possible need for an address argument). This could eventually
> be supported by adding a specific flag for this (e.g.
> MEMBARRIER_CMD_SHM), which would indicate that an extra
> parameter is provided (an address).

That's exactly what I was thinking; eg. it can be optimized in the case
where nothing else with the memory mapped is running.

Cheers,
Rusty.
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