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SubjectRe: sysmips call and glibc atomic set
Ralf, firstly, thank you for the answers :)

Ralf Baechle wrote:

>
> Ok, but since the kernel disables MIPS III you're limited to MIPS II anyway ...
>

This makes sense...

>
>
> Read the ISA manual; sc will fail if the LL-bit in c0_status is cleared
> which will be cleared when the interrupt returns using the eret instruction.
>

I tried to find a MIPSIII manual from mips.com but all I could find was
mips32 and mips64 (which are not the same as MIPSII/MIPSIII/MIPSIV).

>
>
> Not having swap doesn't mean you're safe. Think of any kind of previously
> unmapped page.
>

Is there a reason why it doesn't just force that page to be mapped first?

>
>> QUESTION 2) Wouldn't it be better to pass back the initial value of
>> *arg1 in *arg3 and return zero or negative error code?
>
>
> The semantics of this syscall were previously defined by Risc/OS and later
> on continued to be used by IRIX.
>
>
>> case MIPS_ATOMIC_SET: {
>> /* This is broken in case of page faults and SMP ...
>> Risc/OS faults after maximum 20 tries with EAGAIN. */
>> unsigned int tmp;
>>
>> p = (int *) arg1;
>> errno = verify_area(VERIFY_WRITE, p, sizeof(*p));
>> if (errno)
>> return errno;
>> errno = 0;
>> save_and_cli(flags);
>> errno |= __get_user(tmp, p);
>> errno |= __put_user(arg2, p);
>> restore_flags(flags);
>>
>> if (errno)
>> return tmp;
>>
>> return tmp; /* This is broken ... */
>> }
>>
>> QUESTION 3) I notice that the code for this particular case of sysmips
>> has changed recently. The old code looked more like the 'll/sc' version
>> of glibc above. I would think that the 'll/sc' code would be better on
>> SMP systems.
>
>
> Don't think about SMP without ll/sc. There's algorithems available for
> that but their complexity leaves them a unpractical, theoretical construct.
>
>
>> Is there a good reason why this reverted?
>

Looking at 2.4.0-test5 I see the ll/sc code, but -test12 doesn't use it.
I was just curious at why it was taken out.

>
> Above code will break if the old content of memory has bit 31 set or you take
> pagefaults. The latter problem is a problem even on UP - think multi-
> threading.
>
> Finally, post such things to one of the MIPS-related mailing lists. If
> you're unlucky nobody of the MIPS'ers might see your posting on l-k.
>
> Ralf


--
Joe deBlaquiere
Red Hat, Inc.
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