Alright, I rolled my sleeve and digged into IRIX 6.5, and guess what? sysmips() does NOT have MIPS_ATOMIC_SET (2001) on IRIX! See the header below. So apparently MIPS_ATOMIC_SET was invented for Linux only, probably just to implement _test_and_set(). (It would be interesting to see how IRIX implement _test_and_set() on MIPS I machines. However, the machine I have access uses ll/sc instructions). Given the above understanding, I think we are free to add a new sysmips subcall or even just change the current one if we want to. Through the discussions, I hope we have achived the following concensus: 1. We need a correct syscall support to implement _test_and_set() on MIPS I machine. ll/sc emulation is not considered good enough to eliminate this need due to performance concerns. People who take this route understand that they may have to create extra user land configurations. 2. Nobody seems to object the idea of ll/sc emulation per se, althought it is not currently fully implemented based on my understanding. People who prefer this route will enjoy the same ll/sc-enabled userland but presumably take a performance hit on machines without ll/sc. Anybody still have objections to the above conclusions? ------------- Now back to the fix for syscall: the cvs tree is buggy as it is, as pointed out by Florian. I see several possibilities, which more or less the same as I brought up at the beginning: 1) Florian's fix - introduce a new assembly routine to intercept MIPS_ATOMIC_SET call so that correct return value is returned when there is no error and error value is returned when there is an error. 1.a) A slight modification to 1) - we send a seg fault when there is access error. This solves two problems in 1). With 1), _test_and_set() will not be able to distinguish whether an error has happened or a negative value is returned. So in effect, error is mistakenly ignored. The second benenfit is that with seg fault MIPS I implementation will have the same behavior as MIPS II implementation which uses ll/sc. 2) Add a new subcall to sysmips, MIPS_NEW_ATOMIC_SET. It takes three arguments with the third one being the address to hold return value. Again, I think we should send seg fault on access errors. The advantage of 2) is that we can still use the same sysmips() call without introducing any new files. The disadvantage is that people who use MIPS_ATOMIC_SET directly is still subject to error, either in one form or another. To me, either 1.a) or 2) is fine with me, although I have a slight faovr over 2) (perhaps because I don't like assembly code and the extra "vertical" calling layer introduced in 1.a) -------------- So, please do us all a favor, can everybody who cares let us know : 1) can we agree on the concensus? 2) which fix do you like (naturally assuming you agree with the concensus)? Let us work together and put a closure on this recurring matter! Jun
