Andreas Jaeger wrote: > > "Maciej W. Rozycki" <macro@ds2.pg.gda.pl> writes: > > > Hi, > > > > Here is the second version of the sys__test_and_set() syscall suite. A > > glibc patch is included this time as well. > > > > There are two small changes to the sys__test_and_set() implementation: > > > > 1. verify_area() is now called for the ll/sc version as well. Otherwise > > one could pass a KSEG address and gain unauthorized access. > > > > 2. The fuction now returns immediately without performing a write access > > if the value stored in the memory wouldn't change. This is to avoid the > > need of a potentially costful sc operation; for consistency, this is also > > done for the non-ll/sc version. > > > > The glibc patch should be fairly obvious. There is no inline version of > > the _test_and_set() function for MIPS I anymore -- while previously it > > saved an extra stack frame just to call sysmips(), this would be pointless > > now (well, not quite as long as we fallback to sysmips(), actually, but > > that is a temporary compatibility bit that will soon get removed, I hope). > > Note that while sys__test_and_set() never returns an error, there might > > one happen if someone tries to execute the syscall running a kernel that > > does not support it. Hence we fall back to sysmips(). > > > > The official entry point is _test_and_set(). There is also the > > ___test_and_set() entry point defined, mostly for completeness for MIPS > > II+ systems, to be sure all syscalls actually have their wrappers > > exported. Not to be used under normal circumstances, though. > > > > Andreas, what do you think: Should we fall back to sysmips() as in the > > following patch (at a considerable performance hit -- without the fallback > > the entire ___test_and_set() wrapper is seven instructions long) or just > > require a specific minimum kernel version bail out at the compile time if > > no __NR__test_and_set is defined? Granted, pthreads don't run for > > MIPS/Linux for a long time, so it's possible the user base is not that > > large such an abrupt switch would be impossible. Especially as sysmips() > > seems to be continuously in flux for the last few months. I assume the > > switch to the new syscall would be mandatory for glibc 2.3 in any case. > > Do it the following way: > - Define in sysdeps/unix/sysv/linux/kernel-features a new macro, e.g. > __ASSUME_TEST_AND_SET with the appropriate guards > - Do *both* implementations like this: > #include <kernel-features.h> > #if __ASSUME_TEST_AND_SET > fast code without fallback > #else > slow code that first tries kernel call and then falls back to sysmips > #endif > > This way you get the fast one if you configure glibc with > --enable-kernel=2.4.6 if we assume that 2.4.6 is the first kernel with > those features. > > Check other places in glibc for details how this can be done. > This might be an overkill - the slow code on a newer kernel costs about 1 or 2 cycle longer per call. > > I'm open to constructive feedback. An open question is whether returning > > the result in v1 is clean. I believe it is -- I haven't been convinced > > that storing the result in a memory location passed as the third argument > > is cleaner. Certainly it's not faster and v1 is still dedicated to be a > > result register. It's used by sys_pipe() this way, for example. On a second thought I feel stronger using $v1 is not a good idea - what if the return path of syscall suddenly needs to modify $v1? Also we have generic syscall routine in glibc, what if someday that routine starts to check $v1 as well? I understand the chances of these "what if" are low (and perhaps sys_pipe() is already this way), but I am still convinced we should the right thing. (Whoever invented MIPS_ATOMIC_SET might have been thinking it should never need to return an error code!) I don't see any "dirtiness" in using the third argument. The slowdown in performance should be negligible under the context of the whole system call. A syscall is invented to be here and stay. I personally feel more comfortable to play a little safer rather than a littel faster. Jun
