On Fri, 28 Feb 2014, Heiko Carstens wrote: > On Fri, Feb 28, 2014 at 02:12:22PM +1100, Finn Thain wrote: > > > > > Data read fault at 0x00000000 in Super Data (pc=0x3afec) BAD > > > > > KERNEL BUSERR > > > > > > I think futex_init needs to do set_fs(USER_DS), since futex is about > > > user data. > > > > Rhetorical question: does it make sense to explicitly switch to > > USER_DS when we know that current->mm may be invalid at the time? > > > > Given that the presence of USER_DS or KERNEL_DS at initcall-time > > varies between architectures, perhaps this question must be left to > > arch implementors. > > All architectures start with KERNEL_DS, otherwise the first couple of > system calls within kernel_init() would all fail and the system wouldn't > boot. I was alluding to the assertion that "USER_DS is the normal operating state": https://lkml.org/lkml/2014/1/15/476 > > > Moreover, architectures that have no need for a run-time test for > > usable futex_value ops have no real interest in such questions. > > AFAICT, only mips needs the run-time test. > > > > For every other arch the availability of futex_value ops is known at > > compile-time. We don't need to pursue universal answers to questions > > like these: > > > > 1) the correctness of switching to USER_DS with mm->current == NULL > > I "fixed" s390, so it should survive an early switch to USER_DS and a > subsequent failing user space access now. > > Anyway, > > > The discussion of these questions in various threads on various lists > > did not resolve anything. Again, this is probably because all of these > > questions are actually arch implementation issues. > > > > So it seems to me that the solution to the futex_init() problem is to > > evaluate the futex_cmpxchg_enabled assignment at compile-time where > > possible, and use the run-time test only on mips. Thoughts? > > Such a patch exists already (bottom of message): > > http://marc.info/?l=linux-next&m=138875880028607&w=2 > Thanks. I gather that you don't need this patch on s390 any more. Though it does promise to save some cycles on any architecture that can optimize away all the "if (!futex_cmpxchg_enabled) return" tests. With your patch, the nak'd patch* from Geert would be obsolete unless it was useful to mips (or deemed correct by fiat). Finn * http://marc.info/?l=linux-kernel&m=138675863423989&w=2 -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
