On Fri, 18 Jul 2014, Arnd Bergmann wrote: > On Friday 18 July 2014 14:07:42 Ley Foon Tan wrote: > > On Thu, Jul 17, 2014 at 7:07 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote: > > > The get_user/put_user functions really need to be annotated might_fault(), > > > because that's what they do. > > > > > > The whole point of get_user() is to access an unchecked user space > > > pointer, which can do a number of things based on what the pointer > > > points to: > > > > > > - access a user space variable that resides in memory > > > - access a kernel pointer and fail because of the access_ok() > > > check > > > - access a user space pointer that is not mapped and return > > > through the __ex_table fixup. > > > - access a user space pointer that has a valid VMA but not PTE, > > > causing a page fault to be resolved. > > > > > > It's the last case that breaks here. > > So, do you mean that we can't use get_user/put_user in futex support? > > BTW, some architectures like sh,parisc, m68k use get_user in futex > > function as well. > > Any recommendation way to support futex if we can't use get_user. > > Note, nios2 doesn't have atomic instruction. > > Thanks. > > I looked at it again now and I'm no longer sure about my initial > interpretation. The way it seems to work is that pagefault_disable() > turns the case I mentioned into a simple error through the fixup, > so we return -EFAULT from get_user, and retry the futex from > futex_wake_op(). > > This would however also mean that there is no need for a spinlock > at all, atomicity is already implied by pagefault_disable() here > because you are running on a UP kernel and pagefault_disable() also > means there is no preemption. > > If this understanding is right, we can probably just merge the > m68k implementation into the asm-generic version, as that does > exactly that, and just isn't SMP safe. I'm still unsure whether > I'm missing something here though, as everything else seems to > do this in assembly, even for non-SMP machines that could use > the trivial method that m68k has. For UP relying of pagefault disable should be good enough indeed. I guess the asm for the other UP stuff results from looking at architectures or copying from architectures which did this in ASM :) Thanks, tglx -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
