* Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > On Wed, 7 Jan 2009 22:32:22 +0100 > Ingo Molnar <mingo@xxxxxxx> wrote: > > > > We could do the whole "oldfs = get_fs(); set_fs(KERNEL_DS); .. > > > set_fs(oldfs);" crud, but it would probably be better to just add an > > > architected accessor. Especially since it's going to generally just be a > > > > > > #define get_kernel_careful(val,p) __get_user(val,p) > > > > > > for most architectures. > > > > > > We've needed that before (and yes, we've simply mis-used __get_user() on > > > x86 before rather than add it). > > > > for the oldfs stuff we already have probe_kernel_read(). OTOH, that > > involves pagefault_disable() which is an atomic op > > tisn't. pagefault_disable() is just preempt_count()+=1;barrier() ? okay. Not an atomic (which is plenty fast on Nehalem with 20 cycles anyway), but probe_kernel_read() is expensive nevertheless: ffffffff8027c092 <probe_kernel_read>: ffffffff8027c092: 65 48 8b 04 25 10 00 mov %gs:0x10,%rax ffffffff8027c099: 00 00 ffffffff8027c09b: 53 push %rbx ffffffff8027c09c: 48 8b 98 48 e0 ff ff mov -0x1fb8(%rax),%rbx ffffffff8027c0a3: 48 c7 80 48 e0 ff ff movq $0xffffffffffffffff,-0x1fb8(%rax) ffffffff8027c0aa: ff ff ff ff ffffffff8027c0ae: 65 48 8b 04 25 10 00 mov %gs:0x10,%rax ffffffff8027c0b5: 00 00 ffffffff8027c0b7: ff 80 44 e0 ff ff incl -0x1fbc(%rax) ffffffff8027c0bd: e8 0e dd 0d 00 callq ffffffff80359dd0 <__copy_from_user_inatomic> ffffffff8027c0c2: 65 48 8b 14 25 10 00 mov %gs:0x10,%rdx ffffffff8027c0c9: 00 00 ffffffff8027c0cb: ff 8a 44 e0 ff ff decl -0x1fbc(%rdx) ffffffff8027c0d1: 65 48 8b 14 25 10 00 mov %gs:0x10,%rdx ffffffff8027c0d8: 00 00 ffffffff8027c0da: 48 83 f8 01 cmp $0x1,%rax ffffffff8027c0de: 48 89 9a 48 e0 ff ff mov %rbx,-0x1fb8(%rdx) ffffffff8027c0e5: 48 19 c0 sbb %rax,%rax ffffffff8027c0e8: 48 f7 d0 not %rax ffffffff8027c0eb: 48 83 e0 f2 and $0xfffffffffffffff2,%rax ffffffff8027c0ef: 5b pop %rbx ffffffff8027c0f0: c3 retq ffffffff8027c0f1: 90 nop where __copy_user_inatomic() goes into the full __copy_generic_unrolled(). Not pretty. > Am suspecting that you guys might be over-optimising this > contended-path-were-going-to-spin-anyway code? not sure. Especially for 'good' locking usage - where there are shortly held locks and the spin times are short, the average time to get _out_ of the spinning section is a kind of secondary fastpath as well. Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
