On Wed, Jul 19, 2017 at 10:20:35AM -0700, Thomas Garnier wrote: > On Wed, Jul 19, 2017 at 10:06 AM, Russell King - ARM Linux > <linux@xxxxxxxxxxxxxxx> wrote: > > On Wed, Jul 19, 2017 at 05:58:20PM +0300, Leonard Crestez wrote: > > Probably best to revert. I stopped looking at these patches during > > the discussion, as the discussion seemed to be mainly around other > > architectures, and I thought we had ARM settled. > > > > Looking at this patch now, there's several things I'm not happy with. > > > > The effect of adding a the new TIF flag for FSCHECK amongst the other > > flags is that we end up overflowing the 8-bit constant, and have to > > split the tests, meaning more instructions in the return path. Eg: > > > > - tst r1, #_TIF_SYSCALL_WORK | _TIF_WORK_MASK > > + tst r1, #_TIF_SYSCALL_WORK > > + bne fast_work_pending > > + tst r1, #_TIF_WORK_MASK > > bne fast_work_pending > > > > should be written: > > > > tst r1, #_TIF_SYSCALL_WORK > > tsteq r1, #_TIF_WORK_MASK > > bne fast_work_pending > > > > and: > > > > - tst r1, #_TIF_SYSCALL_WORK | _TIF_WORK_MASK > > + tst r1, #_TIF_SYSCALL_WORK > > + bne fast_work_pending > > + tst r1, #_TIF_WORK_MASK > > > > should be: > > > > tst r1, #_TIF_SYSCALL_WORK > > tsteq r1, #_TIF_WORK_MASK > > > > There's no need for extra branches. > > > > Now, the next issue is that I don't think this TIF-flag approach is > > good for ARM - alignment faults can happen any time due to misaligned > > packets in the networking code, and we really don't want to be doing > > this check in a place that we can loop. > > > > My original suggestion for ARM was to do the address limit check after > > all work had been processed, with interrupts disabled (so no > > possibility of this kind of loop happening.) However, that seems to > > have been replaced with this TIF approach, which is going to cause > > loops - I suspect if the probes code is enabled, this will suffer > > the same problem. Remember, the various probes stuff can walk > > userspace stacks, which means they'll be using set_fs(). > > > > I don't see why we've ended up with this (imho) sub-standard TIF-flag > > approach, and I think it's going to be very problematical. > > > > Can we please go back to the approach I suggested back in March for > > ARM that doesn't suffer from this problem? > > During the extensive thread discussion, Linus asked to move away from > architecture specific changes to this work flag system. I am glad to > fix the assembly as you asked on a separate patch. Well, for the record, I don't think you've got to the bottom of the "infinite loop" potential of Linus' approach. Eg, perf will likely trigger this same issue. Eg, perf record -a -g will attempt to record the callchain both in kernel space and userspace each time a perf interrupt happens. If the perf interrupt frequency is sufficiently high that we have multiple interrupts during the execution of do_work_pending() and its called functions, then that will turn this into an infinite loop yet again. -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up according to speedtest.net. -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
