On Wed, Jul 19, 2017 at 11:35 AM, Russell King - ARM Linux <linux@xxxxxxxxxxxxxxx> wrote: > 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. Do you think it applies to the patch I just sent? The other approach is to check at the entrance, ignore _TIF_FSCHECK on the loop and clear it on exit. > > -- > 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. -- Thomas -- 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
