On 10.01.20 17:56, Will Deacon wrote: > Hi all, > > This is a follow-up RFC to the discussions we had on the mailing list at > the end of last year: > > https://lore.kernel.org/lkml/875zimp0ay.fsf@xxxxxxxxxxxxxxxxxx > > Unfortunately, we didn't get a "silver bullet" solution out of that > long thread, but I've tried to piece together some of the bits and > pieces we discussed and I've ended up with this series, which does at > least solve the pressing problem with the bitops for arm64. > > The rough summary of the series is: > > * Drop the GCC 4.8 workarounds, so that READ_ONCE() is a > straightforward dereference of a cast-to-volatile pointer. > > * Require that the access is either 1, 2, 4 or 8 bytes in size > (even 32-bit architectures tend to use 8-byte accesses here). > > * Introduce __READ_ONCE() for tearing operations with no size > restriction. > > * Drop pointer qualifiers from scalar types, so that volatile scalars > don't generate horrible stack-spilling mess. This is pretty ugly, > but it's also mechanical and wrapped up in a macro. > > * Convert acquire/release accessors to perform the same qualifier > stripping. > > I gave up trying to prevent READ_ONCE() on aggregates because it is > pervasive, particularly within the mm/ layer on things like pmd_t. > Thankfully, these don't tend to be volatile. > > I have more patches in this area because I'm trying to move all the > read_barrier_depends() magic into arch/alpha/, but I'm holding off until > we agree on this part first. > > Cheers, > > Will > > Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > Cc: Segher Boessenkool <segher@xxxxxxxxxxxxxxxxxxx> > Cc: Christian Borntraeger <borntraeger@xxxxxxxxxx> > Cc: Luc Van Oostenryck <luc.vanoostenryck@xxxxxxxxx> > Cc: Arnd Bergmann <arnd@xxxxxxxx> Looks sane on s390. I also checked that the problematic sequence in arch/s390/kvm/gaccess.c is not miscompiled (the binary code for the ipte_lock function is almost the same, just different addresses due to a different start address.) The kernel seems to get slighly larger though. Mostly due to different inlining decisions it seems. Total: Before=14133361, After=14135643, chg +0.02%
