Merge tag 'riscv-for-linus-4.15-rc4-riscv_fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/palmer/linux

Pull RISC-V fixes from Palmer Dabbelt:
 "This contains three small fixes:

   - A fix to a typo in sys_riscv_flush_icache. This only effects error
     handling, but I think it's a small and obvious enough change that
     it's sane outside the merge window.

   - The addition of smp_mb__after_spinlock(), which was recently
     removed due to an incorrect comment. This is largly a comment
     change (as there's a big one now), and while it's necessary for
     complience with the RISC-V memory model the lack of this fence
     shouldn't manifest as a bug on current implementations.
     Nonetheless, it still seems saner to have the fence in 4.15.

   - The removal of some of the HVC_RISCV_SBI driver that snuck into the
     arch port. This is compile-time dead code in 4.15 (as the driver
     isn't in yet), and during the review process we found a better way
     to implement early printk on RISC-V. While this change doesn't do
     anything, it will make staging our HVC driver easier: without this
     change the HVC driver we hope to upstream won't build on 4.15
     (because the 4.15 arch code would reference a function that no
     longer exists).

  I don't think this is the last patch set we'll want for 4.15: I think
  I'll want to remove some of the first-level irqchip driver that snuck
  in as well, which will look a lot like the HVC patch here. This is
  pending some asm-generic cleanup I'm doing that I haven't quite gotten
  clean enough to send out yet, though, but hopefully it'll be ready by
  next week (and still OK for that late)"

 * tag 'riscv-for-linus-4.15-rc4-riscv_fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/palmer/linux:
  RISC-V: Remove unused CONFIG_HVC_RISCV_SBI code
  RISC-V: Resurrect smp_mb__after_spinlock()
  RISC-V: Logical vs Bitwise typo

1 files changed, 19 insertions, 0 deletions

diff --git a/arch/riscv/include/asm/barrier.h b/arch/riscv/include/asm/barrier.h
index 773c4e039cd7..c0319cbf1eec 100644
--- a/arch/riscv/include/asm/barrier.h
+++ b/arch/riscv/include/asm/barrier.h
@@ -38,6 +38,25 @@
 #define smp_rmb()	RISCV_FENCE(r,r)
 #define smp_wmb()	RISCV_FENCE(w,w)
 
+/*
+ * This is a very specific barrier: it's currently only used in two places in
+ * the kernel, both in the scheduler.  See include/linux/spinlock.h for the two
+ * orderings it guarantees, but the "critical section is RCsc" guarantee
+ * mandates a barrier on RISC-V.  The sequence looks like:
+ *
+ *    lr.aq lock
+ *    sc    lock <= LOCKED
+ *    smp_mb__after_spinlock()
+ *    // critical section
+ *    lr    lock
+ *    sc.rl lock <= UNLOCKED
+ *
+ * The AQ/RL pair provides a RCpc critical section, but there's not really any
+ * way we can take advantage of that here because the ordering is only enforced
+ * on that one lock.  Thus, we're just doing a full fence.
+ */
+#define smp_mb__after_spinlock()	RISCV_FENCE(rw,rw)
+
 #include <asm-generic/barrier.h>
 
 #endif /* __ASSEMBLY__ */