RISC-V: Clear load reservations while restoring hart contexts

This is almost entirely a comment.  The bug is unlikely to manifest on
existing hardware because there is a timeout on load reservations, but
manifests on QEMU because there is no timeout.

Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>

diff --git a/arch/riscv/include/asm/asm.h b/arch/riscv/include/asm/asm.h
index 5a02b7d5094081d2d1f7e09d5a16dce23073c4f4..9c992a88d858fe6105f16978849f3a564d42b85f 100644 (file)
--- a/arch/riscv/include/asm/asm.h
+++ b/arch/riscv/include/asm/asm.h
@@ -22,6 +22,7 @@
 
 #define REG_L          __REG_SEL(ld, lw)
 #define REG_S          __REG_SEL(sd, sw)
+#define REG_SC         __REG_SEL(sc.d, sc.w)
 #define SZREG          __REG_SEL(8, 4)
 #define LGREG          __REG_SEL(3, 2)
 

diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
index da7aa88113c2cca00be5902dfff88173067b744b..2d592da1e77620f985a4fd47a2620577c526d59e 100644 (file)
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S
@@ -98,7 +98,26 @@ _save_context:
  */
        .macro RESTORE_ALL
        REG_L a0, PT_SSTATUS(sp)
-       REG_L a2, PT_SEPC(sp)
+       /*
+        * The current load reservation is effectively part of the processor's
+        * state, in the sense that load reservations cannot be shared between
+        * different hart contexts.  We can't actually save and restore a load
+        * reservation, so instead here we clear any existing reservation --
+        * it's always legal for implementations to clear load reservations at
+        * any point (as long as the forward progress guarantee is kept, but
+        * we'll ignore that here).
+        *
+        * Dangling load reservations can be the result of taking a trap in the
+        * middle of an LR/SC sequence, but can also be the result of a taken
+        * forward branch around an SC -- which is how we implement CAS.  As a
+        * result we need to clear reservations between the last CAS and the
+        * jump back to the new context.  While it is unlikely the store
+        * completes, implementations are allowed to expand reservations to be
+        * arbitrarily large.
+        */
+       REG_L  a2, PT_SEPC(sp)
+       REG_SC x0, a2, PT_SEPC(sp)
+
        csrw CSR_SSTATUS, a0
        csrw CSR_SEPC, a2