[PATCH v2] RISC-V: Break load reservations during switch_to

[Palmer Dabbelt <palmer@sifive.com>]

The comment describes why in detail.  This was found because QEMU never
gives up load reservations, the issue is unlikely to manifest on real
hardware.

Thanks to Carlos Eduardo for finding the bug!

Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
---
Changes since v1 <20190605231735.26581-1-palmer@sifive.com>:

* REG_SC is now defined as a helper macro, for any code that wants to SC
  a register-sized value.
* The explicit #ifdef to check that TASK_THREAD_RA_RA is 0 has been
  removed.  Instead we rely on the assembler to catch non-zero SC
  offsets.  I've tested this does actually work.

 arch/riscv/include/asm/asm.h |  1 +
 arch/riscv/kernel/entry.S    | 11 +++++++++++
 2 files changed, 12 insertions(+)

diff --git a/arch/riscv/include/asm/asm.h b/arch/riscv/include/asm/asm.h
index 5ad4cb622bed..946b671f996c 100644
--- a/arch/riscv/include/asm/asm.h
+++ b/arch/riscv/include/asm/asm.h
@@ -30,6 +30,7 @@
 
 #define REG_L		__REG_SEL(ld, lw)
 #define REG_S		__REG_SEL(sd, sw)
+#define REG_SC		__REG_SEL(sc.w, sc.d)
 #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 1c1ecc238cfa..af685782af17 100644
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S
@@ -330,6 +330,17 @@ ENTRY(__switch_to)
 	add   a3, a0, a4
 	add   a4, a1, a4
 	REG_S ra,  TASK_THREAD_RA_RA(a3)
+	/*
+	 * The Linux ABI allows programs to depend on load reservations being
+	 * broken on context switches, but the ISA doesn't require that the
+	 * hardware ever breaks a load reservation.  The only way to break a
+	 * load reservation is with a store conditional, so we emit one here.
+	 * Since nothing ever takes a load reservation on TASK_THREAD_RA_RA we
+	 * know this will always fail, but just to be on the safe side this
+	 * writes the same value that was unconditionally written by the
+	 * previous instruction.
+	 */
+	REG_SC x0, ra, TASK_THREAD_RA_RA(a3)
 	REG_S sp,  TASK_THREAD_SP_RA(a3)
 	REG_S s0,  TASK_THREAD_S0_RA(a3)
 	REG_S s1,  TASK_THREAD_S1_RA(a3)
-- 
2.21.0