[PATCH] RISC-V: Clear load reservations while restoring hart contexts

[PATCH] RISC-V: Clear load reservations while restoring hart contexts

[Palmer Dabbelt]

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>
---
 arch/riscv/include/asm/asm.h |  1 +
 arch/riscv/kernel/entry.S    | 21 ++++++++++++++++++++-
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/include/asm/asm.h b/arch/riscv/include/asm/asm.h
index 5a02b7d50940..9c992a88d858 100644
--- 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 74ccfd464071..9fbb256da55d 100644
--- 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
 
-- 
2.21.0


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Re: [PATCH] RISC-V: Clear load reservations while restoring hart contexts

[Christoph Hellwig]

On Tue, Sep 24, 2019 at 05:15:56PM -0700, Palmer Dabbelt wrote:
> 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.

Given how many different lines of RISC-V hardware are around how do
you know it doesn't affect any existing hardware?

Except for that the patch looks fine to me:

Reviewed-by: Christoph Hellwig <hch@lst.de>

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Re: [PATCH] RISC-V: Clear load reservations while restoring hart contexts

[Paul Walmsley]

On Tue, 24 Sep 2019, Palmer Dabbelt wrote:

> 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>

Thanks, queued for v5.4-rc.


- Paul

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