From 5c7b8e385e4c358c602d10723a16b4d948087492 Mon Sep 17 00:00:00 2001 From: Will Deacon Date: Mon, 4 Jul 2016 17:44:48 +0100 Subject: [PATCH] arm64: percpu: rewrite ll/sc loops in assembly Writing the outer loop of an LL/SC sequence using do {...} while constructs potentially allows the compiler to hoist memory accesses between the STXR and the branch back to the LDXR. On CPUs that do not guarantee forward progress of LL/SC loops when faced with memory accesses to the same ERG (up to 2k) between the failed STXR and the branch back, we may end up livelocking. This patch avoids this issue in our percpu atomics by rewriting the outer loop as part of the LL/SC inline assembly block. Cc: Fixes: ffe0a6f178db ("arm64: percpu: Implement this_cpu operations") Reviewed-by: Mark Rutland Tested-by: Mark Rutland Signed-off-by: Will Deacon --- arch/arm64/include/asm/percpu.h | 120 +++++++++++++++----------------- 1 file changed, 56 insertions(+), 64 deletions(-) diff --git a/arch/arm64/include/asm/percpu.h b/arch/arm64/include/asm/percpu.h index 2fee2f59288c9..5394c8405e660 100644 --- a/arch/arm64/include/asm/percpu.h +++ b/arch/arm64/include/asm/percpu.h @@ -44,48 +44,44 @@ static inline unsigned long __percpu_##op(void *ptr, \ \ switch (size) { \ case 1: \ - do { \ - asm ("//__per_cpu_" #op "_1\n" \ - "ldxrb %w[ret], %[ptr]\n" \ + asm ("//__per_cpu_" #op "_1\n" \ + "1: ldxrb %w[ret], %[ptr]\n" \ #asm_op " %w[ret], %w[ret], %w[val]\n" \ - "stxrb %w[loop], %w[ret], %[ptr]\n" \ - : [loop] "=&r" (loop), [ret] "=&r" (ret), \ - [ptr] "+Q"(*(u8 *)ptr) \ - : [val] "Ir" (val)); \ - } while (loop); \ + " stxrb %w[loop], %w[ret], %[ptr]\n" \ + " cbnz %w[loop], 1b" \ + : [loop] "=&r" (loop), [ret] "=&r" (ret), \ + [ptr] "+Q"(*(u8 *)ptr) \ + : [val] "Ir" (val)); \ break; \ case 2: \ - do { \ - asm ("//__per_cpu_" #op "_2\n" \ - "ldxrh %w[ret], %[ptr]\n" \ + asm ("//__per_cpu_" #op "_2\n" \ + "1: ldxrh %w[ret], %[ptr]\n" \ #asm_op " %w[ret], %w[ret], %w[val]\n" \ - "stxrh %w[loop], %w[ret], %[ptr]\n" \ - : [loop] "=&r" (loop), [ret] "=&r" (ret), \ - [ptr] "+Q"(*(u16 *)ptr) \ - : [val] "Ir" (val)); \ - } while (loop); \ + " stxrh %w[loop], %w[ret], %[ptr]\n" \ + " cbnz %w[loop], 1b" \ + : [loop] "=&r" (loop), [ret] "=&r" (ret), \ + [ptr] "+Q"(*(u16 *)ptr) \ + : [val] "Ir" (val)); \ break; \ case 4: \ - do { \ - asm ("//__per_cpu_" #op "_4\n" \ - "ldxr %w[ret], %[ptr]\n" \ + asm ("//__per_cpu_" #op "_4\n" \ + "1: ldxr %w[ret], %[ptr]\n" \ #asm_op " %w[ret], %w[ret], %w[val]\n" \ - "stxr %w[loop], %w[ret], %[ptr]\n" \ - : [loop] "=&r" (loop), [ret] "=&r" (ret), \ - [ptr] "+Q"(*(u32 *)ptr) \ - : [val] "Ir" (val)); \ - } while (loop); \ + " stxr %w[loop], %w[ret], %[ptr]\n" \ + " cbnz %w[loop], 1b" \ + : [loop] "=&r" (loop), [ret] "=&r" (ret), \ + [ptr] "+Q"(*(u32 *)ptr) \ + : [val] "Ir" (val)); \ break; \ case 8: \ - do { \ - asm ("//__per_cpu_" #op "_8\n" \ - "ldxr %[ret], %[ptr]\n" \ + asm ("//__per_cpu_" #op "_8\n" \ + "1: ldxr %[ret], %[ptr]\n" \ #asm_op " %[ret], %[ret], %[val]\n" \ - "stxr %w[loop], %[ret], %[ptr]\n" \ - : [loop] "=&r" (loop), [ret] "=&r" (ret), \ - [ptr] "+Q"(*(u64 *)ptr) \ - : [val] "Ir" (val)); \ - } while (loop); \ + " stxr %w[loop], %[ret], %[ptr]\n" \ + " cbnz %w[loop], 1b" \ + : [loop] "=&r" (loop), [ret] "=&r" (ret), \ + [ptr] "+Q"(*(u64 *)ptr) \ + : [val] "Ir" (val)); \ break; \ default: \ BUILD_BUG(); \ @@ -150,44 +146,40 @@ static inline unsigned long __percpu_xchg(void *ptr, unsigned long val, switch (size) { case 1: - do { - asm ("//__percpu_xchg_1\n" - "ldxrb %w[ret], %[ptr]\n" - "stxrb %w[loop], %w[val], %[ptr]\n" - : [loop] "=&r"(loop), [ret] "=&r"(ret), - [ptr] "+Q"(*(u8 *)ptr) - : [val] "r" (val)); - } while (loop); + asm ("//__percpu_xchg_1\n" + "1: ldxrb %w[ret], %[ptr]\n" + " stxrb %w[loop], %w[val], %[ptr]\n" + " cbnz %w[loop], 1b" + : [loop] "=&r"(loop), [ret] "=&r"(ret), + [ptr] "+Q"(*(u8 *)ptr) + : [val] "r" (val)); break; case 2: - do { - asm ("//__percpu_xchg_2\n" - "ldxrh %w[ret], %[ptr]\n" - "stxrh %w[loop], %w[val], %[ptr]\n" - : [loop] "=&r"(loop), [ret] "=&r"(ret), - [ptr] "+Q"(*(u16 *)ptr) - : [val] "r" (val)); - } while (loop); + asm ("//__percpu_xchg_2\n" + "1: ldxrh %w[ret], %[ptr]\n" + " stxrh %w[loop], %w[val], %[ptr]\n" + " cbnz %w[loop], 1b" + : [loop] "=&r"(loop), [ret] "=&r"(ret), + [ptr] "+Q"(*(u16 *)ptr) + : [val] "r" (val)); break; case 4: - do { - asm ("//__percpu_xchg_4\n" - "ldxr %w[ret], %[ptr]\n" - "stxr %w[loop], %w[val], %[ptr]\n" - : [loop] "=&r"(loop), [ret] "=&r"(ret), - [ptr] "+Q"(*(u32 *)ptr) - : [val] "r" (val)); - } while (loop); + asm ("//__percpu_xchg_4\n" + "1: ldxr %w[ret], %[ptr]\n" + " stxr %w[loop], %w[val], %[ptr]\n" + " cbnz %w[loop], 1b" + : [loop] "=&r"(loop), [ret] "=&r"(ret), + [ptr] "+Q"(*(u32 *)ptr) + : [val] "r" (val)); break; case 8: - do { - asm ("//__percpu_xchg_8\n" - "ldxr %[ret], %[ptr]\n" - "stxr %w[loop], %[val], %[ptr]\n" - : [loop] "=&r"(loop), [ret] "=&r"(ret), - [ptr] "+Q"(*(u64 *)ptr) - : [val] "r" (val)); - } while (loop); + asm ("//__percpu_xchg_8\n" + "1: ldxr %[ret], %[ptr]\n" + " stxr %w[loop], %[val], %[ptr]\n" + " cbnz %w[loop], 1b" + : [loop] "=&r"(loop), [ret] "=&r"(ret), + [ptr] "+Q"(*(u64 *)ptr) + : [val] "r" (val)); break; default: BUILD_BUG(); -- 2.39.5