From mboxrd@z Thu Jan 1 00:00:00 1970 From: will.deacon@arm.com (Will Deacon) Date: Wed, 19 Oct 2016 11:59:44 +0100 Subject: [PATCH 2/2] arm64: percpu: rewrite ll/sc loops in assembly In-Reply-To: <1476874784-16214-1-git-send-email-will.deacon@arm.com> References: <1476874784-16214-1-git-send-email-will.deacon@arm.com> Message-ID: <1476874784-16214-2-git-send-email-will.deacon@arm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org 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. 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 2fee2f59288c..5394c8405e66 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(); -- 2.1.4