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: <stable@vger.kernel.org>
Fixes: f97fc810798c ("arm64: percpu: Implement this_cpu operations")
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

diff --git a/arch/arm64/include/asm/percpu.h b/arch/arm64/include/asm/percpu.h
index 2fee2f5..5394c84 100644 (file)
--- 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();