[PATCH AUTOSEL 6.0 17/46] arm64: atomics: remove LL/SC trampolines

Sasha Levin
2022-10-11T14:51:59Z

From: Mark Rutland <mark.rutland@arm.com>

[ Upstream commit b2c3ccbd0011bb3b51d0fec24cb3a5812b1ec8ea ]

When CONFIG_ARM64_LSE_ATOMICS=y, each use of an LL/SC atomic results in
a fragment of code being generated in a subsection without a clear
association with its caller. A trampoline in the caller branches to the
LL/SC atomic with with a direct branch, and the atomic directly branches
back into its trampoline.

This breaks backtracing, as any PC within the out-of-line fragment will
be symbolized as an offset from the nearest prior symbol (which may not
be the function using the atomic), and since the atomic returns with a
direct branch, the caller's PC may be missing from the backtrace.

For example, with secondary_start_kernel() hacked to contain
atomic_inc(NULL), the resulting exception can be reported as being taken
from cpus_are_stuck_in_kernel():

| Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
| Mem abort info:
| ESR = 0x0000000096000004
| EC = 0x25: DABT (current EL), IL = 32 bits
| SET = 0, FnV = 0
| EA = 0, S1PTW = 0
| FSC = 0x04: level 0 translation fault
| Data abort info:
| ISV = 0, ISS = 0x00000004
| CM = 0, WnR = 0
| [0000000000000000] user address but active_mm is swapper
| Internal error: Oops: 96000004 [#1] PREEMPT SMP
| Modules linked in:
| CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.19.0-11219-geb555cb5b794-dirty #3
| Hardware name: linux,dummy-virt (DT)
| pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
| pc : cpus_are_stuck_in_kernel+0xa4/0x120
| lr : secondary_start_kernel+0x164/0x170
| sp : ffff80000a4cbe90
| x29: ffff80000a4cbe90 x28: 0000000000000000 x27: 0000000000000000
| x26: 0000000000000000 x25: 0000000000000000 x24: 0000000000000000
| x23: 0000000000000000 x22: 0000000000000000 x21: 0000000000000000
| x20: 0000000000000001 x19: 0000000000000001 x18: 0000000000000008
| x17: 3030383832343030 x16: 3030303030307830 x15: ffff80000a4cbab0
| x14: 0000000000000001 x13: 5d31666130663133 x12: 3478305b20313030
| x11: 3030303030303078 x10: 3020726f73736563 x9 : 726f737365636f72
| x8 : ffff800009ff2ef0 x7 : 0000000000000003 x6 : 0000000000000000
| x5 : 0000000000000000 x4 : 0000000000000000 x3 : 0000000000000100
| x2 : 0000000000000000 x1 : ffff0000029bd880 x0 : 0000000000000000
| Call trace:
| cpus_are_stuck_in_kernel+0xa4/0x120
| __secondary_switched+0xb0/0xb4
| Code: 35ffffa3 17fffc6c d53cd040 f9800011 (885f7c01)
| ---[ end trace 0000000000000000 ]---

This is confusing and hinders debugging, and will be problematic for
CONFIG_LIVEPATCH as these cases cannot be unwound reliably.

This is very similar to recent issues with out-of-line exception fixups,
which were removed in commits:

35d67794b8828333 ("arm64: lib: __arch_clear_user(): fold fixups into body")
4012e0e22739eef9 ("arm64: lib: __arch_copy_from_user(): fold fixups into body")
139f9ab73d60cf76 ("arm64: lib: __arch_copy_to_user(): fold fixups into body")

When the trampolines were introduced in commit:

addfc38672c73efd ("arm64: atomics: avoid out-of-line ll/sc atomics")

The rationale was to improve icache performance by grouping the LL/SC
atomics together. This has never been measured, and this theoretical
benefit is outweighed by other factors:

* As the subsections are collapsed into sections at object file
granularity, these are spread out throughout the kernel and can share
cachelines with unrelated code regardless.

* GCC 12.1.0 has been observed to place the trampoline out-of-line in
specialised __ll_sc_*() functions, introducing more branching than was
intended.

* Removing the trampolines has been observed to shrink a defconfig
kernel Image by 64KiB when building with GCC 12.1.0.

This patch removes the LL/SC trampolines, meaning that the LL/SC atomics
will be inlined into their callers (or placed in out-of line functions
using regular BL/RET pairs). When CONFIG_ARM64_LSE_ATOMICS=y, the LL/SC
atomics are always called in an unlikely branch, and will be placed in a
cold portion of the function, so this should have minimal impact to the
hot paths.

Other than the improved backtracing, there should be no functional
change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20220817155914.3975112-2-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/arm64/include/asm/atomic_ll_sc.h | 40 ++++++---------------------
1 file changed, 9 insertions(+), 31 deletions(-)

diff --git a/arch/arm64/include/asm/atomic_ll_sc.h b/arch/arm64/include/asm/atomic_ll_sc.h
index fe0db8d416fb..906e2d8c254c 100644
--- a/arch/arm64/include/asm/atomic_ll_sc.h
+++ b/arch/arm64/include/asm/atomic_ll_sc.h
@@ -12,19 +12,6 @@

#include <linux/stringify.h>

-#ifdef CONFIG_ARM64_LSE_ATOMICS
-#define __LL_SC_FALLBACK(asm_ops) \
-" b 3f\n" \
-" .subsection 1\n" \
-"3:\n" \
-asm_ops "\n" \
-" b 4f\n" \
-" .previous\n" \
-"4:\n"
-#else
-#define __LL_SC_FALLBACK(asm_ops) asm_ops
-#endif
-
#ifndef CONFIG_CC_HAS_K_CONSTRAINT
#define K
#endif
@@ -43,12 +30,11 @@ __ll_sc_atomic_##op(int i, atomic_t *v) \
int result; \
\
asm volatile("// atomic_" #op "\n" \
- __LL_SC_FALLBACK( \
" prfm pstl1strm, %2\n" \
"1: ldxr %w0, %2\n" \
" " #asm_op " %w0, %w0, %w3\n" \
" stxr %w1, %w0, %2\n" \
- " cbnz %w1, 1b\n") \
+ " cbnz %w1, 1b\n" \
: "=&r" (result), "=&r" (tmp), "+Q" (v->counter) \
: __stringify(constraint) "r" (i)); \
}
@@ -61,13 +47,12 @@ __ll_sc_atomic_##op##_return##name(int i, atomic_t *v) \
int result; \
\
asm volatile("// atomic_" #op "_return" #name "\n" \
- __LL_SC_FALLBACK( \
" prfm pstl1strm, %2\n" \
"1: ld" #acq "xr %w0, %2\n" \
" " #asm_op " %w0, %w0, %w3\n" \
" st" #rel "xr %w1, %w0, %2\n" \
" cbnz %w1, 1b\n" \
- " " #mb ) \
+ " " #mb \
: "=&r" (result), "=&r" (tmp), "+Q" (v->counter) \
: __stringify(constraint) "r" (i) \
: cl); \
@@ -83,13 +68,12 @@ __ll_sc_atomic_fetch_##op##name(int i, atomic_t *v) \
int val, result; \
\
asm volatile("// atomic_fetch_" #op #name "\n" \
- __LL_SC_FALLBACK( \
" prfm pstl1strm, %3\n" \
"1: ld" #acq "xr %w0, %3\n" \
" " #asm_op " %w1, %w0, %w4\n" \
" st" #rel "xr %w2, %w1, %3\n" \
" cbnz %w2, 1b\n" \
- " " #mb ) \
+ " " #mb \
: "=&r" (result), "=&r" (val), "=&r" (tmp), "+Q" (v->counter) \
: __stringify(constraint) "r" (i) \
: cl); \
@@ -142,12 +126,11 @@ __ll_sc_atomic64_##op(s64 i, atomic64_t *v) \
unsigned long tmp; \
\
asm volatile("// atomic64_" #op "\n" \
- __LL_SC_FALLBACK( \
" prfm pstl1strm, %2\n" \
"1: ldxr %0, %2\n" \
" " #asm_op " %0, %0, %3\n" \
" stxr %w1, %0, %2\n" \
- " cbnz %w1, 1b") \
+ " cbnz %w1, 1b" \
: "=&r" (result), "=&r" (tmp), "+Q" (v->counter) \
: __stringify(constraint) "r" (i)); \
}
@@ -160,13 +143,12 @@ __ll_sc_atomic64_##op##_return##name(s64 i, atomic64_t *v) \
unsigned long tmp; \
\
asm volatile("// atomic64_" #op "_return" #name "\n" \
- __LL_SC_FALLBACK( \
" prfm pstl1strm, %2\n" \
"1: ld" #acq "xr %0, %2\n" \
" " #asm_op " %0, %0, %3\n" \
" st" #rel "xr %w1, %0, %2\n" \
" cbnz %w1, 1b\n" \
- " " #mb ) \
+ " " #mb \
: "=&r" (result), "=&r" (tmp), "+Q" (v->counter) \
: __stringify(constraint) "r" (i) \
: cl); \
@@ -182,13 +164,12 @@ __ll_sc_atomic64_fetch_##op##name(s64 i, atomic64_t *v) \
unsigned long tmp; \
\
asm volatile("// atomic64_fetch_" #op #name "\n" \
- __LL_SC_FALLBACK( \
" prfm pstl1strm, %3\n" \
"1: ld" #acq "xr %0, %3\n" \
" " #asm_op " %1, %0, %4\n" \
" st" #rel "xr %w2, %1, %3\n" \
" cbnz %w2, 1b\n" \
- " " #mb ) \
+ " " #mb \
: "=&r" (result), "=&r" (val), "=&r" (tmp), "+Q" (v->counter) \
: __stringify(constraint) "r" (i) \
: cl); \
@@ -240,7 +221,6 @@ __ll_sc_atomic64_dec_if_positive(atomic64_t *v)
unsigned long tmp;

asm volatile("// atomic64_dec_if_positive\n"
- __LL_SC_FALLBACK(
" prfm pstl1strm, %2\n"
"1: ldxr %0, %2\n"
" subs %0, %0, #1\n"
@@ -248,7 +228,7 @@ __ll_sc_atomic64_dec_if_positive(atomic64_t *v)
" stlxr %w1, %0, %2\n"
" cbnz %w1, 1b\n"
" dmb ish\n"
- "2:")
+ "2:"
: "=&r" (result), "=&r" (tmp), "+Q" (v->counter)
:
: "cc", "memory");
@@ -274,7 +254,6 @@ __ll_sc__cmpxchg_case_##name##sz(volatile void *ptr, \
old = (u##sz)old; \
\
asm volatile( \
- __LL_SC_FALLBACK( \
" prfm pstl1strm, %[v]\n" \
"1: ld" #acq "xr" #sfx "\t%" #w "[oldval], %[v]\n" \
" eor %" #w "[tmp], %" #w "[oldval], %" #w "[old]\n" \
@@ -282,7 +261,7 @@ __ll_sc__cmpxchg_case_##name##sz(volatile void *ptr, \
" st" #rel "xr" #sfx "\t%w[tmp], %" #w "[new], %[v]\n" \
" cbnz %w[tmp], 1b\n" \
" " #mb "\n" \
- "2:") \
+ "2:" \
: [tmp] "=&r" (tmp), [oldval] "=&r" (oldval), \
[v] "+Q" (*(u##sz *)ptr) \
: [old] __stringify(constraint) "r" (old), [new] "r" (new) \
@@ -326,7 +305,6 @@ __ll_sc__cmpxchg_double##name(unsigned long old1, \
unsigned long tmp, ret; \
\
asm volatile("// __cmpxchg_double" #name "\n" \
- __LL_SC_FALLBACK( \
" prfm pstl1strm, %2\n" \
"1: ldxp %0, %1, %2\n" \
" eor %0, %0, %3\n" \
@@ -336,7 +314,7 @@ __ll_sc__cmpxchg_double##name(unsigned long old1, \
" st" #rel "xp %w0, %5, %6, %2\n" \
" cbnz %w0, 1b\n" \
" " #mb "\n" \
- "2:") \
+ "2:" \
: "=&r" (tmp), "=&r" (ret), "+Q" (*(unsigned long *)ptr) \
: "r" (old1), "r" (old2), "r" (new1), "r" (new2) \
: cl); \
--
2.35.1