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); \