[PATCH] locking/atomic: scripts: fix fallback ifdeffery
Mark Rutland
mark.rutland at arm.com
Tue Sep 19 10:14:29 PDT 2023
Since commit:
9257959a6e5b4fca ("locking/atomic: scripts: restructure fallback ifdeffery")
The ordering fallbacks for atomic*_read_acquire() and
atomic*_set_release() erroneously fall back to the implictly relaxed
atomic*_read() and atomic*_set() variants respectively, without any
additional barriers. This loses the ACQUIRE and RELEASE ordering
semantics, which can result in a wide variety of problems, even on
strongly-ordered architectures where the implementation of
atomic*_read() and/or atomic*_set() allows the compiler to reorder those
relative to other accesses.
In practice this has been observed to break bit spinlocks on arm64,
resulting in dentry cache corruption.
The fallback logic was intended to allow ACQUIRE/RELEASE/RELAXED ops to
be defined in terms of FULL ops, but where an op had RELAXED ordering by
default, this unintentionally permitted the ACQUIRE/RELEASE ops to be
defined in terms of the implicitly RELAXED default.
This patch corrects the logic to avoid falling back to implicitly
RELAXED ops, resulting in the same behaviour as prior to commit
9257959a6e5b4fca.
I've verified the resulting assembly on arm64 by generating outlined
wrappers of the atomics. Prior to this patch the compiler generates
sequences using relaxed load (LDR) and store (STR) instructions, e.g.
| <outlined_atomic64_read_acquire>:
| ldr x0, [x0]
| ret
|
| <outlined_atomic64_set_release>:
| str x1, [x0]
| ret
With this patch applied the compiler generates sequences using the
intended load-acquire (LDAR) and store-release (STLR) instructions, e.g.
| <outlined_atomic64_read_acquire>:
| ldar x0, [x0]
| ret
|
| <outlined_atomic64_set_release>:
| stlr x1, [x0]
| ret
To make sure that there were no other victims of the ifdeffery rewrite,
I generated outlined copies of all of the {atomic,atomic64,atomic_long}
atomic operations before and after commit 9257959a6e5b4fca. A diff of
the generated assembly on arm64 shows that only the read_acquire() and
set_release() operations were changed, and only lost their intended
ordering:
| [mark at lakrids:~/src/linux]% diff -u \
| <(aarch64-linux-gnu-objdump -d before-9257959a6e5b4fca.o)
| <(aarch64-linux-gnu-objdump -d after-9257959a6e5b4fca.o)
| --- /proc/self/fd/11 2023-09-19 16:51:51.114779415 +0100
| +++ /proc/self/fd/16 2023-09-19 16:51:51.114779415 +0100
| @@ -1,5 +1,5 @@
|
| -before-9257959a6e5b4fca.o: file format elf64-littleaarch64
| +after-9257959a6e5b4fca.o: file format elf64-littleaarch64
|
|
| Disassembly of section .text:
| @@ -9,7 +9,7 @@
| 4: d65f03c0 ret
|
| 0000000000000008 <outlined_atomic_read_acquire>:
| - 8: 88dffc00 ldar w0, [x0]
| + 8: b9400000 ldr w0, [x0]
| c: d65f03c0 ret
|
| 0000000000000010 <outlined_atomic_set>:
| @@ -17,7 +17,7 @@
| 14: d65f03c0 ret
|
| 0000000000000018 <outlined_atomic_set_release>:
| - 18: 889ffc01 stlr w1, [x0]
| + 18: b9000001 str w1, [x0]
| 1c: d65f03c0 ret
|
| 0000000000000020 <outlined_atomic_add>:
| @@ -1230,7 +1230,7 @@
| 1070: d65f03c0 ret
|
| 0000000000001074 <outlined_atomic64_read_acquire>:
| - 1074: c8dffc00 ldar x0, [x0]
| + 1074: f9400000 ldr x0, [x0]
| 1078: d65f03c0 ret
|
| 000000000000107c <outlined_atomic64_set>:
| @@ -1238,7 +1238,7 @@
| 1080: d65f03c0 ret
|
| 0000000000001084 <outlined_atomic64_set_release>:
| - 1084: c89ffc01 stlr x1, [x0]
| + 1084: f9000001 str x1, [x0]
| 1088: d65f03c0 ret
|
| 000000000000108c <outlined_atomic64_add>:
| @@ -2427,7 +2427,7 @@
| 207c: d65f03c0 ret
|
| 0000000000002080 <outlined_atomic_long_read_acquire>:
| - 2080: c8dffc00 ldar x0, [x0]
| + 2080: f9400000 ldr x0, [x0]
| 2084: d65f03c0 ret
|
| 0000000000002088 <outlined_atomic_long_set>:
| @@ -2435,7 +2435,7 @@
| 208c: d65f03c0 ret
|
| 0000000000002090 <outlined_atomic_long_set_release>:
| - 2090: c89ffc01 stlr x1, [x0]
| + 2090: f9000001 str x1, [x0]
| 2094: d65f03c0 ret
|
| 0000000000002098 <outlined_atomic_long_add>:
I've build tested this with a variety of configs for alpha, arm, arm64,
csky, i386, m68k, microblaze, mips, nios2, openrisc, powerpc, riscv,
s390, sh, sparc, x86_64, and xtensa, for which I've seen no issues. I
was unable to build test for ia64 and parisc due to existing build
breakage in v6.6-rc2.
Fixes: 9257959a6e5b4fca ("locking/atomic: scripts: restructure fallback ifdeffery")
Reported-by: Ming Lei <ming.lei at redhat.com>
Link: https://lore.kernel.org/all/ZOWFtqA2om0w5Vmz@fedora/
Reported-by: Darrick J. Wong <djwong at kernel.org>
Link: https://lore.kernel.org/linux-fsdevel/20230912173026.GA3389127@frogsfrogsfrogs/
Signed-off-by: Mark Rutland <mark.rutland at arm.com>
Cc: Baokun Li <libaokun1 at huawei.com>
Cc: Boqun Feng <boqun.feng at gmail.com>
Cc: Darrick J. Wong <djwong at kernel.org>
Cc: Dave Chinner <david at fromorbit.com>
Cc: Kent Overstreet <kent.overstreet at linux.dev>
Cc: Ming Lei <ming.lei at redhat.com>
Cc: Peter Zijlstra <peterz at infradead.org>
Cc: Will Deacon <will at kernel.org>
Cc: Yi Zhang <yi.zhang at redhat.com>
Cc: linux-arm-kernel at lists.infradead.org
---
include/linux/atomic/atomic-arch-fallback.h | 10 +---------
scripts/atomic/gen-atomic-fallback.sh | 2 +-
2 files changed, 2 insertions(+), 10 deletions(-)
Peter, are you happy to queue this in the tip tree? It's a pretty nasty
regresssion in v6.5, and I'd like to get this in as a fix for v6.6 ASAP.
Thanks,
Mark.
diff --git a/include/linux/atomic/atomic-arch-fallback.h b/include/linux/atomic/atomic-arch-fallback.h
index 18f5744dfb5d8..b83ef19da13de 100644
--- a/include/linux/atomic/atomic-arch-fallback.h
+++ b/include/linux/atomic/atomic-arch-fallback.h
@@ -459,8 +459,6 @@ raw_atomic_read_acquire(const atomic_t *v)
{
#if defined(arch_atomic_read_acquire)
return arch_atomic_read_acquire(v);
-#elif defined(arch_atomic_read)
- return arch_atomic_read(v);
#else
int ret;
@@ -508,8 +506,6 @@ raw_atomic_set_release(atomic_t *v, int i)
{
#if defined(arch_atomic_set_release)
arch_atomic_set_release(v, i);
-#elif defined(arch_atomic_set)
- arch_atomic_set(v, i);
#else
if (__native_word(atomic_t)) {
smp_store_release(&(v)->counter, i);
@@ -2575,8 +2571,6 @@ raw_atomic64_read_acquire(const atomic64_t *v)
{
#if defined(arch_atomic64_read_acquire)
return arch_atomic64_read_acquire(v);
-#elif defined(arch_atomic64_read)
- return arch_atomic64_read(v);
#else
s64 ret;
@@ -2624,8 +2618,6 @@ raw_atomic64_set_release(atomic64_t *v, s64 i)
{
#if defined(arch_atomic64_set_release)
arch_atomic64_set_release(v, i);
-#elif defined(arch_atomic64_set)
- arch_atomic64_set(v, i);
#else
if (__native_word(atomic64_t)) {
smp_store_release(&(v)->counter, i);
@@ -4657,4 +4649,4 @@ raw_atomic64_dec_if_positive(atomic64_t *v)
}
#endif /* _LINUX_ATOMIC_FALLBACK_H */
-// 202b45c7db600ce36198eb1f1fc2c2d5268ace2d
+// 2fdd6702823fa842f9cea57a002e6e4476ae780c
diff --git a/scripts/atomic/gen-atomic-fallback.sh b/scripts/atomic/gen-atomic-fallback.sh
index c0c8a85d7c81b..a45154cefa487 100755
--- a/scripts/atomic/gen-atomic-fallback.sh
+++ b/scripts/atomic/gen-atomic-fallback.sh
@@ -102,7 +102,7 @@ gen_proto_order_variant()
fi
# Allow ACQUIRE/RELEASE/RELAXED ops to be defined in terms of FULL ops
- if [ ! -z "${order}" ]; then
+ if [ ! -z "${order}" ] && ! meta_is_implicitly_relaxed "${meta}"; then
printf "#elif defined(arch_${basename})\n"
printf "\t${retstmt}arch_${basename}(${args});\n"
fi
--
2.30.2
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