Re: [PATCH] locking/atomic: scripts: fix fallback ifdeffery

From: Peter Zijlstra
Date: Tue Sep 19 2023 - 15:00:28 EST

On Tue, Sep 19, 2023 at 06:14:29PM +0100, Mark Rutland wrote:
> 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@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@xxxxxxxxxx>
> Link: https://lore.kernel.org/all/ZOWFtqA2om0w5Vmz@fedora/
> Reported-by: Darrick J. Wong <djwong@xxxxxxxxxx>
> Link: https://lore.kernel.org/linux-fsdevel/20230912173026.GA3389127@frogsfrogsfrogs/
> Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Baokun Li <libaokun1@xxxxxxxxxx>
> Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> Cc: Darrick J. Wong <djwong@xxxxxxxxxx>
> Cc: Dave Chinner <david@xxxxxxxxxxxxx>
> Cc: Kent Overstreet <kent.overstreet@xxxxxxxxx>
> Cc: Ming Lei <ming.lei@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Yi Zhang <yi.zhang@xxxxxxxxxx>
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> ---
> 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.

Yes, I'll go queue it in tip/locking/urgent. Thanks!