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

[Darrick J. Wong]

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.

I'll send this out to the arm64 fstestscloud fleet shortly and report
back when it's done.  Thank you for the quick response. :)

--D

> 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
>