Messages in this thread | | | Date | Sat, 3 Apr 2021 15:51:48 +0100 | From | Sergei Trofimovich <> | Subject | Re: [PATCH v2 3/3] hpsa: add an assert to prevent from __packed reintroduction |
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On Fri, 2 Apr 2021 14:40:39 +0000 "Elliott, Robert (Servers)" <elliott@hpe.com> wrote:
> It looks like ia64 implements atomic_t as a 64-bit value and expects atomic_t > to be 64-bit aligned, but does nothing to ensure that. > > For x86, atomic_t is a 32-bit value and atomic64_t is a 64-bit value, and > the definition of atomic64_t is overridden in a way that ensures > 64-bit (8 byte) alignment: > > Generic definitions are in include/linux/types.h: > typedef struct { > int counter; > } atomic_t; > > #define ATOMIC_INIT(i) { (i) } > > #ifdef CONFIG_64BIT > typedef struct { > s64 counter; > } atomic64_t; > #endif > > Override in arch/x86/include/asm/atomic64_32.h: > typedef struct { > s64 __aligned(8) counter; > } atomic64_t; > > Perhaps ia64 needs to take over the definition of both atomic_t and atomic64_t > and do the same?
I don't think it's needed. ia64 is a 64-bit arch with expected natural alignment for s64: alignof(s64)=8.
Also if my understanding is correct adding __aligned(8) would not fix use case of embedding locks into packed structs even on x86_64 (or i386):
$ cat a.c #include <stdio.h> #include <stddef.h>
typedef struct { unsigned long long __attribute__((aligned(8))) l; } lock_t; struct s { char c; lock_t lock; } __attribute__((packed)); int main() { printf ("offsetof(struct s, lock) = %lu\nsizeof(struct s) = %lu\n", offsetof(struct s, lock), sizeof(struct s)); }
$ x86_64-pc-linux-gnu-gcc a.c -o a && ./a offsetof(struct s, lock) = 1 sizeof(struct s) = 9
$ x86_64-pc-linux-gnu-gcc a.c -o a -m32 && ./a offsetof(struct s, lock) = 1 sizeof(struct s) = 9
Note how alignment of 'lock' stays 1 byte in both cases.
8-byte alignment added for i386 in https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=bbf2a330d92c5afccfd17592ba9ccd50f41cf748 is only as a performance optimization (not a correctness fix).
Larger alignment on i386 is preferred because alignof(s64)=4 on that target which might make atomic op span cache lines that leads to performance degradation.
--
Sergei
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