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git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
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0769857a07
As BPF doesn't include any barrier instructions, smp_mb() is implemented
by doing a dummy value returning atomic operation. Such an operation
acts a full barrier as enforced by LKMM and also by the work in progress
BPF memory model.
If the returned value is not used, clang[1] can optimize the value
returning atomic instruction in to a normal atomic instruction which
provides no ordering guarantees.
Mark the variable as volatile so the above optimization is never
performed and smp_mb() works as expected.
[1] https://godbolt.org/z/qzze7bG6z
Fixes: 88d706ba7c ("selftests/bpf: Introduce arena spin lock")
Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Link: https://lore.kernel.org/r/20250710175434.18829-2-puranjay@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
140 lines
4.9 KiB
C++
140 lines
4.9 KiB
C++
// SPDX-License-Identifier: GPL-2.0
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/* Copyright (c) 2025 Meta Platforms, Inc. and affiliates. */
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#ifndef BPF_ATOMIC_H
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#define BPF_ATOMIC_H
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#include <vmlinux.h>
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#include <bpf/bpf_helpers.h>
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#include "bpf_experimental.h"
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extern bool CONFIG_X86_64 __kconfig __weak;
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/*
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* __unqual_typeof(x) - Declare an unqualified scalar type, leaving
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* non-scalar types unchanged,
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*
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* Prefer C11 _Generic for better compile-times and simpler code. Note: 'char'
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* is not type-compatible with 'signed char', and we define a separate case.
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*
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* This is copied verbatim from kernel's include/linux/compiler_types.h, but
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* with default expression (for pointers) changed from (x) to (typeof(x)0).
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*
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* This is because LLVM has a bug where for lvalue (x), it does not get rid of
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* an extra address_space qualifier, but does in case of rvalue (typeof(x)0).
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* Hence, for pointers, we need to create an rvalue expression to get the
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* desired type. See https://github.com/llvm/llvm-project/issues/53400.
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*/
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#define __scalar_type_to_expr_cases(type) \
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unsigned type : (unsigned type)0, signed type : (signed type)0
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#define __unqual_typeof(x) \
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typeof(_Generic((x), \
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char: (char)0, \
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__scalar_type_to_expr_cases(char), \
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__scalar_type_to_expr_cases(short), \
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__scalar_type_to_expr_cases(int), \
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__scalar_type_to_expr_cases(long), \
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__scalar_type_to_expr_cases(long long), \
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default: (typeof(x))0))
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/* No-op for BPF */
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#define cpu_relax() ({})
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#define READ_ONCE(x) (*(volatile typeof(x) *)&(x))
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#define WRITE_ONCE(x, val) ((*(volatile typeof(x) *)&(x)) = (val))
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#define cmpxchg(p, old, new) __sync_val_compare_and_swap((p), old, new)
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#define try_cmpxchg(p, pold, new) \
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({ \
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__unqual_typeof(*(pold)) __o = *(pold); \
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__unqual_typeof(*(p)) __r = cmpxchg(p, __o, new); \
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if (__r != __o) \
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*(pold) = __r; \
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__r == __o; \
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})
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#define try_cmpxchg_relaxed(p, pold, new) try_cmpxchg(p, pold, new)
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#define try_cmpxchg_acquire(p, pold, new) try_cmpxchg(p, pold, new)
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#define smp_mb() \
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({ \
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volatile unsigned long __val; \
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__sync_fetch_and_add(&__val, 0); \
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})
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#define smp_rmb() \
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({ \
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if (!CONFIG_X86_64) \
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smp_mb(); \
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else \
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barrier(); \
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})
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#define smp_wmb() \
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({ \
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if (!CONFIG_X86_64) \
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smp_mb(); \
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else \
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barrier(); \
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})
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/* Control dependency provides LOAD->STORE, provide LOAD->LOAD */
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#define smp_acquire__after_ctrl_dep() ({ smp_rmb(); })
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#define smp_load_acquire(p) \
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({ \
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__unqual_typeof(*(p)) __v = READ_ONCE(*(p)); \
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if (!CONFIG_X86_64) \
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smp_mb(); \
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barrier(); \
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__v; \
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})
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#define smp_store_release(p, val) \
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({ \
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if (!CONFIG_X86_64) \
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smp_mb(); \
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barrier(); \
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WRITE_ONCE(*(p), val); \
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})
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#define smp_cond_load_relaxed_label(p, cond_expr, label) \
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({ \
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typeof(p) __ptr = (p); \
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__unqual_typeof(*(p)) VAL; \
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for (;;) { \
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VAL = (__unqual_typeof(*(p)))READ_ONCE(*__ptr); \
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if (cond_expr) \
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break; \
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cond_break_label(label); \
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cpu_relax(); \
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} \
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(typeof(*(p)))VAL; \
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})
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#define smp_cond_load_acquire_label(p, cond_expr, label) \
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({ \
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__unqual_typeof(*p) __val = \
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smp_cond_load_relaxed_label(p, cond_expr, label); \
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smp_acquire__after_ctrl_dep(); \
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(typeof(*(p)))__val; \
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})
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#define atomic_read(p) READ_ONCE((p)->counter)
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#define atomic_cond_read_relaxed_label(p, cond_expr, label) \
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smp_cond_load_relaxed_label(&(p)->counter, cond_expr, label)
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#define atomic_cond_read_acquire_label(p, cond_expr, label) \
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smp_cond_load_acquire_label(&(p)->counter, cond_expr, label)
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#define atomic_try_cmpxchg_relaxed(p, pold, new) \
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try_cmpxchg_relaxed(&(p)->counter, pold, new)
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#define atomic_try_cmpxchg_acquire(p, pold, new) \
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try_cmpxchg_acquire(&(p)->counter, pold, new)
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#endif /* BPF_ATOMIC_H */
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