github-mirror/linux
1
0
Fork 0
mirror of https://github.com/torvalds/linux.git synced 2026-05-27 00:22:00 +02:00
Code Issues Actions Packages Projects Releases Wiki Activity Graph

bpf: arm64: Optimize recursion detection by not using atomics

Browse Source 
BPF programs detect recursion using a per-CPU 'active' flag in struct
bpf_prog. The trampoline currently sets/clears this flag with atomic
operations.

On some arm64 platforms (e.g., Neoverse V2 with LSE), per-CPU atomic
operations are relatively slow. Unlike x86_64 - where per-CPU updates
can avoid cross-core atomicity, arm64 LSE atomics are always atomic
across all cores, which is unnecessary overhead for strictly per-CPU
state.

This patch removes atomics from the recursion detection path on arm64 by
changing 'active' to a per-CPU array of four u8 counters, one per
context: {NMI, hard-irq, soft-irq, normal}. The running context uses a
non-atomic increment/decrement on its element.  After increment,
recursion is detected by reading the array as a u32 and verifying that
only the expected element changed; any change in another element
indicates inter-context recursion, and a value > 1 in the same element
indicates same-context recursion.

For example, starting from {0,0,0,0}, a normal-context trigger changes
the array to {0,0,0,1}.  If an NMI arrives on the same CPU and triggers
the program, the array becomes {1,0,0,1}. When the NMI context checks
the u32 against the expected mask for normal (0x00000001), it observes
0x01000001 and correctly reports recursion. Same-context recursion is
detected analogously.

Acked-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Link: https://lore.kernel.org/r/20251219184422.2899902-3-puranjay@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Puranjay Mohan 2025-12-19 10:44:18 -08:00 committed by Alexei Starovoitov
parent 93f0d09697
commit c3e34f88f9
2 changed files with 31 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

32
include/linux/bpf.h
View File

@ -1746,6 +1746,8 @@ struct bpf_prog_aux {
struct bpf_map __rcu *st_ops_assoc;
};
#define BPF_NR_CONTEXTS 4 /* normal, softirq, hardirq, NMI */
struct bpf_prog {
u16 pages; /* Number of allocated pages */
u16 jited:1, /* Is our filter JIT'ed? */
@ -1772,7 +1774,7 @@ struct bpf_prog {
u8 tag[BPF_TAG_SIZE];
};
struct bpf_prog_stats __percpu *stats;
int __percpu *active;
u8 __percpu *active; /* u8[BPF_NR_CONTEXTS] for recursion protection */
unsigned int (*bpf_func)(const void *ctx,
const struct bpf_insn *insn);
struct bpf_prog_aux *aux; /* Auxiliary fields */
@ -2006,12 +2008,36 @@ struct bpf_struct_ops_common_value {
static inline bool bpf_prog_get_recursion_context(struct bpf_prog *prog)
{
return this_cpu_inc_return(*(prog->active)) == 1;
#ifdef CONFIG_ARM64
u8 rctx = interrupt_context_level();
u8 *active = this_cpu_ptr(prog->active);
u32 val;
preempt_disable();
active[rctx]++;
val = le32_to_cpu(*(__le32 *)active);
preempt_enable();
if (val != BIT(rctx * 8))
return false;
return true;
#else
return this_cpu_inc_return(*(int __percpu *)(prog->active)) == 1;
#endif
}
static inline void bpf_prog_put_recursion_context(struct bpf_prog *prog)
{
this_cpu_dec(*(prog->active));
#ifdef CONFIG_ARM64
u8 rctx = interrupt_context_level();
u8 *active = this_cpu_ptr(prog->active);
preempt_disable();
active[rctx]--;
preempt_enable();
#else
this_cpu_dec(*(int __percpu *)(prog->active));
#endif
}
#if defined(CONFIG_BPF_JIT) && defined(CONFIG_BPF_SYSCALL)

3
kernel/bpf/core.c
View File

@ -112,7 +112,8 @@ struct bpf_prog *bpf_prog_alloc_no_stats(unsigned int size, gfp_t gfp_extra_flag
vfree(fp);
return NULL;
}
fp->active = alloc_percpu_gfp(int, bpf_memcg_flags(GFP_KERNEL | gfp_extra_flags));
fp->active = __alloc_percpu_gfp(sizeof(u8[BPF_NR_CONTEXTS]), 4,
bpf_memcg_flags(GFP_KERNEL | gfp_extra_flags));
if (!fp->active) {
vfree(fp);
kfree(aux);