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Message-ID: <0816933d-7e58-0773-1441-891823983ff9@huawei.com>
Date: Wed, 26 Jul 2017 12:11:52 +0800
From: Li Kun <hw.likun@...wei.com>
To: Ard Biesheuvel <ard.biesheuvel@...aro.org>
CC: <linux-arm-kernel@...ts.infradead.org>,
<kernel-hardening@...ts.openwall.com>, <will.deacon@....com>,
<keescook@...omium.org>, <mark.rutland@....com>,
<labbott@...oraproject.org>
Subject: Re: [PATCH v2] arm64: kernel: implement fast refcount checking
Hi Ard，
on 2017/7/26 2:15, Ard Biesheuvel wrote:
> +#define REFCOUNT_OP(op, asm_op, cond, l, clobber...) \
> +__LL_SC_INLINE int \
> +__LL_SC_PREFIX(__refcount_##op(int i, atomic_t *r)) \
> +{ \
> + unsigned long tmp; \
> + int result; \
> + \
> + asm volatile("// refcount_" #op "\n" \
> +" prfm pstl1strm, %2\n" \
> +"1: ldxr %w0, %2\n" \
> +" " #asm_op " %w0, %w0, %w[i]\n" \
> +" st" #l "xr %w1, %w0, %2\n" \
> +" cbnz %w1, 1b\n" \
> + REFCOUNT_CHECK(cond) \
> + : "=&r" (result), "=&r" (tmp), "+Q" (r->counter) \
> + : REFCOUNT_INPUTS(r) [i] "Ir" (i) \
> + clobber); \
> + \
> + return result; \
> +} \
> +__LL_SC_EXPORT(__refcount_##op);
> +
> +REFCOUNT_OP(add_lt, adds, mi, , REFCOUNT_CLOBBERS);
> +REFCOUNT_OP(sub_lt_neg, adds, mi, l, REFCOUNT_CLOBBERS);
> +REFCOUNT_OP(sub_le_neg, adds, ls, l, REFCOUNT_CLOBBERS);
> +REFCOUNT_OP(sub_lt, subs, mi, l, REFCOUNT_CLOBBERS);
> +REFCOUNT_OP(sub_le, subs, ls, l, REFCOUNT_CLOBBERS);
> +
I'm not quite sure if we use b.lt to judge whether the result of adds is
less than zero is correct or not.
The b.lt means N!=V, take an extreme example, if we operate like below,
the b.lt will also be true.
refcount_set(&ref_c,0x80000000);
refcount_dec_and_test(&ref_c);
maybe we should use PL/NE/MI/EQ to judge the LT_ZERO or LE_ZERO condition ?
--
Best Regards
Li Kun
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