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SubjectRe: [PATCH v3 05/10] lib/refcount: Improve performance of generic REFCOUNT_FULL code
On Mon, Oct 07, 2019 at 04:46:58PM +0100, Will Deacon wrote:
> Rewrite the generic REFCOUNT_FULL implementation so that the saturation
> point is moved to INT_MIN / 2. This allows us to defer the sanity checks
> until after the atomic operation, which removes many uses of cmpxchg()
> in favour of atomic_fetch_{add,sub}().

It also radicaly changes behaviour, and afaict is subtly broken, see
below.

> Some crude perf results obtained from lkdtm show substantially less
> overhead, despite the checking:
>
> $ perf stat -r 3 -B -- echo {ATOMIC,REFCOUNT}_TIMING >/sys/kernel/debug/provoke-crash/DIRECT
>
> # arm64
> ATOMIC_TIMING: 46.50451 +- 0.00134 seconds time elapsed ( +- 0.00% )
> REFCOUNT_TIMING (REFCOUNT_FULL, mainline): 77.57522 +- 0.00982 seconds time elapsed ( +- 0.01% )
> REFCOUNT_TIMING (REFCOUNT_FULL, this series): 48.7181 +- 0.0256 seconds time elapsed ( +- 0.05% )
>
> # x86
> ATOMIC_TIMING: 31.6225 +- 0.0776 seconds time elapsed ( +- 0.25% )
> REFCOUNT_TIMING (!REFCOUNT_FULL, mainline/x86 asm): 31.6689 +- 0.0901 seconds time elapsed ( +- 0.28% )
> REFCOUNT_TIMING (REFCOUNT_FULL, mainline): 53.203 +- 0.138 seconds time elapsed ( +- 0.26% )
> REFCOUNT_TIMING (REFCOUNT_FULL, this series): 31.7408 +- 0.0486 seconds time elapsed ( +- 0.15% )

I would _really_ like words on how this is racy and how it probably
doesn't matter.

> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Elena Reshetova <elena.reshetova@intel.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> Tested-by: Hanjun Guo <guohanjun@huawei.com>
> Tested-by: Jan Glauber <jglauber@marvell.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
> include/linux/refcount.h | 87 ++++++++++++++++------------------------
> 1 file changed, 34 insertions(+), 53 deletions(-)
>
> diff --git a/include/linux/refcount.h b/include/linux/refcount.h
> index e719b5b1220e..7f9aa6511142 100644
> +++ b/include/linux/refcount.h
> @@ -47,8 +47,8 @@ static inline unsigned int refcount_read(const refcount_t *r)
> #ifdef CONFIG_REFCOUNT_FULL
> #include <linux/bug.h>
>
> +#define REFCOUNT_MAX INT_MAX
> +#define REFCOUNT_SATURATED (INT_MIN / 2)
>
> /*
> * Variant of atomic_t specialized for reference counts.
> @@ -109,25 +109,19 @@ static inline unsigned int refcount_read(const refcount_t *r)
> */
> static inline __must_check bool refcount_add_not_zero(int i, refcount_t *r)
> {
> + int old = refcount_read(r);
>
> do {
> + if (!old)
> + break;
> + } while (!atomic_try_cmpxchg_relaxed(&r->refs, &old, old + i));
>
> + if (unlikely(old < 0 || old + i < 0)) {

So this is obviously racy against itself and other operations.
Particularly refcount_read(), as the sole API member that actually
exposes the value, is affected.

Yes, it shouldn't happen and we'll have trouble if we ever hit this, but
are all refcount_read() users sane enough to not cause further trouble?

> + refcount_set(r, REFCOUNT_SATURATED);
> + WARN_ONCE(1, "refcount_t: saturated; leaking memory.\n");
> + }
>
> + return old;
> }
>
> /**
> @@ -148,7 +142,13 @@ static inline __must_check bool refcount_add_not_zero(int i, refcount_t *r)
> */
> static inline void refcount_add(int i, refcount_t *r)
> {
> + int old = atomic_fetch_add_relaxed(i, &r->refs);
> +
> + WARN_ONCE(!old, "refcount_t: addition on 0; use-after-free.\n");

This is a change in behaviour vs the old one; the previous verion would
not change the value this will.

Is it important, I don't know, but it's not documented.

> + if (unlikely(old <= 0 || old + i <= 0)) {
> + refcount_set(r, REFCOUNT_SATURATED);
> + WARN_ONCE(old, "refcount_t: saturated; leaking memory.\n");
> + }
> }
>
> /**

> @@ -224,26 +208,19 @@ static inline void refcount_inc(refcount_t *r)
> */
> static inline __must_check bool refcount_sub_and_test(int i, refcount_t *r)
> {
> + int old = atomic_fetch_sub_release(i, &r->refs);
>
> + if (old == i) {
> smp_acquire__after_ctrl_dep();
> return true;
> }
>
> + if (unlikely(old - i < 0)) {
> + refcount_set(r, REFCOUNT_SATURATED);
> + WARN_ONCE(1, "refcount_t: underflow; use-after-free.\n");
> + }

I'm failing to see how this preserves REFCOUNT_SATURATED for
non-underflow. AFAICT this should have:

if (unlikely(old == REFCOUNT_SATURATED || old - i < 0))

> + return false;
> }
>
> /**
> @@ -276,9 +253,13 @@ static inline __must_check bool refcount_dec_and_test(refcount_t *r)
> */
> static inline void refcount_dec(refcount_t *r)
> {
> + int old = atomic_fetch_sub_release(1, &r->refs);
>
> + if (unlikely(old <= 1)) {

Idem.

> + refcount_set(r, REFCOUNT_SATURATED);
> + WARN_ONCE(1, "refcount_t: decrement hit 0; leaking memory.\n");
> + }
> +}

Also, things like refcount_dec_not_one() might need fixing to preserve
REFCOUNT_SATURATED, because they're not expecting that value to actually
change, but you do!

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