Messages in this thread Patch in this message | | | Date | Wed, 9 Oct 2019 11:25:08 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v3 05/10] lib/refcount: Improve performance of generic REFCOUNT_FULL code |
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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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