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locking/refcount: Improve performance of generic REFCOUNT_FULL code 

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}().

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% )

Signed-off-by: Will Deacon <will@kernel.org>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Tested-by: Hanjun Guo <guohanjun@huawei.com>
Tested-by: Jan Glauber <jglauber@marvell.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Elena Reshetova <elena.reshetova@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20191121115902.2551-6-will@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Will Deacon 2019-11-21 11:58:57 +00:00 committed by Ingo Molnar
parent 77e9971c79
commit dcb786493f
1 changed files with 75 additions and 56 deletions
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131
include/linux/refcount.h
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@ -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 (UINT_MAX - 1)
#define REFCOUNT_SATURATED UINT_MAX
#define REFCOUNT_MAX INT_MAX
#define REFCOUNT_SATURATED (INT_MIN / 2)
/*
* Variant of atomic_t specialized for reference counts.
@ -56,9 +56,47 @@ static inline unsigned int refcount_read(const refcount_t *r)
* The interface matches the atomic_t interface (to aid in porting) but only
* provides the few functions one should use for reference counting.
*
* It differs in that the counter saturates at REFCOUNT_SATURATED and will not
* move once there. This avoids wrapping the counter and causing 'spurious'
* use-after-free issues.
* Saturation semantics
* ====================
*
* refcount_t differs from atomic_t in that the counter saturates at
* REFCOUNT_SATURATED and will not move once there. This avoids wrapping the
* counter and causing 'spurious' use-after-free issues. In order to avoid the
* cost associated with introducing cmpxchg() loops into all of the saturating
* operations, we temporarily allow the counter to take on an unchecked value
* and then explicitly set it to REFCOUNT_SATURATED on detecting that underflow
* or overflow has occurred. Although this is racy when multiple threads
* access the refcount concurrently, by placing REFCOUNT_SATURATED roughly
* equidistant from 0 and INT_MAX we minimise the scope for error:
*
* INT_MAX REFCOUNT_SATURATED UINT_MAX
* 0 (0x7fff_ffff) (0xc000_0000) (0xffff_ffff)
* +--------------------------------+----------------+----------------+
* <---------- bad value! ---------->
*
* (in a signed view of the world, the "bad value" range corresponds to
* a negative counter value).
*
* As an example, consider a refcount_inc() operation that causes the counter
* to overflow:
*
* int old = atomic_fetch_add_relaxed(r);
* // old is INT_MAX, refcount now INT_MIN (0x8000_0000)
* if (old < 0)
* atomic_set(r, REFCOUNT_SATURATED);
*
* If another thread also performs a refcount_inc() operation between the two
* atomic operations, then the count will continue to edge closer to 0. If it
* reaches a value of 1 before /any/ of the threads reset it to the saturated
* value, then a concurrent refcount_dec_and_test() may erroneously free the
* underlying object. Given the precise timing details involved with the
* round-robin scheduling of each thread manipulating the refcount and the need
* to hit the race multiple times in succession, there doesn't appear to be a
* practical avenue of attack even if using refcount_add() operations with
* larger increments.
*
* Memory ordering
* ===============
*
* Memory ordering rules are slightly relaxed wrt regular atomic_t functions
* and provide only what is strictly required for refcounts.
@ -109,25 +147,19 @@ static inline unsigned int refcount_read(const refcount_t *r)
*/
static inline __must_check bool refcount_add_not_zero(int i, refcount_t *r)
{
unsigned int new, val = atomic_read(&r->refs);
int old = refcount_read(r);
do {
if (!val)
return false;
if (!old)
break;
} while (!atomic_try_cmpxchg_relaxed(&r->refs, &old, old + i));
if (unlikely(val == REFCOUNT_SATURATED))
return true;
if (unlikely(old < 0 || old + i < 0)) {
refcount_set(r, REFCOUNT_SATURATED);
WARN_ONCE(1, "refcount_t: saturated; leaking memory.\n");
}
new = val + i;
if (new < val)
new = REFCOUNT_SATURATED;
} while (!atomic_try_cmpxchg_relaxed(&r->refs, &val, new));
WARN_ONCE(new == REFCOUNT_SATURATED,
"refcount_t: saturated; leaking memory.\n");
return true;
return old;
}
/**
@ -148,7 +180,13 @@ static inline __must_check bool refcount_add_not_zero(int i, refcount_t *r)
*/
static inline void refcount_add(int i, refcount_t *r)
{
WARN_ONCE(!refcount_add_not_zero(i, r), "refcount_t: addition on 0; use-after-free.\n");
int old = atomic_fetch_add_relaxed(i, &r->refs);
WARN_ONCE(!old, "refcount_t: addition on 0; use-after-free.\n");
if (unlikely(old <= 0 || old + i <= 0)) {
refcount_set(r, REFCOUNT_SATURATED);
WARN_ONCE(old, "refcount_t: saturated; leaking memory.\n");
}
}
/**
@ -166,23 +204,7 @@ static inline void refcount_add(int i, refcount_t *r)
*/
static inline __must_check bool refcount_inc_not_zero(refcount_t *r)
{
unsigned int new, val = atomic_read(&r->refs);
do {
new = val + 1;
if (!val)
return false;
if (unlikely(!new))
return true;
} while (!atomic_try_cmpxchg_relaxed(&r->refs, &val, new));
WARN_ONCE(new == REFCOUNT_SATURATED,
"refcount_t: saturated; leaking memory.\n");
return true;
return refcount_add_not_zero(1, r);
}
/**
@ -199,7 +221,7 @@ static inline __must_check bool refcount_inc_not_zero(refcount_t *r)
*/
static inline void refcount_inc(refcount_t *r)
{
WARN_ONCE(!refcount_inc_not_zero(r), "refcount_t: increment on 0; use-after-free.\n");
refcount_add(1, r);
}
/**
@ -224,26 +246,19 @@ static inline void refcount_inc(refcount_t *r)
*/
static inline __must_check bool refcount_sub_and_test(int i, refcount_t *r)
{
unsigned int new, val = atomic_read(&r->refs);
int old = atomic_fetch_sub_release(i, &r->refs);
do {
if (unlikely(val == REFCOUNT_SATURATED))
return false;
new = val - i;
if (new > val) {
WARN_ONCE(new > val, "refcount_t: underflow; use-after-free.\n");
return false;
}
} while (!atomic_try_cmpxchg_release(&r->refs, &val, new));
if (!new) {
if (old == i) {
smp_acquire__after_ctrl_dep();
return true;
}
return false;
if (unlikely(old < 0 || old - i < 0)) {
refcount_set(r, REFCOUNT_SATURATED);
WARN_ONCE(1, "refcount_t: underflow; use-after-free.\n");
}
return false;
}
/**
@ -276,9 +291,13 @@ static inline __must_check bool refcount_dec_and_test(refcount_t *r)
*/
static inline void refcount_dec(refcount_t *r)
{
WARN_ONCE(refcount_dec_and_test(r), "refcount_t: decrement hit 0; leaking memory.\n");
}
int old = atomic_fetch_sub_release(1, &r->refs);
if (unlikely(old <= 1)) {
refcount_set(r, REFCOUNT_SATURATED);
WARN_ONCE(1, "refcount_t: decrement hit 0; leaking memory.\n");
}
}
#else /* CONFIG_REFCOUNT_FULL */
#define REFCOUNT_MAX INT_MAX