[PATCH 03/12] random: use lockless techniques in the interrupt path

Theodore Ts'o
2012-07-06T22:46:45Z

The real-time Linux folks don't like add_interrupt_randomness() taking
a spinlock since it is called in the low-level interrupt routine.
This also allows us to reduce the overhead in the fast path, for the
random driver, which is the interrupt collection path.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@vger.kernel.org
---
drivers/char/random.c | 78 +++++++++++++++++++++++++--------------------------
1 file changed, 39 insertions(+), 39 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 556088b..09a11d8 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -418,9 +418,9 @@ struct entropy_store {
/* read-write data: */
spinlock_t lock;
unsigned add_ptr;
+ unsigned input_rotate;
int entropy_count;
int entropy_total;
- int input_rotate;
unsigned int initialized:1;
__u8 last_data[EXTRACT_SIZE];
};
@@ -468,26 +468,24 @@ static __u32 const twist_table[8] = {
* it's cheap to do so and helps slightly in the expected case where
* the entropy is concentrated in the low-order bits.
*/
-static void mix_pool_bytes_extract(struct entropy_store *r, const void *in,
- int nbytes, __u8 out[64])
+static void __mix_pool_bytes(struct entropy_store *r, const void *in,
+ int nbytes, __u8 out[64])
{
unsigned long i, j, tap1, tap2, tap3, tap4, tap5;
int input_rotate;
int wordmask = r->poolinfo->poolwords - 1;
const char *bytes = in;
__u32 w;
- unsigned long flags;

- /* Taps are constant, so we can load them without holding r->lock. */
tap1 = r->poolinfo->tap1;
tap2 = r->poolinfo->tap2;
tap3 = r->poolinfo->tap3;
tap4 = r->poolinfo->tap4;
tap5 = r->poolinfo->tap5;

- spin_lock_irqsave(&r->lock, flags);
- input_rotate = r->input_rotate;
- i = r->add_ptr;
+ smp_rmb();
+ input_rotate = ACCESS_ONCE(r->input_rotate);
+ i = ACCESS_ONCE(r->add_ptr);

/* mix one byte at a time to simplify size handling and churn faster */
while (nbytes--) {
@@ -514,19 +512,23 @@ static void mix_pool_bytes_extract(struct entropy_store *r, const void *in,
input_rotate += i ? 7 : 14;
}

- r->input_rotate = input_rotate;
- r->add_ptr = i;
+ ACCESS_ONCE(r->input_rotate) = input_rotate;
+ ACCESS_ONCE(r->add_ptr) = i;
+ smp_wmb();

if (out)
for (j = 0; j < 16; j++)
((__u32 *)out)[j] = r->pool[(i - j) & wordmask];
-
- spin_unlock_irqrestore(&r->lock, flags);
}

-static void mix_pool_bytes(struct entropy_store *r, const void *in, int bytes)
+static void mix_pool_bytes(struct entropy_store *r, const void *in,
+ int nbytes, __u8 out[64])
{
- mix_pool_bytes_extract(r, in, bytes, NULL);
+ unsigned long flags;
+
+ spin_lock_irqsave(&r->lock, flags);
+ __mix_pool_bytes(r, in, nbytes, out);
+ spin_unlock_irqrestore(&r->lock, flags);
}

struct fast_pool {
@@ -564,23 +566,22 @@ static void fast_mix(struct fast_pool *f, const void *in, int nbytes)
*/
static void credit_entropy_bits(struct entropy_store *r, int nbits)
{
- unsigned long flags;
- int entropy_count;
+ int entropy_count, orig;

if (!nbits)
return;

- spin_lock_irqsave(&r->lock, flags);
-
DEBUG_ENT("added %d entropy credits to %s\n", nbits, r->name);
- entropy_count = r->entropy_count;
+retry:
+ entropy_count = orig = ACCESS_ONCE(r->entropy_count);
entropy_count += nbits;
if (entropy_count < 0) {
DEBUG_ENT("negative entropy/overflow\n");
entropy_count = 0;
} else if (entropy_count > r->poolinfo->POOLBITS)
entropy_count = r->poolinfo->POOLBITS;
- r->entropy_count = entropy_count;
+ if (cmpxchg(&r->entropy_count, orig, entropy_count) != orig)
+ goto retry;

if (!r->initialized && nbits > 0) {
r->entropy_total += nbits;
@@ -593,7 +594,6 @@ static void credit_entropy_bits(struct entropy_store *r, int nbits)
wake_up_interruptible(&random_read_wait);
kill_fasync(&fasync, SIGIO, POLL_IN);
}
- spin_unlock_irqrestore(&r->lock, flags);
}

/*********************************************************************
@@ -680,7 +680,7 @@ static void add_timer_randomness(struct timer_rand_state *state, unsigned num)
sample.cycles = get_cycles();

sample.num = num;
- mix_pool_bytes(&input_pool, &sample, sizeof(sample));
+ mix_pool_bytes(&input_pool, &sample, sizeof(sample), NULL);

/*
* Calculate number of bits of randomness we probably added.
@@ -764,7 +764,7 @@ void add_interrupt_randomness(int irq, int irq_flags)
fast_pool->last = now;

r = nonblocking_pool.initialized ? &input_pool : &nonblocking_pool;
- mix_pool_bytes(r, &fast_pool->pool, sizeof(fast_pool->pool));
+ __mix_pool_bytes(r, &fast_pool->pool, sizeof(fast_pool->pool), NULL);
/*
* If we don't have a valid cycle counter, and we see
* back-to-back timer interrupts, then skip giving credit for
@@ -829,7 +829,7 @@ static void xfer_secondary_pool(struct entropy_store *r, size_t nbytes)

bytes = extract_entropy(r->pull, tmp, bytes,
random_read_wakeup_thresh / 8, rsvd);
- mix_pool_bytes(r, tmp, bytes);
+ mix_pool_bytes(r, tmp, bytes, NULL);
credit_entropy_bits(r, bytes*8);
}
}
@@ -890,9 +890,11 @@ static void extract_buf(struct entropy_store *r, __u8 *out)
int i;
__u32 hash[5], workspace[SHA_WORKSPACE_WORDS];
__u8 extract[64];
+ unsigned long flags;

/* Generate a hash across the pool, 16 words (512 bits) at a time */
sha_init(hash);
+ spin_lock_irqsave(&r->lock, flags);
for (i = 0; i < r->poolinfo->poolwords; i += 16)
sha_transform(hash, (__u8 *)(r->pool + i), workspace);

@@ -905,7 +907,8 @@ static void extract_buf(struct entropy_store *r, __u8 *out)
* brute-forcing the feedback as hard as brute-forcing the
* hash.
*/
- mix_pool_bytes_extract(r, hash, sizeof(hash), extract);
+ __mix_pool_bytes(r, hash, sizeof(hash), extract);
+ spin_unlock_irqrestore(&r->lock, flags);

/*
* To avoid duplicates, we atomically extract a portion of the
@@ -928,11 +931,10 @@ static void extract_buf(struct entropy_store *r, __u8 *out)
}

static ssize_t extract_entropy(struct entropy_store *r, void *buf,
- size_t nbytes, int min, int reserved)
+ size_t nbytes, int min, int reserved)
{
ssize_t ret = 0, i;
__u8 tmp[EXTRACT_SIZE];
- unsigned long flags;

xfer_secondary_pool(r, nbytes);
nbytes = account(r, nbytes, min, reserved);
@@ -941,6 +943,8 @@ static ssize_t extract_entropy(struct entropy_store *r, void *buf,
extract_buf(r, tmp);

if (fips_enabled) {
+ unsigned long flags;
+
spin_lock_irqsave(&r->lock, flags);
if (!memcmp(tmp, r->last_data, EXTRACT_SIZE))
panic("Hardware RNG duplicated output!\n");
@@ -1034,22 +1038,18 @@ EXPORT_SYMBOL(get_random_bytes);
static void init_std_data(struct entropy_store *r)
{
int i;
- ktime_t now;
- unsigned long flags;
+ ktime_t now = ktime_get_real();
+ unsigned long rv;

- spin_lock_irqsave(&r->lock, flags);
r->entropy_count = 0;
r->entropy_total = 0;
- spin_unlock_irqrestore(&r->lock, flags);
-
- now = ktime_get_real();
- mix_pool_bytes(r, &now, sizeof(now));
- for (i = r->poolinfo->POOLBYTES; i > 0; i -= sizeof flags) {
- if (!arch_get_random_long(&flags))
+ mix_pool_bytes(r, &now, sizeof(now), NULL);
+ for (i = r->poolinfo->POOLBYTES; i > 0; i -= sizeof(rv)) {
+ if (!arch_get_random_long(&rv))
break;
- mix_pool_bytes(r, &flags, sizeof(flags));
+ mix_pool_bytes(r, &rv, sizeof(rv), NULL);
}
- mix_pool_bytes(r, utsname(), sizeof(*(utsname())));
+ mix_pool_bytes(r, utsname(), sizeof(*(utsname())), NULL);
}

static int rand_initialize(void)
@@ -1186,7 +1186,7 @@ write_pool(struct entropy_store *r, const char __user *buffer, size_t count)
count -= bytes;
p += bytes;

- mix_pool_bytes(r, buf, bytes);
+ mix_pool_bytes(r, buf, bytes, NULL);
cond_resched();
}

--
1.7.11.1.108.gb129051