random: use lockless techniques in the interrupt path

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

1 files changed, 39 insertions, 39 deletions

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 9fcceace239c..315feb1f59f3 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();
         }