random: always update the entropy pool under the spinlock

Instead of using lockless techniques introduced in commit
902c098a3663, use spin_trylock to try to grab entropy pool's lock.  If
we can't get the lock, then just try again on the next interrupt.

Based on discussions with George Spelvin.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: George Spelvin <linux@horizon.com>

1 files changed, 23 insertions, 21 deletions

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 0a7ac0a7b252..922a2e4089f9 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -495,9 +495,8 @@ static void _mix_pool_bytes(struct entropy_store *r, const void *in,
         tap4 = r->poolinfo->tap4;
         tap5 = r->poolinfo->tap5;
 
-        smp_rmb();
-        input_rotate = ACCESS_ONCE(r->input_rotate);
-        i = ACCESS_ONCE(r->add_ptr);
+        input_rotate = r->input_rotate;
+        i = r->add_ptr;
 
         /* mix one byte at a time to simplify size handling and churn faster */
         while (nbytes--) {
@@ -524,9 +523,8 @@ static void _mix_pool_bytes(struct entropy_store *r, const void *in,
                 input_rotate = (input_rotate + (i ? 7 : 14)) & 31;
         }
 
-        ACCESS_ONCE(r->input_rotate) = input_rotate;
-        ACCESS_ONCE(r->add_ptr) = i;
-        smp_wmb();
+        r->input_rotate = input_rotate;
+        r->add_ptr = i;
 
         if (out)
                 for (j = 0; j < 16; j++)
@@ -845,7 +843,7 @@ void add_interrupt_randomness(int irq, int irq_flags)
         __u32                   input[4], c_high, j_high;
         __u64                   ip;
         unsigned long           seed;
-        int                     credit;
+        int                     credit = 0;
 
         c_high = (sizeof(cycles) > 4) ? cycles >> 32 : 0;
         j_high = (sizeof(now) > 4) ? now >> 32 : 0;
@@ -860,36 +858,40 @@ void add_interrupt_randomness(int irq, int irq_flags)
         if ((fast_pool->count & 63) && !time_after(now, fast_pool->last + HZ))
                 return;
 
-        fast_pool->last = now;
-
         r = nonblocking_pool.initialized ? &input_pool : &nonblocking_pool;
+        if (!spin_trylock(&r->lock)) {
+                fast_pool->count--;
+                return;
+        }
+        fast_pool->last = now;
         __mix_pool_bytes(r, &fast_pool->pool, sizeof(fast_pool->pool), NULL);
 
         /*
+         * If we have architectural seed generator, produce a seed and
+         * add it to the pool.  For the sake of paranoia count it as
+         * 50% entropic.
+         */
+        if (arch_get_random_seed_long(&seed)) {
+                __mix_pool_bytes(r, &seed, sizeof(seed), NULL);
+                credit += sizeof(seed) * 4;
+        }
+        spin_unlock(&r->lock);
+
+        /*
          * If we don't have a valid cycle counter, and we see
          * back-to-back timer interrupts, then skip giving credit for
          * any entropy, otherwise credit 1 bit.
          */
-        credit = 1;
+        credit++;
         if (cycles == 0) {
                 if (irq_flags & __IRQF_TIMER) {
                         if (fast_pool->last_timer_intr)
-                                credit = 0;
+                                credit--;
                         fast_pool->last_timer_intr = 1;
                 } else
                         fast_pool->last_timer_intr = 0;
         }
 
-        /*
-         * If we have architectural seed generator, produce a seed and
-         * add it to the pool.  For the sake of paranoia count it as
-         * 50% entropic.
-         */
-        if (arch_get_random_seed_long(&seed)) {
-                __mix_pool_bytes(r, &seed, sizeof(seed), NULL);
-                credit += sizeof(seed) * 4;
-        }
-
         credit_entropy_bits(r, credit);
 }