[PATCH v3] random: remove batched entropy locking

["Jason A. Donenfeld" <Jason@zx2c4.com>]

Rather than use spinlocks to protect batched entropy, we can instead
disable interrupts locally, since we're dealing with per-cpu data, and
manage resets with a basic generation counter. At the same time, we
can't quite do this on PREEMPT_RT, where we still want spinlocks-as-
mutexes semantics. So we use a local_lock_t, which provides the right
behavior for each. Because this is a per-cpu lock, that generation
counter is still doing the necessary CPU-to-CPU communication.

This should improve performance a bit. It will also fix the linked splat
that Jonathan received with a PROVE_RAW_LOCK_NESTING=y.

Note that Sebastian has pointed out a few other areas where
using spinlock_t in an IRQ context is potentially problematic for
PREEMPT_RT. This patch handles one of those cases, and we'll hopefully
have additional patches for other cases.

Suggested-by: Andy Lutomirski <luto@kernel.org>
Reported-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Tested-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/lkml/YfMa0QgsjCVdRAvJ@latitude/
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 drivers/char/random.c | 55 ++++++++++++++++++++++---------------------
 1 file changed, 28 insertions(+), 27 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 455615ac169a..3e54b90a3ff8 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1732,13 +1732,16 @@ struct ctl_table random_table[] = {
 };
 #endif	/* CONFIG_SYSCTL */
 
+static atomic_t batch_generation = ATOMIC_INIT(0);
+
 struct batched_entropy {
 	union {
 		u64 entropy_u64[CHACHA_BLOCK_SIZE / sizeof(u64)];
 		u32 entropy_u32[CHACHA_BLOCK_SIZE / sizeof(u32)];
 	};
+	local_lock_t lock;
 	unsigned int position;
-	spinlock_t batch_lock;
+	int generation;
 };
 
 /*
@@ -1750,7 +1753,7 @@ struct batched_entropy {
  * point prior.
  */
 static DEFINE_PER_CPU(struct batched_entropy, batched_entropy_u64) = {
-	.batch_lock = __SPIN_LOCK_UNLOCKED(batched_entropy_u64.lock),
+	.lock = INIT_LOCAL_LOCK(batched_entropy_u64.lock)
 };
 
 u64 get_random_u64(void)
@@ -1759,41 +1762,54 @@ u64 get_random_u64(void)
 	unsigned long flags;
 	struct batched_entropy *batch;
 	static void *previous;
+	int next_gen;
 
 	warn_unseeded_randomness(&previous);
 
-	batch = raw_cpu_ptr(&batched_entropy_u64);
-	spin_lock_irqsave(&batch->batch_lock, flags);
-	if (batch->position % ARRAY_SIZE(batch->entropy_u64) == 0) {
+	batch = this_cpu_ptr(&batched_entropy_u64);
+	local_lock_irqsave(&batch->lock, flags);
+
+	next_gen = atomic_read(&batch_generation);
+	if (batch->position % ARRAY_SIZE(batch->entropy_u64) == 0 ||
+	    next_gen != batch->generation) {
 		extract_crng((u8 *)batch->entropy_u64);
 		batch->position = 0;
+		batch->generation = next_gen;
 	}
+
 	ret = batch->entropy_u64[batch->position++];
-	spin_unlock_irqrestore(&batch->batch_lock, flags);
+	local_unlock_irqrestore(&batch->lock, flags);
 	return ret;
 }
 EXPORT_SYMBOL(get_random_u64);
 
 static DEFINE_PER_CPU(struct batched_entropy, batched_entropy_u32) = {
-	.batch_lock = __SPIN_LOCK_UNLOCKED(batched_entropy_u32.lock),
+	.lock = INIT_LOCAL_LOCK(batched_entropy_u32.lock)
 };
+
 u32 get_random_u32(void)
 {
 	u32 ret;
 	unsigned long flags;
 	struct batched_entropy *batch;
 	static void *previous;
+	int next_gen;
 
 	warn_unseeded_randomness(&previous);
 
-	batch = raw_cpu_ptr(&batched_entropy_u32);
-	spin_lock_irqsave(&batch->batch_lock, flags);
-	if (batch->position % ARRAY_SIZE(batch->entropy_u32) == 0) {
+	batch = this_cpu_ptr(&batched_entropy_u32);
+	local_lock_irqsave(&batch->lock, flags);
+
+	next_gen = atomic_read(&batch_generation);
+	if (batch->position % ARRAY_SIZE(batch->entropy_u32) == 0 ||
+	    next_gen != batch->generation) {
 		extract_crng((u8 *)batch->entropy_u32);
 		batch->position = 0;
+		batch->generation = next_gen;
 	}
+
 	ret = batch->entropy_u32[batch->position++];
-	spin_unlock_irqrestore(&batch->batch_lock, flags);
+	local_unlock_irqrestore(&batch->lock, flags);
 	return ret;
 }
 EXPORT_SYMBOL(get_random_u32);
@@ -1804,22 +1820,7 @@ EXPORT_SYMBOL(get_random_u32);
  * next usage. */
 static void invalidate_batched_entropy(void)
 {
-	int cpu;
-	unsigned long flags;
-
-	for_each_possible_cpu(cpu) {
-		struct batched_entropy *batched_entropy;
-
-		batched_entropy = per_cpu_ptr(&batched_entropy_u32, cpu);
-		spin_lock_irqsave(&batched_entropy->batch_lock, flags);
-		batched_entropy->position = 0;
-		spin_unlock(&batched_entropy->batch_lock);
-
-		batched_entropy = per_cpu_ptr(&batched_entropy_u64, cpu);
-		spin_lock(&batched_entropy->batch_lock);
-		batched_entropy->position = 0;
-		spin_unlock_irqrestore(&batched_entropy->batch_lock, flags);
-	}
+	atomic_inc(&batch_generation);
 }
 
 /**
-- 
2.35.0