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Subject[PATCH 5.17 009/111] random: remove batched entropy locking
Date
From: "Jason A. Donenfeld" <Jason@zx2c4.com>

commit 77760fd7f7ae3dfd03668204e708d1568d75447d upstream.

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.

Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
Reviewed-by: Eric Biggers <ebiggers@google.com>
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/
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/char/random.c | 55 +++++++++++++++++++++++++-------------------------
1 file changed, 28 insertions(+), 27 deletions(-)

--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1731,13 +1731,16 @@ static int __init random_sysctls_init(vo
device_initcall(random_sysctls_init);
#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;
};

/*
@@ -1749,7 +1752,7 @@ 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)
@@ -1758,67 +1761,65 @@ u64 get_random_u64(void)
unsigned long flags;
struct batched_entropy *batch;
static void *previous;
+ int next_gen;

warn_unseeded_randomness(&previous);

+ local_lock_irqsave(&batched_entropy_u64.lock, flags);
batch = raw_cpu_ptr(&batched_entropy_u64);
- spin_lock_irqsave(&batch->batch_lock, flags);
- if (batch->position % ARRAY_SIZE(batch->entropy_u64) == 0) {
+
+ 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(&batched_entropy_u64.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);

+ local_lock_irqsave(&batched_entropy_u32.lock, flags);
batch = raw_cpu_ptr(&batched_entropy_u32);
- spin_lock_irqsave(&batch->batch_lock, flags);
- if (batch->position % ARRAY_SIZE(batch->entropy_u32) == 0) {
+
+ 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(&batched_entropy_u32.lock, flags);
return ret;
}
EXPORT_SYMBOL(get_random_u32);

/* It's important to invalidate all potential batched entropy that might
* be stored before the crng is initialized, which we can do lazily by
- * simply resetting the counter to zero so that it's re-extracted on the
- * next usage. */
+ * bumping the generation counter.
+ */
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);
}

/**

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