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 | 58 +++++++++++++++++++++++--------------------------- 1 file changed, 27 insertions(+), 31 deletions(-)
--- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -1770,13 +1770,15 @@ struct ctl_table random_table[] = { }; #endif /* CONFIG_SYSCTL */
+static atomic_t batch_generation = ATOMIC_INIT(0); + struct batched_entropy { union { u64 entropy_u64[CHACHA20_BLOCK_SIZE / sizeof(u64)]; u32 entropy_u32[CHACHA20_BLOCK_SIZE / sizeof(u32)]; }; unsigned int position; - spinlock_t batch_lock; + int generation; };
/* @@ -1787,9 +1789,7 @@ struct batched_entropy { * wait_for_random_bytes() should be called and return 0 at least once at any * point prior. */ -static DEFINE_PER_CPU(struct batched_entropy, batched_entropy_u64) = { - .batch_lock = __SPIN_LOCK_UNLOCKED(batched_entropy_u64.lock), -}; +static DEFINE_PER_CPU(struct batched_entropy, batched_entropy_u64);
u64 get_random_u64(void) { @@ -1797,67 +1797,63 @@ u64 get_random_u64(void) unsigned long flags; struct batched_entropy *batch; static void *previous; + int next_gen;
warn_unseeded_randomness(&previous);
+ local_irq_save(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_irq_restore(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), -}; +static DEFINE_PER_CPU(struct batched_entropy, batched_entropy_u32); + 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_irq_save(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_irq_restore(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); }
/**