[v5] random: defer fast pool mixing to worker

Commit Message

Jason A. Donenfeld Feb. 11, 2022, 1:08 p.m. UTC

On PREEMPT_RT, it's problematic to take spinlocks from hard irq
handlers. We can fix this by deferring to a workqueue the dumping of
the fast pool into the input pool.

We accomplish this with some careful rules on fast_pool->count:

  - When it's incremented to >= 64, we schedule the work.
  - If the top bit is set, we never schedule the work, even if >= 64.
  - The worker is responsible for setting it back to 0 when it's done.

There are two small issues around using workqueues for this purpose that
we work around.

The first issue is that mix_interrupt_randomness() might be migrated to
another CPU during CPU hotplug. This issue is rectified by checking that
it hasn't been migrated (after disabling migration). If it has been
migrated, then we set the count to zero, so that when the CPU comes
online again, it can requeue the work. As part of this, we switch to
using an atomic_t, so that the increment in the irq handler doesn't wipe
out the zeroing if the CPU comes back online while this worker is
running.

The second issue is that, though relatively minor in effect, we probably
want to make sure we get a consistent view of the pool onto the stack,
in case it's interrupted by an irq while reading. To do this, we simply
read count before and after the memcpy and make sure they're the same.
If they're not, we try again. This isn't a seqlock or anything heavy
like that because we're guaranteed to be on the same core as the irq
handler interrupting, which means that interruption either happens in
full or doesn't at all. The likelihood of actually hitting this is very
low, as we're talking about a 2 or 4 word mov.

Cc: Sebastian Andrzej Siewior <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Theodore Ts'o <[e-mail]>
Cc: Sultan Alsawaf <[e-mail]>
Cc: Jonathan Neuschäfer <[e-mail]>
Reviewed-by: Dominik Brodowski <[e-mail]>
Signed-off-by: Jason A. Donenfeld <[e-mail]>
---
Sebastian - I merged the fixes into the base commit and now I'm
resending this. I also incorporated your nit about the acquire/release
comments. Let me know if you think this needs further changes.

 drivers/char/random.c | 72 ++++++++++++++++++++++++++++++++++---------
 1 file changed, 58 insertions(+), 14 deletions(-)

Comments

Sebastian Andrzej Siewior Feb. 11, 2022, 3 p.m. UTC | #1

On 2022-02-11 14:08:07 [+0100], Jason A. Donenfeld wrote:
…
> +static void mix_interrupt_randomness(struct work_struct *work)
> +{
> +	struct fast_pool *fast_pool = container_of(work, struct fast_pool, mix);
> +	unsigned long pool[ARRAY_SIZE(fast_pool->pool)];
> +	unsigned int count_snapshot;
> +	size_t i;
> +
> +	/* Check to see if we're running on the wrong CPU due to hotplug. */
> +	migrate_disable();
> +	if (fast_pool != this_cpu_ptr(&irq_randomness)) {
> +		migrate_enable();
> +		/*
> +		 * If we are unlucky enough to have been moved to another CPU,
> +		 * then we set our count to zero atomically so that when the
> +		 * CPU comes back online, it can enqueue work again. The
> +		 * _release here pairs with the atomic_inc_return_acquire in
> +		 * add_interrupt_randomness().
> +		 */
> +		atomic_set_release(&fast_pool->count, 0);
> +		return;
> +	}
> +
> +	/*
> +	 * Copy the pool to the stack so that the mixer always has a
> +	 * consistent view. It's extremely unlikely but possible that
> +	 * this 2 or 4 word read is interrupted by an irq, but in case
> +	 * it is, we double check that count stays the same.
> +	 */
> +	do {
> +		count_snapshot = (unsigned int)atomic_read(&fast_pool->count);
> +		for (i = 0; i < ARRAY_SIZE(pool); ++i)
> +			pool[i] = READ_ONCE(fast_pool->pool[i]);
> +	} while (count_snapshot != (unsigned int)atomic_read(&fast_pool->count));

Which what I wrote in the last mail, can't we just have a cmpxchg loop
here?

Sebastian

Patch

diff --git a/drivers/char/random.c b/drivers/char/random.c
index c42c07a7eb56..43c7e6c0a1b7 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1162,10 +1162,12 @@  EXPORT_SYMBOL_GPL(add_bootloader_randomness);
 
 struct fast_pool {
 	unsigned long pool[16 / sizeof(long)];
+	struct work_struct mix;
 	unsigned long last;
+	atomic_t count;
 	u16 reg_idx;
-	u8 count;
 };
+#define FAST_POOL_MIX_INFLIGHT (1U << 31)
 
 /*
  * This is a fast mixing routine used by the interrupt randomness
@@ -1214,12 +1216,57 @@  static u32 get_reg(struct fast_pool *f, struct pt_regs *regs)
 	return *ptr;
 }
 
+static void mix_interrupt_randomness(struct work_struct *work)
+{
+	struct fast_pool *fast_pool = container_of(work, struct fast_pool, mix);
+	unsigned long pool[ARRAY_SIZE(fast_pool->pool)];
+	unsigned int count_snapshot;
+	size_t i;
+
+	/* Check to see if we're running on the wrong CPU due to hotplug. */
+	migrate_disable();
+	if (fast_pool != this_cpu_ptr(&irq_randomness)) {
+		migrate_enable();
+		/*
+		 * If we are unlucky enough to have been moved to another CPU,
+		 * then we set our count to zero atomically so that when the
+		 * CPU comes back online, it can enqueue work again. The
+		 * _release here pairs with the atomic_inc_return_acquire in
+		 * add_interrupt_randomness().
+		 */
+		atomic_set_release(&fast_pool->count, 0);
+		return;
+	}
+
+	/*
+	 * Copy the pool to the stack so that the mixer always has a
+	 * consistent view. It's extremely unlikely but possible that
+	 * this 2 or 4 word read is interrupted by an irq, but in case
+	 * it is, we double check that count stays the same.
+	 */
+	do {
+		count_snapshot = (unsigned int)atomic_read(&fast_pool->count);
+		for (i = 0; i < ARRAY_SIZE(pool); ++i)
+			pool[i] = READ_ONCE(fast_pool->pool[i]);
+	} while (count_snapshot != (unsigned int)atomic_read(&fast_pool->count));
+
+	/* We take care to zero out the count only after we're done reading the pool. */
+	atomic_set(&fast_pool->count, 0);
+	fast_pool->last = jiffies;
+	migrate_enable();
+
+	mix_pool_bytes(pool, sizeof(pool));
+	credit_entropy_bits(1);
+	memzero_explicit(pool, sizeof(pool));
+}
+
 void add_interrupt_randomness(int irq)
 {
 	struct fast_pool *fast_pool = this_cpu_ptr(&irq_randomness);
 	struct pt_regs *regs = get_irq_regs();
 	unsigned long now = jiffies;
 	cycles_t cycles = random_get_entropy();
+	unsigned int new_count;
 
 	if (cycles == 0)
 		cycles = get_reg(fast_pool, regs);
@@ -1235,12 +1282,13 @@  void add_interrupt_randomness(int irq)
 	}
 
 	fast_mix((u32 *)fast_pool->pool);
-	++fast_pool->count;
+	/* The _acquire here pairs with the atomic_set_release in mix_interrupt_randomness(). */
+	new_count = (unsigned int)atomic_inc_return_acquire(&fast_pool->count);
 
 	if (unlikely(crng_init == 0)) {
-		if (fast_pool->count >= 64 &&
+		if (new_count >= 64 &&
 		    crng_fast_load(fast_pool->pool, sizeof(fast_pool->pool)) > 0) {
-			fast_pool->count = 0;
+			atomic_set(&fast_pool->count, 0);
 			fast_pool->last = now;
 
 			/*
@@ -1254,20 +1302,16 @@  void add_interrupt_randomness(int irq)
 		return;
 	}
 
-	if ((fast_pool->count < 64) && !time_after(now, fast_pool->last + HZ))
+	if (new_count & FAST_POOL_MIX_INFLIGHT)
 		return;
 
-	if (!spin_trylock(&input_pool.lock))
+	if (new_count < 64 && !time_after(now, fast_pool->last + HZ))
 		return;
 
-	fast_pool->last = now;
-	_mix_pool_bytes(&fast_pool->pool, sizeof(fast_pool->pool));
-	spin_unlock(&input_pool.lock);
-
-	fast_pool->count = 0;
-
-	/* Award one bit for the contents of the fast pool. */
-	credit_entropy_bits(1);
+	if (unlikely(!fast_pool->mix.func))
+		INIT_WORK(&fast_pool->mix, mix_interrupt_randomness);
+	atomic_or(FAST_POOL_MIX_INFLIGHT, &fast_pool->count);
+	queue_work_on(raw_smp_processor_id(), system_highpri_wq, &fast_pool->mix);
 }
 EXPORT_SYMBOL_GPL(add_interrupt_randomness);