random: avoid reading two cache lines on irq randomness

commit 9ee0507e896b45af6d65408c77815800bce30008 upstream.

In order to avoid reading and dirtying two cache lines on every IRQ,
move the work_struct to the bottom of the fast_pool struct. add_
interrupt_randomness() always touches .pool and .count, which are
currently split, because .mix pushes everything down. Instead, move .mix
to the bottom, so that .pool and .count are always in the first cache
line, since .mix is only accessed when the pool is full.

Fixes: 58340f8e952b ("random: defer fast pool mixing to worker")
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/char/random.c b/drivers/char/random.c
index c594d9e92fc934a2ddf3309577d50bb288e4baef..144ae87bc33377443fb92ca30e36b9134ff0eed2 100644 (file)
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -903,10 +903,10 @@ EXPORT_SYMBOL_GPL(unregister_random_vmfork_notifier);
 #endif
 
 struct fast_pool {
-       struct work_struct mix;
        unsigned long pool[4];
        unsigned long last;
        unsigned int count;
+       struct work_struct mix;
 };
 
 static DEFINE_PER_CPU(struct fast_pool, irq_randomness) = {