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    Subject[PATCH 5.4 160/240] random: use static branch for crng_ready()
    Date
    From: "Jason A. Donenfeld" <Jason@zx2c4.com>

    commit f5bda35fba615ace70a656d4700423fa6c9bebee upstream.

    Since crng_ready() is only false briefly during initialization and then
    forever after becomes true, we don't need to evaluate it after, making
    it a prime candidate for a static branch.

    One complication, however, is that it changes state in a particular call
    to credit_init_bits(), which might be made from atomic context, which
    means we must kick off a workqueue to change the static key. Further
    complicating things, credit_init_bits() may be called sufficiently early
    on in system initialization such that system_wq is NULL.

    Fortunately, there exists the nice function execute_in_process_context(),
    which will immediately execute the function if !in_interrupt(), and
    otherwise defer it to a workqueue. During early init, before workqueues
    are available, in_interrupt() is always false, because interrupts
    haven't even been enabled yet, which means the function in that case
    executes immediately. Later on, after workqueues are available,
    in_interrupt() might be true, but in that case, the work is queued in
    system_wq and all goes well.

    Cc: Theodore Ts'o <tytso@mit.edu>
    Cc: Sultan Alsawaf <sultan@kerneltoast.com>
    Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
    Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    drivers/char/random.c | 16 ++++++++++++----
    1 file changed, 12 insertions(+), 4 deletions(-)

    --- a/drivers/char/random.c
    +++ b/drivers/char/random.c
    @@ -77,8 +77,9 @@ static enum {
    CRNG_EMPTY = 0, /* Little to no entropy collected */
    CRNG_EARLY = 1, /* At least POOL_EARLY_BITS collected */
    CRNG_READY = 2 /* Fully initialized with POOL_READY_BITS collected */
    -} crng_init = CRNG_EMPTY;
    -#define crng_ready() (likely(crng_init >= CRNG_READY))
    +} crng_init __read_mostly = CRNG_EMPTY;
    +static DEFINE_STATIC_KEY_FALSE(crng_is_ready);
    +#define crng_ready() (static_branch_likely(&crng_is_ready) || crng_init >= CRNG_READY)
    /* Various types of waiters for crng_init->CRNG_READY transition. */
    static DECLARE_WAIT_QUEUE_HEAD(crng_init_wait);
    static struct fasync_struct *fasync;
    @@ -108,6 +109,11 @@ bool rng_is_initialized(void)
    }
    EXPORT_SYMBOL(rng_is_initialized);

    +static void crng_set_ready(struct work_struct *work)
    +{
    + static_branch_enable(&crng_is_ready);
    +}
    +
    /* Used by wait_for_random_bytes(), and considered an entropy collector, below. */
    static void try_to_generate_entropy(void);

    @@ -267,7 +273,7 @@ static void crng_reseed(void)
    ++next_gen;
    WRITE_ONCE(base_crng.generation, next_gen);
    WRITE_ONCE(base_crng.birth, jiffies);
    - if (!crng_ready())
    + if (!static_branch_likely(&crng_is_ready))
    crng_init = CRNG_READY;
    spin_unlock_irqrestore(&base_crng.lock, flags);
    memzero_explicit(key, sizeof(key));
    @@ -782,6 +788,7 @@ static void extract_entropy(void *buf, s

    static void credit_init_bits(size_t nbits)
    {
    + static struct execute_work set_ready;
    unsigned int new, orig, add;
    unsigned long flags;

    @@ -797,6 +804,7 @@ static void credit_init_bits(size_t nbit

    if (orig < POOL_READY_BITS && new >= POOL_READY_BITS) {
    crng_reseed(); /* Sets crng_init to CRNG_READY under base_crng.lock. */
    + execute_in_process_context(crng_set_ready, &set_ready);
    process_random_ready_list();
    wake_up_interruptible(&crng_init_wait);
    kill_fasync(&fasync, SIGIO, POLL_IN);
    @@ -1306,7 +1314,7 @@ SYSCALL_DEFINE3(getrandom, char __user *
    if (count > INT_MAX)
    count = INT_MAX;

    - if (!(flags & GRND_INSECURE) && !crng_ready()) {
    + if (!crng_ready() && !(flags & GRND_INSECURE)) {
    int ret;

    if (flags & GRND_NONBLOCK)

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