random: use static branch for crng_ready()
authorJason A. Donenfeld <Jason@zx2c4.com>
Tue, 3 May 2022 13:30:45 +0000 (15:30 +0200)
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Sat, 25 Jun 2022 09:46:40 +0000 (11:46 +0200)
commitb0eabe217548a1526ef5a43364bea4073a12db1a
tree271f82b0d3412d352f4b0ba16a74bd9161212b40
parent37fe03f2708ed67eb642613860b2b4db6d0e08b7
random: use static branch for crng_ready()

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