Messages in this thread | | | Date | Thu, 26 Oct 2017 09:27:27 +1100 | From | "Tobin C. Harding" <> | Subject | Re: [PATCH v7] printk: hash addresses printed with %p |
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On Wed, Oct 25, 2017 at 06:00:21AM +0200, Jason A. Donenfeld wrote: > On Wed, Oct 25, 2017 at 5:49 AM, Tobin C. Harding <me@tobin.cc> wrote: > > static_branch_disable(&no_ptr_secret) : Doesn't sleep, just atomic read > > and set and maybe a WARN_ONCE. > > Are you sure about that? I just looked myself, and though there is a > !HAVE_JUMP_LABEL ifdef that does what you described, there's also a > HAVE_JUMP_LABEL that takes a mutex, which sleeps: > > static_branch_disable > static_key_disable > cpus_read_lock > percpu_down_read > percpu_down_read_preempt_disable > might_sleep > > > Now for the 'executes from process context' stuff. > > Er, sorry, I meant to write non-process context in my original > message, which is generally where you're worried about sleeping. > > > If the callback mechanism is utilized (i.e print before randomness is > > ready) then the call back will be executed the next time the randomness > > pool gets added to > > So it sounds to me like this might be called in non-process context. > Disaster. I realize the static_key thing was my idea in the original > email, so sorry for leading you astray. But moving to do this in > early_initcall wound up fixing other issues too, so all and all a net > good in going this direction. > > Two options: you stick with static_branch, because it's cool and speed > is fun, and work around all of the above with a call to queue_work so > that static_branch_enable is called only from process context.
This definitely sounds more fun, the static_branch stuff is dead sexy.
> Or, you give up on static_key, because it's not actually super > necessary, and instead just use an atomic, and reason that using `if > (unlikely(!atomic_read(&whatever)))` is probably good enough. In this > option, the code would be pretty much the same as v7, except you'd > s/static_branch/atomic_t/, and change the helpers, etc. This is > probably the more reasonable way.
How good is unlikely()?
It doesn't _feel_ right adding a check on every call to printk just to check for a condition that was only true for the briefest time when the kernel booted. But if unlikely() is good then I guess it doesn't hurt.
I'm leaning towards the option 1, but then all those text books I read are telling me to implement the simplest solution first then if we need to go faster implement the more complex solution.
This is a pretty airy fairy discussion now, but if you have an opinion I'd love to hear it.
thanks, Tobin.
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