Messages in this thread | | | Date | Mon, 1 Aug 2022 15:44:12 +0200 | From | "Jason A. Donenfeld" <> | Subject | Re: [PATCH v4] lib/vsprintf: defer filling siphash key on RT |
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Hey again,
On Mon, Aug 01, 2022 at 03:36:32PM +0200, Jason A. Donenfeld wrote: > Hi Sebastian, > > On Mon, Aug 01, 2022 at 02:46:35PM +0200, Sebastian Andrzej Siewior wrote: > > On 2022-08-01 14:39:46 [+0200], Jason A. Donenfeld wrote: > > > On RT, we can't call get_random_bytes() from inside of the raw locks > > > that callers of vsprintf might take, because get_random_bytes() takes > > > normal spinlocks. So on those RT systems, defer the siphash key > > > generation to a worker. > > > > > > Also, avoid using a static_branch, as this isn't the fast path. > > > Using static_branch_likely() to signal that ptr_key has been filled is a > > > bit much given that it is not a fast path. > > > > > > Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de> > > > Reported-by: Mike Galbraith <efault@gmx.de> > > > Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com> > > > --- > > > Sebastian - feel free to take this and tweak it as needed. Sending this > > > mostly as something illustrative of what the "simpler" thing would be > > > that I had in mind. -Jason > > > > Can have the same behaviour regardless of CONFIG_PREEMPT_RT? Here > > lockdep _may_ yell with !RT because it is broken for RT. > > If we agree that we drop the first %p print here, can we do this on > > both (regardless of CONFIG_PREEMPT_RT)? > > "Lockdep may yell" -- but this would be when lockdep is turned on to > catch RT bugs, not to catch non-RT bugs. The actual bug only exists on > RT. This is an RT problem. Stop pretending that this is a real issue > outside of RT. It isn't. This is *only* an RT issue. So why would we > make things worse for an issue that doesn't actually exist on non-RT? > > I too generally prefer having only one code path and not two. But the > way this patch is written, the worker function just gets reused with a > straight call on the non-RT case, so it doesn't actually require > duplicating code. > > Jason
By the way, another option that would be fine with me would be to make random.c use all raw spinlocks. From a non-RT perspective, that wouldn't change the codegen at all, so it doesn't make a huge difference to me. From an RT perspective, it would presumably fix a lot of these issues, and enable randomness to be available in any context, which is maybe what we want anyway. From an RT-safety point of view, I suspect doing this might actually be okay, because the locks are only ever protecting operations that are fixed duration CPU-bound, like generating a chacha block or something, not waiting for some I/O.
Thoughts on that?
Jason
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