On Thu, 23 May 2024 12:49:50 +0800 Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> wrote: > On Wed, May 22, 2024 at 03:53:23PM -0700, Linus Torvalds wrote: > > > > That said, looking at the code in question, there are other oddities > > going on. Even the "we found a favorite new rng" case looks rather > > strange. The thread we use - nice and asynchronous - seems to sleep > > only if the randomness source is emptied. > > > > What if you have a really good source of hw randomness? That looks > > like a busy loop to me, but hopefully I'm missing something obvious. > > Yes that does look strange. So I dug up the original patch at > > https://lore.kernel.org/all/20140317165012.GC1763@xxxxxx/ > > and therein lies the answer. It's relying on random.c to push back > when the amount of new entropy exceeds what it needs. IOW we will > sleep via add_hwgenerator_randomness when random.c decides that > enough is enough. Yes, I thought that this was the obvious choice, the lesser evil. If it just discarded the excess and returned immediately I had expected some kernel threads to spin constantly. > In fact the rate is much less now compared to > when the patch was first applied. You mean the rate of required entropy? Doesn't that make things worse? Maybe redesign the API to use a pull scheme? RNGs register at the randomness facility with a callback that can be used when there is a need for fresh entropy? Torsten
