On Sat, Aug 15, 2020 at 10:27:54AM +0200, Peter Zijlstra wrote: > On Sat, Aug 15, 2020 at 01:14:53AM +0200, Thomas Gleixner wrote: > > > > As a matter of fact I assume^Wdeclare that removing struct rcu_head which > > provides a fallback is not an option at all. I know that you want to, > > but it wont work ever. Dream on, but as we agreed on recently there is > > this thing called reality which ruins everything. > > It never was going to work, freeing memory can never hard rely on the > success of allocating memory. In neither case does the freeing of memory rely hard-rely on the success of allocating memory. This is because there is a fallback in both cases should allocation fail. Given an rcu_head structure, we use that, and accept the extra cache misses at callback-invocation time. Otherwise, without an rcu_head structure, the allocation parameters are carefully chosen to avoid indefinite sleeping, meaning that the allocation attempt either succeeds or fails within a reasonable amount of time. And upon failure we invoke synchronize_rcu(), then immediately free. Which is slow, but then again life is like that under OOM conditions. And yes, this means that the price of leaving the rcu_head structure out of the structure to be freed is that you must call kvfree_free() from a sleepable context. If you don't like being restricted to sleepable context, you can always supply the rcu_head structure. Thanx, Paul
