"Paul E. McKenney" <paulmck@xxxxxxxxxx> writes: > On Wed, Apr 14, 2021 at 08:39:04PM +0200, Toke Høiland-Jørgensen wrote: >> "Paul E. McKenney" <paulmck@xxxxxxxxxx> writes: >> >> > On Wed, Apr 14, 2021 at 10:59:23AM -0700, Alexei Starovoitov wrote: >> >> On Wed, Apr 14, 2021 at 10:52 AM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: >> >> > >> >> > > > > > if (num_online_cpus() > 1) >> >> > > > > > synchronize_rcu(); >> >> > >> >> > In CONFIG_PREEMPT_NONE=y and CONFIG_PREEMPT_VOLUNTARY=y kernels, this >> >> > synchronize_rcu() will be a no-op anyway due to there only being the >> >> > one CPU. Or are these failures all happening in CONFIG_PREEMPT=y kernels, >> >> > and in tests where preemption could result in the observed failures? >> >> > >> >> > Could you please send your .config file, or at least the relevant portions >> >> > of it? >> >> >> >> That's my understanding as well. I assumed Toke has preempt=y. >> >> Otherwise the whole thing needs to be root caused properly. >> > >> > Given that there is only a single CPU, I am still confused about what >> > the tests are expecting the membarrier() system call to do for them. >> >> It's basically a proxy for waiting until the objects are freed on the >> kernel side, as far as I understand... > > There are in-kernel objects that are freed via call_rcu(), and the idea > is to wait until these objects really are freed? Or am I still missing > out on what is going on? Something like that? Although I'm not actually sure these are using call_rcu()? One of them needs __put_task_struct() to run, and the other waits for map freeing, with this comment: /* we need to either wait for or force synchronize_rcu(), before * checking for "still exists" condition, otherwise map could still be * resolvable by ID, causing false positives. * * Older kernels (5.8 and earlier) freed map only after two * synchronize_rcu()s, so trigger two, to be entirely sure. */ CHECK(kern_sync_rcu(), "sync_rcu", "failed\n"); CHECK(kern_sync_rcu(), "sync_rcu", "failed\n"); -Toke
