On Wed, Apr 14, 2021 at 03:13:38PM -0700, Andrii Nakryiko wrote: > On Wed, Apr 14, 2021 at 2:25 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > > > On Wed, Apr 14, 2021 at 09:18:09PM +0200, Toke Høiland-Jørgensen wrote: > > > "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"); > > > > OK, so the issue is that the membarrier() system call is designed to force > > ordering only within a user process, and you need it in the kernel. > > > > Give or take my being puzzled as to why the membarrier() system call > > doesn't do it for you on a CONFIG_PREEMPT_NONE=y system, this brings > > us back to the question Alexei asked me in the first place, what is the > > best way to invoke an in-kernel synchronize_rcu() from userspace? > > > > You guys gave some reasonable examples. Here are a few others: > > > > o Bring a CPU online, then force it offline, or vice versa. > > But in this case, sys_membarrier() would do what you need > > given more than one CPU. > > > > o Use the membarrier() system call, but require that the tests > > run on systems with at least two CPUs. > > > > o Create a kernel module whose init function does a > > synchronize_rcu() and then returns failure. This will > > avoid the overhead of removing that kernel module. > > > > o Create a sysfs or debugfs interface that does a > > synchronize_rcu(). > > > > But I am still concerned that you are needing more than synchronize_rcu() > > can do. Otherwise, the membarrier() system call would work just fine > > on a single CPU on your CONFIG_PREEMPT_VOLUNTARY=y kernel. > > Selftests know internals of kernel implementation and wait for some > objects to be freed with call_rcu(). So I think at this point the best > way is just to go back to map-in-map or socket local storage. > Map-in-map will probably work on older kernels, so I'd stick with that > (plus all the code is there in the referenced commit). The performance > and number of syscalls performed doesn't matter, really. Ah! If they need to wait for objects to be freed with call_rcu(), then they need to make the kernel execute an rcu_barrier(). One way to make this happen is to unmount an ext4 filesystem. This would explain why the membarrier() system call wasn't doing the job on single-CPU systems even in kernels built with CONFIG_PREEMPT_VOLUNTARY=y. But if you have a more direct way to wait the required period of time, so much the better! Thanx, Paul
