On Thu, Jul 13, 2023 at 10:02:17AM +0800, Gao Xiang wrote: > > > On 2023/7/13 08:32, Joel Fernandes wrote: > > On Wed, Jul 12, 2023 at 02:20:56PM -0700, Sandeep Dhavale wrote: > > [..] > > > > As such this patch looks correct to me, one thing I noticed is that > > > > you can check rcu_is_watching() like the lockdep-enabled code does. > > > > That will tell you also if a reader-section is possible because in > > > > extended-quiescent-states, RCU readers should be non-existent or > > > > that's a bug. > > > > > > > Please correct me if I am wrong, reading from the comment in > > > kernel/rcu/update.c rcu_read_lock_held_common() > > > .. > > > * The reason for this is that RCU ignores CPUs that are > > > * in such a section, considering these as in extended quiescent state, > > > * so such a CPU is effectively never in an RCU read-side critical section > > > * regardless of what RCU primitives it invokes. > > > > > > It seems rcu will treat this as lock not held rather than a fact that > > > lock is not held. Is my understanding correct? > > > > If RCU treats it as a lock not held, that is a fact for RCU ;-). Maybe you > > mean it is not a fact for erofs? > > I'm not sure if I get what you mean, EROFS doesn't take any RCU read lock > here: The key point is that we need lockdep to report errors when rcu_read_lock(), rcu_dereference(), and friends are used when RCU is not watching. We also need lockdep to report an error when someone uses rcu_dereference() when RCU is not watching, but also forgets the rcu_read_lock(). And this is the job of rcu_read_lock_held(), which is one reason why that rcu_is_watching() is needed. > z_erofs_decompressqueue_endio() is actually a "bio->bi_end_io", previously > which can be called under two scenarios: > > 1) under softirq context, which is actually part of device I/O compleltion; > > 2) under threaded context, like what dm-verity or likewise calls. > > But EROFS needs to decompress in a threaded context anyway, so we trigger > a workqueue to resolve the case 1). > > Recently, someone reported there could be some case 3) [I think it was > introduced recently but I have no time to dig into it]: > > case 3: under RCU read lock context, which is shown by this: > https://lore.kernel.org/r/4a8254eb-ac39-1e19-3d82-417d3a7b9f94@xxxxxxxxxxxxxxxxx/T/#u > > and such RCU read lock is taken in __blk_mq_run_dispatch_ops(). > > But as the commit shown, we only need to trigger a workqueue for case 1) > and 3) due to performance reasons. Just out of curiosity, exactly how much is it costing to trigger the workqueue? > Hopefully I show it more clear. One additional question... What is your plan for kernels built with CONFIG_PREEMPT_COUNT=n? After all, in such kernels, there is no way that I know of for code to determine whether it is in an RCU read-side critical section, holding a spinlock, or running with preemption disabled. Thanx, Paul
