On Sat, Jan 14, 2017 at 12:00:22AM -0800, Paul E. McKenney wrote: > It now looks like this: > > ------------------------------------------------------------------------ > > Note that the code was buggy even before this commit, as it was subject > to failure on real-time systems that forced all expedited grace periods > to run as normal grace periods (for example, using the rcu_normal ksysfs > parameter). The callchain from the failure case is as follows: > > early_amd_iommu_init() > |-> acpi_put_table(ivrs_base); > |-> acpi_tb_put_table(table_desc); > |-> acpi_tb_invalidate_table(table_desc); > |-> acpi_tb_release_table(...) > |-> acpi_os_unmap_memory > |-> acpi_os_unmap_iomem > |-> acpi_os_map_cleanup > |-> synchronize_rcu_expedited > > The kernel showing this callchain was built with CONFIG_PREEMPT_RCU=y, > which caused the code to try using workqueues before they were > initialized, which did not go well. > > ------------------------------------------------------------------------ > > Does that work?
Yap, thanks. > Fair point, but this wording appears in almost all of my patches. ;-) :-) > My rationale is that it provides a clear transition from describing the > problem to introducing the solution. Fair enough. > Exactly, but yes, worth a comment. > > The header comment for rcu_scheduler_starting() is now as follows: > > /* > * During boot, we forgive RCU lockdep issues. After this function is > * invoked, we start taking RCU lockdep issues seriously. Note that unlike > * Tree RCU, Tiny RCU transitions directly from RCU_SCHEDULER_INACTIVE > * to RCU_SCHEDULER_RUNNING, skipping the RCU_SCHEDULER_INIT stage. > * The reason for this is that Tiny RCU does not need kthreads, so does > * not have to care about the fact that the scheduler is half-initialized > * at a certain phase of the boot process. > */ Good. > I believe that this would not buy very much, but if this variable starts > showing up on profiles, then perhaps a jump label would be appropriate. > As a separate patch, though! Yeah, let's keep that opportunity in the bag, just in case. > Thank you for your review and comments! Thanks for the fix. Btw, I'll build one more test kernel for people with your final version here: https://lkml.kernel.org/r/1484383554-18095-2-git-send-email-paul...@linux.vnet.ibm.com backported to 4.9. I say 4.9 because the reports started then, probably because of 8b355e3bc140 ("rcu: Drive expedited grace periods from workqueue") Which means, you probably should tag your fix CC:stable and add Fixes: 8b355e3bc140 ("rcu: Drive expedited grace periods from workqueue") to it too. Hmmm. -- Regards/Gruss, Boris. Good mailing practices for 400: avoid top-posting and trim the reply.
