On Tue, Sep 21, 2021 at 11:12:50PM +0200, Thomas Gleixner wrote: > Valentin reported warnings about suspicious RCU usage on RT kernels. Those > happen when offloading of RCU callbacks is enabled: > > WARNING: suspicious RCU usage > 5.13.0-rt1 #20 Not tainted > ----------------------------- > kernel/rcu/tree_plugin.h:69 Unsafe read of RCU_NOCB offloaded state! > > rcu_rdp_is_offloaded (kernel/rcu/tree_plugin.h:69 kernel/rcu/tree_plugin.h:58) > rcu_core (kernel/rcu/tree.c:2332 kernel/rcu/tree.c:2398 kernel/rcu/tree.c:2777) > rcu_cpu_kthread (./include/linux/bottom_half.h:32 kernel/rcu/tree.c:2876) > > The reason is that rcu_rdp_is_offloaded() is invoked without one of the > required protections on RT enabled kernels because local_bh_disable() does > not disable preemption on RT. > > Valentin proposed to add a local lock to the code in question, but that's > suboptimal in several aspects: > > 1) local locks add extra code to !RT kernels for no value. > > 2) All possible callsites have to audited and amended when affected > possible at an outer function level due to lock nesting issues. > > 3) As the local lock has to be taken at the outer functions it's required > to release and reacquire them in the inner code sections which might > voluntary schedule, e.g. rcu_do_batch(). > > Both callsites of rcu_rdp_is_offloaded() which trigger this check invoke > rcu_rdp_is_offloaded() in the variable declaration section right at the top > of the functions. But the actual usage of the result is either within a > section which provides the required protections or after such a section. > > So the obvious solution is to move the invocation into the code sections > which provide the proper protections, which solves the problem for RT and > does not have any impact on !RT kernels. Also while at it, I'm asking again: traditionally softirqs could assume that manipulating a local state was safe against !irq_count() code fiddling with the same state on the same CPU. Now with preemptible softirqs, that assumption can be broken anytime. RCU was fortunate enough to have a warning for that. But who knows how many issues like this are lurking? Thanks.
