> > > > On 4/28/2025 6:59 AM, Z qiang wrote: > >> > >> Le Mon, Apr 28, 2025 at 05:54:03PM +0800, Zqiang a écrit : > >>> For Preempt-RT kernel, when enable CONFIG_PROVE_RCU Kconfig, > >>> disable local bh in rcuc kthreads will not affect preempt_count(), > >>> this resulted in the following splat: > >>> > >>> WARNING: suspicious RCU usage > >>> kernel/rcu/tree_plugin.h:36 Unsafe read of RCU_NOCB offloaded state! > >>> stack backtrace: > >>> CPU: 0 UID: 0 PID: 22 Comm: rcuc/0 > >>> Call Trace: > >>> [ 0.407907] <TASK> > >>> [ 0.407910] dump_stack_lvl+0xbb/0xd0 > >>> [ 0.407917] dump_stack+0x14/0x20 > >>> [ 0.407920] lockdep_rcu_suspicious+0x133/0x210 > >>> [ 0.407932] rcu_rdp_is_offloaded+0x1c3/0x270 > >>> [ 0.407939] rcu_core+0x471/0x900 > >>> [ 0.407942] ? lockdep_hardirqs_on+0xd5/0x160 > >>> [ 0.407954] rcu_cpu_kthread+0x25f/0x870 > >>> [ 0.407959] ? __pfx_rcu_cpu_kthread+0x10/0x10 > >>> [ 0.407966] smpboot_thread_fn+0x34c/0xa50 > >>> [ 0.407970] ? trace_preempt_on+0x54/0x120 > >>> [ 0.407977] ? __pfx_smpboot_thread_fn+0x10/0x10 > >>> [ 0.407982] kthread+0x40e/0x840 > >>> [ 0.407990] ? __pfx_kthread+0x10/0x10 > >>> [ 0.407994] ? rt_spin_unlock+0x4e/0xb0 > >>> [ 0.407997] ? rt_spin_unlock+0x4e/0xb0 > >>> [ 0.408000] ? __pfx_kthread+0x10/0x10 > >>> [ 0.408006] ? __pfx_kthread+0x10/0x10 > >>> [ 0.408011] ret_from_fork+0x40/0x70 > >>> [ 0.408013] ? __pfx_kthread+0x10/0x10 > >>> [ 0.408018] ret_from_fork_asm+0x1a/0x30 > >>> [ 0.408042] </TASK> > >>> > >>> Currently, triggering an rdp offloaded state change need the > >>> corresponding rdp's CPU goes offline, and at this time the rcuc > >>> kthreads has already in parking state. this means the corresponding > >>> rcuc kthreads can safely read offloaded state of rdp while it's > >>> corresponding cpu is online. > >>> > >>> This commit therefore add rdp->rcu_cpu_kthread_task check for > >>> Preempt-RT kernels. > >>> > >>> Signed-off-by: Zqiang <qiang.zhang1...@gmail.com> > >>> --- > >>> kernel/rcu/tree_plugin.h | 4 +++- > >>> 1 file changed, 3 insertions(+), 1 deletion(-) > >>> > >>> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > >>> index 003e549f6514..fe728eded36e 100644 > >>> --- a/kernel/rcu/tree_plugin.h > >>> +++ b/kernel/rcu/tree_plugin.h > >>> @@ -31,7 +31,9 @@ static bool rcu_rdp_is_offloaded(struct rcu_data *rdp) > >>> lockdep_is_held(&rcu_state.nocb_mutex) || > >>> (!(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible()) && > >>> rdp == this_cpu_ptr(&rcu_data)) || > >>> - rcu_current_is_nocb_kthread(rdp)), > >>> + rcu_current_is_nocb_kthread(rdp) || > >>> + (IS_ENABLED(CONFIG_PREEMPT_RT) && > >>> + current == rdp->rcu_cpu_kthread_task)), > >> > >> Isn't it safe also on !CONFIG_PREEMPT_RT ? > > > > For !CONFIG_PREEMPT_RT and in rcuc kthreads, it's also safe, > > but the following check will passed : > > > > (!(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible()) && > > rdp == this_cpu_ptr(&rcu_data)) > > I think the fact that it already passes for !PREEMPT_RT does not matter, > because > it simplifies the code so drop the PREEMPT_RT check? > > Or will softirq_count() not work? It appears to have special casing for > PREEMPT_RT's local_bh_disable(): > > ( ( !(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible()) || softirq_count() > ) > && rdp == this_cpu_ptr(&rcu_data)) )
Thank you for Joel's reply, I also willing to accept such modifications and resend :) . Thanks Zqiang > > thanks, > > - Joel > > > > >
