Le Wed, May 07, 2025 at 12:06:29PM -0400, Joel Fernandes a écrit : > > > On 5/7/2025 7:26 AM, Zqiang wrote: > > For built with CONFIG_PROVE_RCU=y and CONFIG_PREEMPT_RT=y kernels, > > Disable BH does not change the SOFTIRQ corresponding bits in > > preempt_count(), but change current->softirq_disable_cnt, 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 softirq_count() check for > > Preempt-RT kernels. > > > > Suggested-by: Joel Fernandes <joelagn...@nvidia.com> > > Signed-off-by: Zqiang <qiang.zhang1...@gmail.com> > > --- > > kernel/rcu/tree_plugin.h | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > > index 003e549f6514..a91b2322a0cd 100644 > > --- a/kernel/rcu/tree_plugin.h > > +++ b/kernel/rcu/tree_plugin.h > > @@ -29,7 +29,7 @@ static bool rcu_rdp_is_offloaded(struct rcu_data *rdp) > > (IS_ENABLED(CONFIG_HOTPLUG_CPU) && lockdep_is_cpus_held()) || > > lockdep_is_held(&rdp->nocb_lock) || > > lockdep_is_held(&rcu_state.nocb_mutex) || > > - (!(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible()) && > > + ((!(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible()) || > > softirq_count()) && > > rdp == this_cpu_ptr(&rcu_data)) || > This looks good to me. Frederic told me he'll further review and give final > green signal. Then I'll pull this particular one. > > One thing I was wondering -- it would be really nice if preemptible() itself > checked for softirq_count() by default. Or adding something like a > really_preemptible() which checks for both CONFIG_PREEMPT_COUNT and > softirq_count() along with preemptible(). I feel like this always comes back > to > bite us in different ways, and not knowing atomicity complicates various code > paths. > > Maybe a summer holidays project? ;)
I thought about that too but I think this is semantically incorrect. In PREEMPT_RT, softirqs are actually preemptible. Thanks. > > - Joel > -- Frederic Weisbecker SUSE Labs
