Re: [PATCH v2] srcu: Fix flush srcu structure's->sup work warning in cleanup_srcu_struct()

["Paul E. McKenney" <paulmck@xxxxxxxxxx>]

On Thu, Mar 23, 2023 at 09:46:21PM +0800, Zqiang wrote:
> When unloading rcutorture kmod will trigger the following callstack:
> 
> insmod rcutorture.ko
> rmmod rcutorture.ko
> 
> [  209.437327] WARNING: CPU: 0 PID: 508 at kernel/workqueue.c:3167 __flush_work+0x50a/0x540
> [  209.437346] Modules linked in: rcutorture(-) torture [last unloaded: rcutorture]
> [  209.437382] CPU: 0 PID: 508 Comm: rmmod Tainted: G  W  6.3.0-rc1-yocto-standard+
> [  209.437406] RIP: 0010:__flush_work+0x50a/0x540
> .....
> [  209.437758]  flush_delayed_work+0x36/0x90
> [  209.437776]  cleanup_srcu_struct+0x68/0x2e0
> [  209.437817]  srcu_module_notify+0x71/0x140
> [  209.437854]  blocking_notifier_call_chain+0x9d/0xd0
> [  209.437880]  __x64_sys_delete_module+0x223/0x2e0
> [  209.438046]  do_syscall_64+0x43/0x90
> [  209.438062]  entry_SYSCALL_64_after_hwframe+0x72/0xdc
> 
> flush_delayed_work()
> ->__flush_work()
>    ->if (WARN_ON(!work->func))
>         return false;
> 
> For srcu objects defined with DEFINE_SRCU() or DEFINE_STATIC_SRCU(),
> when compiling and loading as modules, the srcu_module_coming() is
> invoked, allocate memory for srcu structure's->sda and initialize
> sda structure, due to not fully initialize srcu structure's->sup,
> so at this time the sup structure's->work.work.func is null, if not
> invoke init_srcu_struct_fields() before unloading modules, the
> __flush_work() be invoked in srcu_module_going() and find work->func
> is empty, will raise the warning above.
> 
> This commit add the check of srcu_sup structure's->srcu_gp_seq_needed
> to determine whether the check_init_srcu_struct() has been invoked to
> initialize srcu objects in srcu_module_going(), if not initialize, there
> are no pending or running works, so there is no need to flush, only invoke
> free_percpu() to release srcu structure's->sda.
> 
> Co-developed-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

Thank you for the testing, bug-finding, and problem-solving!

In theory, you would need a Signed-off-by here from me as well, but
in practice bisectability means that this must be folded into this:

e7c778489040 ("srcu: Use static init for statically allocated in-module srcu_struct")

This will of course be with attribution.

> Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>

But this is still a bit more complex than needed.  How about something
like this?

							Thanx, Paul

------------------------------------------------------------------------

/* Initialize any global-scope srcu_struct structures used by this module. */
static int srcu_module_coming(struct module *mod)
{
	int i;
	struct srcu_struct *ssp;
	struct srcu_struct **sspp = mod->srcu_struct_ptrs;

	for (i = 0; i < mod->num_srcu_structs; i++) {
		ssp = *(sspp++);
		ssp->sda = alloc_percpu(struct srcu_data);
		if (WARN_ON_ONCE(!ssp->sda))
			return -ENOMEM;
	}
	return 0;
}

/* Clean up any global-scope srcu_struct structures used by this module. */
static void srcu_module_going(struct module *mod)
{
	int i;
	struct srcu_struct *ssp;
	struct srcu_struct **sspp = mod->srcu_struct_ptrs;

	for (i = 0; i < mod->num_srcu_structs; i++) {
		ssp = *(sspp++);
		if (!rcu_seq_state(smp_load_acquire(&ssp->srcu_sup->srcu_gp_seq_needed)) &&
		    !WARN_ON_ONCE(!ssp->srcu_sup->sda_is_static))
				cleanup_srcu_struct(ssp);
		free_percpu(ssp->sda);
	}
}