Re: [PATCH 7/9] srcu: Detect duplicate callback enqueue at registration

[Paul E. McKenney]

On Fri, Oct 23, 2026 at 12:30:57PM -0700, Paul E. McKenney wrote:
> On Fri, Oct 23, 2026 at 06:18:39PM +0800, Kunwu Chan wrote:
> > Move the duplicate-callback check from srcu_do_enqueue() to
> > __call_srcu() entry so that detected duplicate registrations
> > are rejected before they reach the deferred-list enqueue path.
> > Report the callback address, function symbol, and object
> > information, rate-limited to the first three occurrences.
> > 
> > Signed-off-by: Kunwu Chan <chentao@xxxxxxxxxx>
> 
> Getting the stackof the call_srcu() once double-enqueue is detected
> would be a very good thing!
> 
> One question below.
> 
> > ---
> >  kernel/rcu/srcutree.c | 20 ++++++++++++++------
> >  1 file changed, 14 insertions(+), 6 deletions(-)
> > 
> > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > index 1993ddbdb828..3838d9fdf532 100644
> > --- a/kernel/rcu/srcutree.c
> > +++ b/kernel/rcu/srcutree.c
> > @@ -27,6 +27,7 @@
> >  #include <linux/smp.h>
> >  #include <linux/delay.h>
> >  #include <linux/module.h>
> > +#include <linux/mm.h>
> >  #include <linux/slab.h>
> >  #include <linux/srcu.h>
> >  
> > @@ -1511,12 +1512,6 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp,
> >  static void srcu_do_enqueue(struct srcu_struct *ssp, struct rcu_head *rhp,
> >  			    rcu_callback_t func, bool do_norm)
> >  {
> > -	if (debug_rcu_head_queue(rhp)) {
> > -		/* Probable double call_srcu(), so leak the callback. */
> > -		WRITE_ONCE(rhp->func, srcu_leak_callback);
> > -		WARN_ONCE(1, "call_srcu(): Leaked duplicate callback\n");
> > -		return;
> > -	}
> >  	rhp->func = func;
> >  	(void)srcu_gp_start_if_needed(ssp, rhp, do_norm);
> >  }
> > @@ -1529,6 +1524,8 @@ static void srcu_do_enqueue(struct srcu_struct *ssp, struct rcu_head *rhp,
> >  static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
> >  			rcu_callback_t func, bool do_norm)
> >  {
> > +	static atomic_t doublefrees;
> > +
> >  	/* Misaligned rcu_head! */
> >  	WARN_ON_ONCE((unsigned long)rhp & (sizeof(void *) - 1));
> >  
> > @@ -1539,6 +1536,16 @@ static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
> >  	if (WARN_ON_ONCE(ssp->srcu_reader_flavor == SRCU_READ_FLAVOR_ATOMIC))
> >  		return; // Leak the callback rather than corrupt SRCU state.
> >  
> > +	if (debug_rcu_head_queue(rhp)) {
> > +		if (atomic_inc_return(&doublefrees) < 4) {
> 
> Why this hard-coded 4?  Why not 3 or 5?  Should this be adjustable via
> Kconfig or kernel boot parameter?  Or, alternatively, should the below
> pr_err() instead be pr_err_ratelimited()?
> 
> > +			pr_err("%s(): Duplicate call_srcu() CB %p->%pS()\n",
> > +			       __func__, rhp, rhp->func);
> > +			mem_dump_obj(rhp);
> 
> Does any of this automatically dump out the stack that was recorded
> in earlier patches in this series?

As in shouldn't there be a kasan_print_aux_stacks() in there somewhere?
I did not see an call from mem_dump_obj(), but I could easily have
missed it.

							Thanx, Paul

> > +		}
> > +		WRITE_ONCE(rhp->func, srcu_leak_callback);
> > +		return;
> > +	}
> > +
> >  	/* Record the caller: the irq_work's stack says nothing about it. */
> >  	kasan_record_aux_stack(rhp);
> >  
> > @@ -1554,6 +1561,7 @@ static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
> >  		if (READ_ONCE(sndp->draining) && !in_nmi()) {
> >  			WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU),
> >  				  "call_srcu() re-entered during callback drain; leaking callback\n");
> > +			debug_rcu_head_unqueue(rhp);
> >  			return;
> >  		}
> >  		sdp = this_cpu_ptr(ssp->sda);
> > -- 
> > 2.43.0
> >