Re: [PATCH] rcu: Fix rcu_read_unlock() deadloop due to softirq

[Joel Fernandes <joelagnelf@nvidia.com>]

On Thu, Dec 18, 2025 at 03:49:50PM +0800, Yao Kai wrote:
> Commit 5f5fa7ea89dc ("rcu: Don't use negative nesting depth in
> __rcu_read_unlock()") removes the recursion-protection code from
> __rcu_read_unlock(). Therefore, we could invoke the deadloop in
> raise_softirq_irqoff() with ftrace enabled as follows:
> 
> WARNING: CPU: 0 PID: 0 at kernel/trace/trace.c:3021 __ftrace_trace_stack.constprop.0+0x172/0x180
> Modules linked in: my_irq_work(O)
> CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Tainted: G O 6.18.0-rc7-dirty #23 PREEMPT(full)
> Tainted: [O]=OOT_MODULE
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
> RIP: 0010:__ftrace_trace_stack.constprop.0+0x172/0x180
> RSP: 0018:ffffc900000034a8 EFLAGS: 00010002
> RAX: 0000000000000000 RBX: 0000000000000004 RCX: 0000000000000000
> RDX: 0000000000000003 RSI: ffffffff826d7b87 RDI: ffffffff826e9329
> RBP: 0000000000090009 R08: 0000000000000005 R09: ffffffff82afbc4c
> R10: 0000000000000008 R11: 0000000000011d7a R12: 0000000000000000
> R13: ffff888003874100 R14: 0000000000000003 R15: ffff8880038c1054
> FS:  0000000000000000(0000) GS:ffff8880fa8ea000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 000055b31fa7f540 CR3: 00000000078f4005 CR4: 0000000000770ef0
> PKRU: 55555554
> Call Trace:
>  <IRQ>
>  trace_buffer_unlock_commit_regs+0x6d/0x220
>  trace_event_buffer_commit+0x5c/0x260
>  trace_event_raw_event_softirq+0x47/0x80
>  raise_softirq_irqoff+0x6e/0xa0
>  rcu_read_unlock_special+0xb1/0x160
>  unwind_next_frame+0x203/0x9b0
>  __unwind_start+0x15d/0x1c0
>  arch_stack_walk+0x62/0xf0
>  stack_trace_save+0x48/0x70
>  __ftrace_trace_stack.constprop.0+0x144/0x180
>  trace_buffer_unlock_commit_regs+0x6d/0x220
>  trace_event_buffer_commit+0x5c/0x260
>  trace_event_raw_event_softirq+0x47/0x80
>  raise_softirq_irqoff+0x6e/0xa0
>  rcu_read_unlock_special+0xb1/0x160
>  unwind_next_frame+0x203/0x9b0
>  __unwind_start+0x15d/0x1c0
>  arch_stack_walk+0x62/0xf0
>  stack_trace_save+0x48/0x70
>  __ftrace_trace_stack.constprop.0+0x144/0x180
>  trace_buffer_unlock_commit_regs+0x6d/0x220
>  trace_event_buffer_commit+0x5c/0x260
>  trace_event_raw_event_softirq+0x47/0x80
>  raise_softirq_irqoff+0x6e/0xa0
>  rcu_read_unlock_special+0xb1/0x160
>  unwind_next_frame+0x203/0x9b0
>  __unwind_start+0x15d/0x1c0
>  arch_stack_walk+0x62/0xf0
>  stack_trace_save+0x48/0x70
>  __ftrace_trace_stack.constprop.0+0x144/0x180
>  trace_buffer_unlock_commit_regs+0x6d/0x220
>  trace_event_buffer_commit+0x5c/0x260
>  trace_event_raw_event_softirq+0x47/0x80
>  raise_softirq_irqoff+0x6e/0xa0
>  rcu_read_unlock_special+0xb1/0x160
>  __is_insn_slot_addr+0x54/0x70
>  kernel_text_address+0x48/0xc0
>  __kernel_text_address+0xd/0x40
>  unwind_get_return_address+0x1e/0x40
>  arch_stack_walk+0x9c/0xf0
>  stack_trace_save+0x48/0x70
>  __ftrace_trace_stack.constprop.0+0x144/0x180
>  trace_buffer_unlock_commit_regs+0x6d/0x220
>  trace_event_buffer_commit+0x5c/0x260
>  trace_event_raw_event_softirq+0x47/0x80
>  __raise_softirq_irqoff+0x61/0x80
>  __flush_smp_call_function_queue+0x115/0x420
>  __sysvec_call_function_single+0x17/0xb0
>  sysvec_call_function_single+0x8c/0xc0
>  </IRQ>
> 
> Commit b41642c87716 ("rcu: Fix rcu_read_unlock() deadloop due to IRQ work")
> fixed the infinite loop in rcu_read_unlock_special() for IRQ work by
> setting a flag before calling irq_work_queue_on(). We fix this issue by
> setting the same flag before calling raise_softirq_irqoff() and rename the
> flag to defer_qs_pending for more common.
> 
> Fixes: 5f5fa7ea89dc ("rcu: Don't use negative nesting depth in __rcu_read_unlock()")
> Reported-by: Tengda Wu <wutengda2@huawei.com>
> Signed-off-by: Yao Kai <yaokai34@huawei.com>

Good change! It is the exact same pattern and both IRQ work and softirq
raising are for deferred QS purposes.

Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>

thanks,

 - Joel



> ---
>  kernel/rcu/tree.h        |  2 +-
>  kernel/rcu/tree_plugin.h | 15 +++++++++------
>  2 files changed, 10 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> index b8bbe7960cda..2265b9c2906e 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -203,7 +203,7 @@ struct rcu_data {
>  					/*  during and after the last grace */
>  					/* period it is aware of. */
>  	struct irq_work defer_qs_iw;	/* Obtain later scheduler attention. */
> -	int defer_qs_iw_pending;	/* Scheduler attention pending? */
> +	int defer_qs_pending;		/* irqwork or softirq pending? */
>  	struct work_struct strict_work;	/* Schedule readers for strict GPs. */
>  
>  	/* 2) batch handling */
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index dbe2d02be824..95ad967adcf3 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -487,8 +487,8 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
>  	union rcu_special special;
>  
>  	rdp = this_cpu_ptr(&rcu_data);
> -	if (rdp->defer_qs_iw_pending == DEFER_QS_PENDING)
> -		rdp->defer_qs_iw_pending = DEFER_QS_IDLE;
> +	if (rdp->defer_qs_pending == DEFER_QS_PENDING)
> +		rdp->defer_qs_pending = DEFER_QS_IDLE;
>  
>  	/*
>  	 * If RCU core is waiting for this CPU to exit its critical section,
> @@ -645,7 +645,7 @@ static void rcu_preempt_deferred_qs_handler(struct irq_work *iwp)
>  	 * 5. Deferred QS reporting does not happen.
>  	 */
>  	if (rcu_preempt_depth() > 0)
> -		WRITE_ONCE(rdp->defer_qs_iw_pending, DEFER_QS_IDLE);
> +		WRITE_ONCE(rdp->defer_qs_pending, DEFER_QS_IDLE);
>  }
>  
>  /*
> @@ -747,7 +747,10 @@ static void rcu_read_unlock_special(struct task_struct *t)
>  			// Using softirq, safe to awaken, and either the
>  			// wakeup is free or there is either an expedited
>  			// GP in flight or a potential need to deboost.
> -			raise_softirq_irqoff(RCU_SOFTIRQ);
> +			if (rdp->defer_qs_pending != DEFER_QS_PENDING) {
> +				rdp->defer_qs_pending = DEFER_QS_PENDING;
> +				raise_softirq_irqoff(RCU_SOFTIRQ);
> +			}
>  		} else {
>  			// Enabling BH or preempt does reschedule, so...
>  			// Also if no expediting and no possible deboosting,
> @@ -755,11 +758,11 @@ static void rcu_read_unlock_special(struct task_struct *t)
>  			// tick enabled.
>  			set_need_resched_current();
>  			if (IS_ENABLED(CONFIG_IRQ_WORK) && irqs_were_disabled &&
> -			    needs_exp && rdp->defer_qs_iw_pending != DEFER_QS_PENDING &&
> +			    needs_exp && rdp->defer_qs_pending != DEFER_QS_PENDING &&
>  			    cpu_online(rdp->cpu)) {
>  				// Get scheduler to re-evaluate and call hooks.
>  				// If !IRQ_WORK, FQS scan will eventually IPI.
> -				rdp->defer_qs_iw_pending = DEFER_QS_PENDING;
> +				rdp->defer_qs_pending = DEFER_QS_PENDING;
>  				irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu);
>  			}
>  		}
> -- 
> 2.43.0
>