> On Jan 7, 2026, at 8:02 PM, Joel Fernandes <[email protected]> wrote:
>
>
>
>> On Jan 7, 2026, at 6:15 PM, Frederic Weisbecker <[email protected]> wrote:
>>
>> Le Thu, Jan 01, 2026 at 11:34:10AM -0500, Joel Fernandes a écrit :
>>> From: Yao Kai <[email protected]>
>>>
>>> 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 <[email protected]>
>>> Signed-off-by: Yao Kai <[email protected]>
>>> Reviewed-by: Joel Fernandes <[email protected]>
>>> Signed-off-by: Joel Fernandes <[email protected]>
>>
>> Looks good but, BTW, what happens if rcu_qs() is called
>> before rcu_preempt_deferred_qs() had a chance to be called?
>
> Could you provide an example of when that can happen?
>
> If rcu_qs() results in reporting of a quiescent state up the node tree before
> the deferred reporting work had a chance to act, then indeed we should be
> clearing the flag (after canceling the pending raise_softirq_irqoff()).
>
>>> 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 <[email protected]>
>>> Signed-off-by: Yao Kai <[email protected]>
>>> Reviewed-by: Joel Fernandes <[email protected]>
>>> Signed-off-by: Joel Fernandes <[email protected]>
>>
>> Looks good but, BTW, what happens if rcu_qs() is called
>> before rcu_preempt_deferred_qs() had a chance to be called?
>
> Could you provide an example of when that can happen?
>
> As far as I can see, even if that were to happen, which I think you are right
> it can happen, we will still go through the path to report deferred quiescent
> states and cancel the pending work (reset the flag).
>
>> current->rcu_read_unlock_special.b.need_qs is reset by rcu_qs()
>> so subsequent calls to rcu_read_unlock() won't issue
>> rcu_read_unlock_special()
>> (unless the task is blocked). And further calls to rcu_preempt_deferred_qs()
>> through rcu_core() will be ignored as well.
>
> I am not sure if this implies that deferred quiescent state gets cancelled
> because we have already called unlock once. We have to go through the
> deferred quiescent state path on all subsequent quiescent state reporting,
> even if need_qs reset. How else will the GP complete.
>>
>> But rdp->defer_qs_pending will remain in the DEFER_QS_PENDING state until
>> the next grace period. And if rcu_read_unlock_special() is called again
>> during the next GP on unfortunate place needing deferred qs, the state
>> machine
>> will spuriously assume that either rcu_core or the irq_work are pending, when
>> none are anymore.
>>
>> The state should be reset by rcu_qs().
>
> In fact I would say if a deferred QS is pending, we should absolutely not
> reset its state from rcu_qs..
>
> Maybe we should reset it from rcu_report_qs_rdp/rnp?
>
> Unfortunately, all of this is coming from me being on a phone and not at a
> computer, so I will revise my response, but probably tomorrow, because today
> the human body is not cooperating.
>
> thanks,
>
> - Joel
>
>
>> current->rcu_read_unlock_special.b.need_qs is reset by rcu_qs()
>> so subsequent calls to rcu_read_unlock() won't issue
>> rcu_read_unlock_special()
>> (unless the task is blocked). And further calls to rcu_preempt_deferred_qs()
>> through rcu_core() will be ignored as well.
>
> I am not sure if this implies that deferred quiescent state gets cancelled
> because we have already called unlock once. We have to go through the
> deferred quiescent state path on all subsequent quiescent state reporting,
> even if need_qs reset. How else will the GP complete.
>>
>> But rdp->defer_qs_pending will remain in the DEFER_QS_PENDING state until
>> the next grace period. And if rcu_read_unlock_special() is called again
>> during the next GP on unfortunate place needing deferred qs, the state
>> machine
>> will spuriously assume that either rcu_core or the irq_work are pending, when
>> none are anymore.
>>
>> The state should be reset by rcu_qs().
>
> In fact I would say if a deferred QS is pending, we should absolutely not
> reset its state from rcu_qs..
>
> Maybe we should reset it from rcu_report_qs_rdp/rnp?
>
> thanks,
By the way, when I last tried to do it from rcu_qs, it was not fixing the
original bug with the IRQ work recursion.
I found that it was always resetting the flag. But probably it is not even the
right place to do it in the first place.
Thanks,
- Joel
>
> - Joel
>
>
>>
>> Thanks.
>>
>> --
>> Frederic Weisbecker
>> SUSE Labs
>>
