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

[Joel Fernandes <joelagnelf@nvidia.com>]

On 12/19/2025 10:14 AM, Joel Fernandes wrote:
> 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>
Could you please update the following comment in tree.h to say "An IRQ work or
softirq":

/*
 * An IRQ work (deferred_qs_iw) is used by RCU to get the scheduler's attention.
 * to report quiescent states at the soonest possible time.
 * The request can be in one of the following states:
 * - DEFER_QS_IDLE: An IRQ work is yet to be scheduled.
 * - DEFER_QS_PENDING: An IRQ work was scheduled but either not yet run, or it
 *                     ran and we still haven't reported a quiescent state.
 */
#define DEFER_QS_IDLE           0
#define DEFER_QS_PENDING        1

And resend the patch with my Review tag?

thanks,

 - Joel