Re: [PATCH -next 1/8] rcu: Fix rcu_read_unlock() deadloop due to softirq

[Frederic Weisbecker <frederic@kernel.org>]

Le Thu, Jan 01, 2026 at 11:34:10AM -0500, Joel Fernandes a écrit :
> From: Yao Kai <yaokai34@huawei.com>
> 
> 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>
> Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>
> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>

Looks good but, BTW, what happens if rcu_qs() is called
before rcu_preempt_deferred_qs() had a chance to be called?

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.

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().

Thanks.

-- 
Frederic Weisbecker
SUSE Labs