Le Wed, Jan 07, 2026 at 08:02:43PM -0500, Joel Fernandes a écrit : > > > > 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?
It can happen because rcu_qs() is called before rcu_preempt_deferred_qs() in rcu_softirq_qs(). Inverting the calls could help but IRQs must be disabled to ensure there is no read side between rcu_preempt_deferred_qs() and rcu_qs(). I'm not aware of other ways to trigger that, except perhaps this: https://lore.kernel.org/rcu/[email protected]/T/#u Either we fix those sites and make sure that rcu_preempt_deferred_qs() is always called before rcu_qs() in the same IRQ disabled section (or there are other fields set in ->rcu_read_unlock_special for later clearance). If we do that we must WARN_ON_ONCE(rdp->defer_qs_pending == DEFER_QS_PENDING) in rcu_qs(). Or we reset rdp->defer_qs_pending from rcu_qs(), which sounds more robust. Ah an alternative is to make rdp::defer_qs_pending a field in union rcu_special which, sadly, would need to be expanded as a u64. Thanks. -- Frederic Weisbecker SUSE Labs
