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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: >> >> 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 >> >> >> 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 >> Signed-off-by: Yao Kai > > Good change! It is the exact same pattern and both IRQ work and softirq > raising are for deferred QS purposes. > > Reviewed-by: Joel Fernandes 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