[PATCH 6.19 006/781] rcu: Fix rcu_read_unlock() deadloop due to softirq

Greg Kroah-Hartman
2026-02-25T01:27:07Z

From: Yao Kai <yaokai34@huawei.com>

[ Upstream commit d41e37f26b3157b3f1d10223863519a943aa239b ]

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>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/rcu/tree.h | 2 +-
kernel/rcu/tree_plugin.h | 15 +++++++++------
2 files changed, 10 insertions(+), 7 deletions(-)

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index b8bbe7960cda7..2265b9c2906e1 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -203,7 +203,7 @@ struct rcu_data {
/* during and after the last grace */
/* period it is aware of. */
struct irq_work defer_qs_iw; /* Obtain later scheduler attention. */
- int defer_qs_iw_pending; /* Scheduler attention pending? */
+ int defer_qs_pending; /* irqwork or softirq pending? */
struct work_struct strict_work; /* Schedule readers for strict GPs. */

/* 2) batch handling */
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index dbe2d02be824a..95ad967adcf3c 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -487,8 +487,8 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
union rcu_special special;

rdp = this_cpu_ptr(&rcu_data);
- if (rdp->defer_qs_iw_pending == DEFER_QS_PENDING)
- rdp->defer_qs_iw_pending = DEFER_QS_IDLE;
+ if (rdp->defer_qs_pending == DEFER_QS_PENDING)
+ rdp->defer_qs_pending = DEFER_QS_IDLE;

/*
* If RCU core is waiting for this CPU to exit its critical section,
@@ -645,7 +645,7 @@ static void rcu_preempt_deferred_qs_handler(struct irq_work *iwp)
* 5. Deferred QS reporting does not happen.
*/
if (rcu_preempt_depth() > 0)
- WRITE_ONCE(rdp->defer_qs_iw_pending, DEFER_QS_IDLE);
+ WRITE_ONCE(rdp->defer_qs_pending, DEFER_QS_IDLE);
}

/*
@@ -747,7 +747,10 @@ static void rcu_read_unlock_special(struct task_struct *t)
// Using softirq, safe to awaken, and either the
// wakeup is free or there is either an expedited
// GP in flight or a potential need to deboost.
- raise_softirq_irqoff(RCU_SOFTIRQ);
+ if (rdp->defer_qs_pending != DEFER_QS_PENDING) {
+ rdp->defer_qs_pending = DEFER_QS_PENDING;
+ raise_softirq_irqoff(RCU_SOFTIRQ);
+ }
} else {
// Enabling BH or preempt does reschedule, so...
// Also if no expediting and no possible deboosting,
@@ -755,11 +758,11 @@ static void rcu_read_unlock_special(struct task_struct *t)
// tick enabled.
set_need_resched_current();
if (IS_ENABLED(CONFIG_IRQ_WORK) && irqs_were_disabled &&
- needs_exp && rdp->defer_qs_iw_pending != DEFER_QS_PENDING &&
+ needs_exp && rdp->defer_qs_pending != DEFER_QS_PENDING &&
cpu_online(rdp->cpu)) {
// Get scheduler to re-evaluate and call hooks.
// If !IRQ_WORK, FQS scan will eventually IPI.
- rdp->defer_qs_iw_pending = DEFER_QS_PENDING;
+ rdp->defer_qs_pending = DEFER_QS_PENDING;
irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu);
}
}
--
2.51.0