Re: [PATCH] rcu: Use an intermediate irq_work to start process_srcu()

[Boqun Feng <boqun@kernel.org>]

On Fri, Mar 20, 2026 at 09:47:51PM +0100, Andrea Righi wrote:
> Hi Boqun,
> 
> On Fri, Mar 20, 2026 at 11:14:00AM -0700, Boqun Feng wrote:
> > Since commit c27cea4416a3 ("rcu: Re-implement RCU Tasks Trace in terms
> > of SRCU-fast") we switched to SRCU in BPF. However as BPF instrument can
> > happen basically everywhere (including where a scheduler lock is held),
> > call_srcu() now needs to avoid acquiring scheduler lock because
> > otherwise it could cause deadlock [1]. Fix this by following what the
> > previous RCU Tasks Trace did: using an irq_work to delay the queuing of
> > the work to start process_srcu().
> > 
> > [boqun: Apply Joel's feedback]
> > 
> > Reported-by: Andrea Righi <arighi@nvidia.com>
> > Closes: https://lore.kernel.org/all/abjzvz_tL_siV17s@gpd4/
> > Fixes: commit c27cea4416a3 ("rcu: Re-implement RCU Tasks Trace in terms of SRCU-fast")
> > Link: https://lore.kernel.org/rcu/3c4c5a29-24ea-492d-aeee-e0d9605b4183@nvidia.com/ [1]
> > Suggested-by: Zqiang <qiang.zhang@linux.dev>
> > Signed-off-by: Boqun Feng <boqun@kernel.org>
> > ---
> > @Zqiang, I put your name as Suggested-by because you proposed the same
> > idea, let me know if you rather not have it.
> > 
> > @Joel, I did two updates (including your test feedback, other one is
> > call irq_work_sync() when we clean the srcu_struct), please give it a
> > try.
> 
> I'm getting this at boot with this patch applied (testing directly from
> Joel's branch rcu/dev):
> 
> [    0.639477] DEBUG_LOCKS_WARN_ON(lockdep_hardirq_context())

My bad, this is missing:

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 73aef361a524..e08aaacad695 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -1993,13 +1993,14 @@ static void srcu_irq_work(struct irq_work *work)
        struct srcu_struct *ssp;
        struct srcu_usage *sup;
        unsigned long delay;
+       unsigned long flags;

        sup = container_of(work, struct srcu_usage, irq_work);
        ssp = sup->srcu_ssp;

-       raw_spin_lock_irq_rcu_node(ssp->srcu_sup);
+       raw_spin_lock_irqsave_rcu_node(ssp->srcu_sup, flags);
        delay = srcu_get_delay(ssp);
-       raw_spin_unlock_irq_rcu_node(ssp->srcu_sup);
+       raw_spin_unlock_irqrestore_rcu_node(ssp->srcu_sup, flags);

        queue_delayed_work(rcu_gp_wq, &sup->work, !!delay);
 }

Regards,
Boqun

> [    0.639479] WARNING: kernel/locking/lockdep.c:4404 at lockdep_hardirqs_on_prepare+0x15e/0x1a0, CPU#0: swapper/0/1
> [    0.639507] Modules linked in:
> [    0.639507] CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted 7.0.0-rc3-virtme #5 PREEMPT(full)
> [    0.639507] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
> [    0.639507] RIP: 0010:lockdep_hardirqs_on_prepare+0x165/0x1a0
> [    0.639507] Code: cc 90 e8 0e b1 4e 00 85 c0 74 0a 8b 35 54 08 43 02 85 f6 74 31 90 5b c3 cc cc cc cc 48 8d 3d d2 3c 44 02 48 c7 c6 b9 58 c5 b2 <67> 48 0f b9 3a eb ac 48 8d 3d cd 3c 44 02 48 c7 c6 37 55 c5 b2 67
> [    0.639507] RSP: 0018:ffffd26700003f58 EFLAGS: 00010046
> [    0.639507] RAX: 0000000000000001 RBX: ffffffffb35719f8 RCX: 0000000000000001
> [    0.639507] RDX: 0000000000000000 RSI: ffffffffb2c558b9 RDI: ffffffffb36bf810
> [    0.639507] RBP: ffffffffb35718e0 R08: 0000000000000001 R09: 0000000000000000
> [    0.639507] R10: 0000000000000001 R11: 000000007cb360a8 R12: 0000000000000000
> [    0.639507] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
> [    0.639507] FS:  0000000000000000(0000) GS:ffff8a7e076f6000(0000) knlGS:0000000000000000
> [    0.639507] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [    0.639507] CR2: ffff8a7dbffff000 CR3: 0000000023452000 CR4: 0000000000750ef0
> [    0.639507] PKRU: 55555554
> [    0.639507] Call Trace:
> [    0.639507]  <IRQ>
> [    0.639507]  trace_hardirqs_on+0x18/0x100
> [    0.639507]  _raw_spin_unlock_irq+0x28/0x50
> [    0.639507]  srcu_irq_work+0x63/0x90
> [    0.639507]  irq_work_single+0x69/0x90
> [    0.639507]  irq_work_run_list+0x26/0x40
> [    0.639507]  irq_work_run+0x18/0x30
> [    0.639507]  __sysvec_irq_work+0x35/0x1b0
> [    0.639507]  ? irq_exit_rcu+0xe/0x20
> [    0.639507]  sysvec_irq_work+0x6e/0x80
> [    0.639507]  </IRQ>
> [    0.639507]  <TASK>
> [    0.639507]  asm_sysvec_irq_work+0x1a/0x20
> [    0.639507] RIP: 0010:_raw_spin_unlock_irqrestore+0x36/0x70
> [    0.639507] Code: f5 53 48 8b 74 24 10 48 89 fb 48 83 c7 18 e8 b1 3d 26 ff 48 89 df e8 d9 a1 26 ff f7 c5 00 02 00 00 75 17 9c 58 f6 c4 02 75 2b <65> ff 0d 33 62 f0 01 74 16 5b 5d c3 cc cc cc cc e8 f5 83 35 ff 9c
> [    0.639507] RSP: 0018:ffffd26700013d48 EFLAGS: 00000246
> [    0.639507] RAX: 0000000000000092 RBX: ffffffffb35719f8 RCX: ffffffffb2017e0b
> [    0.639507] RDX: ffff8a7d80338000 RSI: 0000000000000000 RDI: ffffffffb2017e0b
> [    0.639507] RBP: 0000000000000282 R08: 0000000000000000 R09: 0000000000000001
> [    0.639507] R10: 0000000000000001 R11: 000000007cb360a8 R12: ffff8a7dbb628a40
> [    0.639507] R13: 0000000000000000 R14: ffffffffb3571940 R15: 0000000000000001
> [    0.639507]  ? _raw_spin_unlock_irqrestore+0x4b/0x70
> [    0.639507]  ? _raw_spin_unlock_irqrestore+0x4b/0x70
> [    0.639507]  srcu_gp_start_if_needed+0x37a/0x520
> [    0.639507]  ? __pfx_rcu_init_tasks_generic+0x10/0x10
> [    0.639507]  __synchronize_srcu+0xf6/0x1b0
> [    0.639507]  ? __pfx_wakeme_after_rcu+0x10/0x10
> [    0.639507]  ? __pfx_rcu_init_tasks_generic+0x10/0x10
> [    0.639507]  rcu_init_tasks_generic+0xfe/0x120
> [    0.639507]  do_one_initcall+0x6f/0x300
> [    0.639507]  kernel_init_freeable+0x24b/0x2b0
> [    0.639507]  ? __pfx_kernel_init+0x10/0x10
> [    0.639507]  kernel_init+0x1a/0x130
> [    0.639507]  ret_from_fork+0x2bd/0x370
> [    0.639507]  ? __pfx_kernel_init+0x10/0x10
> [    0.639507]  ret_from_fork_asm+0x1a/0x30
> [    0.639507]  </TASK>
> [    0.639507] irq event stamp: 6418
> [    0.639507] hardirqs last  enabled at (6417): [<ffffffffb2017e0b>] _raw_spin_unlock_irqrestore+0x4b/0x70
> [    0.639507] hardirqs last disabled at (6418): [<ffffffffb20013fe>] sysvec_irq_work+0xe/0x80
> [    0.639507] softirqs last  enabled at (6406): [<ffffffffb11ce7b6>] __irq_exit_rcu+0x96/0xc0
> [    0.639507] softirqs last disabled at (6401): [<ffffffffb11ce7b6>] __irq_exit_rcu+0x96/0xc0
> [    0.639507] ---[ end trace 0000000000000000 ]---
> 
> Thanks,
> -Andrea