Re: [WARNING] RCU stall in sock_def_readable()

[Steven Rostedt]

On Thu, 16 Apr 2026 17:16:11 -0700
"Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote:

> One "hail Mary" thought is to revert this guy and see if it helps:
> 
> d41e37f26b31 ("rcu: Fix rcu_read_unlock() deadloop due to softirq")
> 
> This commit fixes a bug, so we cannot revert it in mainline, but there
> is some reason to believe that there are more bugs beyond the one that
> it fixed, and it might have (through no fault of its own) made those
> other bugs more probable.
> 
> Worth a try, anyway!

Hail mary's are worth a try, but the reason they call it a hail mary is
because it is unlikely to succeed :-p

run test ssh -t root@tracetest "trace-cmd record -p function -e syscalls /work/c/hackbench_64 50"
ssh -t root@tracetest "trace-cmd record -p function -e syscalls /work/c/hackbench_64 50" ... [  209.590500] rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
[  209.592620] rcu:     Tasks blocked on level-0 rcu_node (CPUs 0-3): P3151/1:b..l
[  209.595266] rcu:     (detected by 0, t=6502 jiffies, g=29673, q=186 ncpus=4)
[  209.597557] task:hackbench_64    state:R  running task     stack:0     pid:3151  tgid:3151  ppid:3144   task_flags:0x400000 flags:0x00080000
[  209.601871] Call Trace:
[  209.602852]  <TASK>
[  209.603752]  __schedule+0x4ac/0x12f0
[  209.605172]  preempt_schedule_common+0x26/0xe0
[  209.606755]  ? preempt_schedule_thunk+0x16/0x30
[  209.608337]  preempt_schedule_thunk+0x16/0x30
[  209.609973]  ? _raw_spin_unlock_irqrestore+0x39/0x70
[  209.611688]  _raw_spin_unlock_irqrestore+0x5d/0x70
[  209.613408]  sock_def_readable+0x9c/0x2b0
[  209.614841]  unix_stream_sendmsg+0x2d7/0x710
[  209.616420]  sock_write_iter+0x185/0x190
[  209.617934]  vfs_write+0x457/0x5b0
[  209.619242]  ksys_write+0xc8/0xf0
[  209.620532]  do_syscall_64+0x117/0x1660
[  209.621936]  ? irqentry_exit+0xd9/0x690
[  209.623319]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
[  209.625199] RIP: 0033:0x7f603e8e5190
[  209.626628] RSP: 002b:00007ffd003f99c8 EFLAGS: 00000202 ORIG_RAX: 0000000000000001
[  209.629304] RAX: ffffffffffffffda RBX: 00007ffd003f9b58 RCX: 00007f603e8e5190
[  209.631710] RDX: 0000000000000001 RSI: 00007ffd003f99ef RDI: 0000000000000006
[  209.634200] RBP: 00007ffd003f9a40 R08: 0011861580000000 R09: 0000000000000000
[  209.636638] R10: 00007f603e8064d0 R11: 0000000000000202 R12: 0000000000000000
[  209.639050] R13: 00007ffd003f9b70 R14: 00005637df126dd8 R15: 00007f603ea10020
[  209.641600]  </TASK>
Detected kernel crash!


That was with the revert :-(

-- Steve