In CONFIG_PREEMPT=n kernel a softlockup was observed while the for loop in exit_sem. Apparently it's possible for the loop to take quite a long time and it doesn't have a scheduling point in it. Since the codes is executing under an rcu read section this may also cause rcu stalls, which in turn block synchronize_rcu operations, which more or less de-stabilises the whole system.
Fix this by introducing a cond_resched at the beginning of the loop. Signed-off-by: Nikolay Borisov <[email protected]> --- So this patch fixes the following: NMI watchdog: BUG: soft lockup - CPU#10 stuck for 23s! [httpd:18119] CPU: 10 PID: 18119 Comm: httpd Tainted: G O 4.4.20-clouder2 #6 Hardware name: Supermicro X10DRi/X10DRi, BIOS 1.1 04/14/2015 task: ffff88348d695280 ti: ffff881c95550000 task.ti: ffff881c95550000 RIP: 0010:[<ffffffff81614bc7>] [<ffffffff81614bc7>] _raw_spin_lock+0x17/0x30 RSP: 0018:ffff881c95553e40 EFLAGS: 00000246 RAX: 0000000000000000 RBX: ffff883161b1eea8 RCX: 000000000000000d RDX: 0000000000000001 RSI: 000000000000000e RDI: ffff883161b1eea4 RBP: ffff881c95553ea0 R08: ffff881c95553e68 R09: ffff883fef376f88 R10: ffff881fffb58c20 R11: ffffea0072556600 R12: ffff883161b1eea0 R13: ffff88348d695280 R14: ffff883dec427000 R15: ffff8831621672a0 FS: 0000000000000000(0000) GS:ffff881fffb40000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007f3b3723e020 CR3: 0000000001c0a000 CR4: 00000000001406e0 Stack: ffffffff8129717c ffff88348d695280 ffff883161b1eea4 ffffea000f2359c0 000480094089b340 ffff881c95553e68 ffff881c95553e68 ffff88348d695280 0000000000000000 00007ffe01ca0170 ffff883217c74280 ffff883217c742e8 Call Trace: [<ffffffff8129717c>] ? exit_sem+0x7c/0x280 [<ffffffff81055548>] do_exit+0x338/0xb40 [<ffffffff81055dd3>] do_group_exit+0x43/0xd0 [<ffffffff81055e74>] SyS_exit_group+0x14/0x20 [<ffffffff81614f5b>] entry_SYSCALL_64_fastpath+0x16/0x6e ipc/sem.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/ipc/sem.c b/ipc/sem.c index 1f97a24871c5..8563535cbd0e 100644 --- a/ipc/sem.c +++ b/ipc/sem.c @@ -2088,6 +2088,8 @@ void exit_sem(struct task_struct *tsk) struct list_head tasks; int semid, i; + cond_resched(); + rcu_read_lock(); un = list_entry_rcu(ulp->list_proc.next, struct sem_undo, list_proc); -- 2.5.0
