[Question] softlockup in __fsnotify_update_child_dentry_flags

[yangerkun <yangerkun@xxxxxxxxxx>]

Hi,

Run process parallel which each code show as below(ARM64 + 2T ram):

#include<stdlib.h>
#include<stdio.h>
#include<string.h>
#include<time.h>


int main()
{
    const char *forname="_dOeSnotExist_.db";
    int i;
    char filename[100]="";
    struct timespec time1 = {0, 0};
    for(;;)
    {
        clock_gettime(CLOCK_REALTIME, &time1);
        for(i=0; i < 10000; i++) {

sprintf(filename,"/%d%d%d%s",time1.tv_sec,time1.tv_nsec,i,forname);
            access(filename,0);
            memset(filename,'\0',100);
        }
    }
    return 0;


}

Sometimes later, system will report softlockup with stack:

#10 [ffff00002ffc39f0] el1_irq at ffff000008083474
     PC: ffff00000814570c  [queued_spin_lock_slowpath+420]
     LR: ffff000008146ca4  [do_raw_spin_lock+260]
     SP: ffff00002ffc3a00  PSTATE: 80000005
    X29: ffff00002ffc3a00  X28: ffff802fa1f62f40  X27: 0000000000000002
    X26: ffff00002ffc3bac  X25: ffff801fac95bd60  X24: ffff00002ffc3bb0
    X23: ffff801fa8a9ce80  X22: ffff00002ffc3bb8  X21: 0000000000000002
    X20: 0000000000d80000  X19: ffff801fa8a9ced8  X18: 0000000000000000
    X17: 0000000000d80000  X16: 0000000000d80000  X15: 0000000000000000
    X14: 0000000000000000  X13: 0000000000000000  X12: 0000000000000000
    X11: 0000000000000000  X10: ffff801fa614c200   X9: ffff801ffbe68e00
     X8: 0000000000000000   X7: ffff809ffbfd4e00   X6: 0000000000040000
     X5: ffff00000976b788   X4: ffff801ffbe68e00   X3: ffff0000093c7000
     X2: 0000000000000000   X1: 0000000000000000   X0: ffff801ffbe68e08
#11 [ffff00002ffc3a00] queued_spin_lock_slowpath at ffff000008145708
#12 [ffff00002ffc3a20] do_raw_spin_lock at ffff000008146ca0
#13 [ffff00002ffc3a40] _raw_spin_lock at ffff000008d0da6c
#14 [ffff00002ffc3a60] lockref_get_not_dead at ffff0000085abca8
#15 [ffff00002ffc3a80] legitimize_path at ffff00000832aee0
#16 [ffff00002ffc3ab0] unlazy_walk at ffff00000832b09c
#17 [ffff00002ffc3ad0] lookup_fast at ffff00000832ba2c
#18 [ffff00002ffc3b40] walk_component at ffff00000832bc30
#19 [ffff00002ffc3bd0] path_lookupat at ffff00000832c5d8
#20 [ffff00002ffc3c40] filename_lookup at ffff00000832e464
#21 [ffff00002ffc3d70] user_path_at_empty at ffff00000832e67c
#22 [ffff00002ffc3db0] do_faccessat at ffff000008317750
#23 [ffff00002ffc3e40] __arm64_sys_faccessat at ffff00000831795c
#24 [ffff00002ffc3e60] el0_svc_common at ffff000008097514
#25 [ffff00002ffc3ea0] el0_svc_handler at ffff00000809762c
#26 [ffff00002ffc3ff0] el0_svc at ffff000008084144


We find the lock of lockref has been catched with cpu 40. And since 
there is too much negative dentry in root dentry's d_subdirs, traversing 
will spend so long time with holding d_lock of root dentry. So other 
thread waiting for the lockref.lock will softlockup.

For this problem, thought?

crash> bt -c 40
PID: 1      TASK: ffff801faf851f80  CPU: 40  COMMAND: "systemd"
 #0 [ffff000009c9bda0] crash_save_cpu at ffff0000081a280c
 #1 [ffff000009c9bf60] handle_IPI at ffff000008096850
 #2 [ffff000009c9bfd0] gic_handle_irq at ffff0000080818d0
--- <IRQ stack> ---
 #3 [ffff000009d5bcb0] el1_irq at ffff000008083474
     PC: ffff00000836c7a8  [__fsnotify_update_child_dentry_flags+160]
     LR: ffff00000836c770  [__fsnotify_update_child_dentry_flags+104]
     SP: ffff000009d5bcc0  PSTATE: 20000005
    X29: ffff000009d5bcc0  X28: ffff809fb33f4150  X27: ffff801f9e33e5e0
    X26: ffff000009b28f68  X25: ffff801f9e33e668  X24: ffff801fa8a9ced8
    X23: 0000000000000184  X22: ffff801fa8a9ce80  X21: ffff801fa8a9cf38
    X20: ffff801ec2499360  X19: ffff801ec24a3a48  X18: 0000000000000000
    X17: 0000000000240001  X16: 0000000000240001  X15: 0000000000000000
    X14: 0000000000000000  X13: 0000000000000000  X12: 0000000000000000
    X11: ffff809fb14276c8  X10: 0000000000000001   X9: ffff809ffbf38e00
     X8: 0000000000000000   X7: ffff802ffbeb7e00   X6: 0000000000a40000
     X5: 0000000000440000   X4: ffff809ffbf38e00   X3: ffff0000093c7000
     X2: 0000809ff2b71000   X1: 0000000000000000   X0: ffff801ec24a3af0
 #4 [ffff000009d5bcc0] __fsnotify_update_child_dentry_flags at 
ffff00000836c7a4
 #5 [ffff000009d5bd10] __fsnotify_update_child_dentry_flags at 
ffff00000836cb74
 #6 [ffff000009d5bd30] fsnotify_recalc_mask at ffff00000836d910
 #7 [ffff000009d5bd50] fsnotify_add_mark_locked at ffff00000836e0e4
 #8 [ffff000009d5bdd0] __arm64_sys_inotify_add_watch at ffff0000083704c8
 #9 [ffff000009d5be60] el0_svc_common at ffff000008097514
#10 [ffff000009d5bea0] el0_svc_handler at ffff00000809762c
#11 [ffff000009d5bff0] el0_svc at ffff000008084144