Re: shell hung in fork system call

> On Jul 9, 2023, at 19:25, Konstantin Belousov <kostikbel@gmail.com> wrote:
> 
> On Sun, Jul 09, 2023 at 10:41:27PM +0000, John F Carr wrote:
>> Kernel and system at a146207d66f320ed239c1059de9df854b66b55b7 plus some irrelevant local changes, four 64 bit ARM processors, make.conf sets CPUTYPE?=cortex-a57.
>> 
>> I typed ^C while /bin/sh was starting a pipeline and my shell got hung in the middle of fork().
>> 
>>> From the terminal:
>> 
>> # git log --oneline --|more
>> ^C^C^C
>> load: 3.26  cmd: sh 95505 [fork] 5308.67r 0.00u 0.03s 0% 2860k
>> mi_switch+0x198 sleepq_switch+0xfc sleepq_timedwait+0x40 _sleep+0x264 fork1+0x67c sys_fork+0x34 do_el0_sync+0x4c8 handle_el0_sync+0x44 
>> load: 3.16  cmd: sh 95505 [fork] 5311.75r 0.00u 0.03s 0% 2860k
>> mi_switch+0x198 sleepq_switch+0xfc sleepq_timedwait+0x40 _sleep+0x264 fork1+0x67c sys_fork+0x34 do_el0_sync+0x4c8 handle_el0_sync+0x44 
>> 
>> According to ps -d on another terminal the shell has no children:
>> 
>>  PID TT  STAT       TIME COMMAND
>> [...]
>>  873 u0  IWs     0:00.00 `-- login [pam] (login)
>>  874 u0  I       0:00.17   `-- -sh (sh)
>> 95504 u0  I       0:00.01     `-- su -
>> 95505 u0  D+      0:00.05       `-- -su (sh)
>> [...]
>> 
>> Nothing on the (115200 bps serial) console.  No change in system performance.
>> 
>> The system is busy copying a large amount of data from the network to a ZFS pool on spinning disks.  The git|more pipeline could have taken some time to get going while I/O requests worked their way through the queue.  It would not have touched the busy pool, only the zroot pool on an SSD.
>> 
>> Has anything changed recently that might cause this?
> 
> There was some change around fork, but your sleep seems to be not from
> that change.  Can you show the wait channel for the process?  Do something
> like
> $ ps alxww
> 

 UID   PID  PPID  C PRI NI   VSZ   RSS MWCHAN   STAT TT        TIME COMMAND
   0 95505 95504  2  20  0 13508  2876 fork     D+   u0     0:00.13 -su (sh)

This is probably the same information displayed as [fork] in the output from ^T.

Does it correspond to the source line

	pause("fork", hz / 2);

?