Hi Oleg! I've tried both 5.3.18 and 5.10.0. The behavior is the same. The important thing is to run "exec strace -p ..." on the second terminal to create the loop A->B->A. So the last line from the first strace we see is: ptrace(PTRACE_SEIZE, 1990, NULL, PTRACE_O_TRACESYSGOOD|PTRACE_O_TRACEEXEC|PTRACE_O_TRACEEXIT I.e. it printed the syscall prior to its execution and hanged after the execution. izh@suse2:~> ps awux|grep strace izh 1891 0.0 0.0 24752 3828 pts/1 ts+ 19:52 0:00 strace -p 1990 izh 1990 0.0 0.0 24752 3628 pts/0 t+ 19:53 0:00 strace -p 1891 izh@suse2:~> kill 1990 1891 izh@suse2:~> kill -9 1990 1891 izh@suse2:~> sudo cat /proc/1891/stack [sudo] password for root: [<0>] ptrace_stop+0x14a/0x260 [<0>] ptrace_do_notify+0x91/0xb0 [<0>] ptrace_notify+0x4e/0x70 [<0>] do_exit+0x910/0xb70 [<0>] do_group_exit+0x3a/0xa0 [<0>] get_signal+0x124/0x800 [<0>] arch_do_signal_or_restart+0xa9/0x290 [<0>] exit_to_user_mode_prepare+0xe7/0x1a0 [<0>] syscall_exit_to_user_mode+0x18/0x40 [<0>] entry_SYSCALL_64_after_hwframe+0x44/0xa9 izh@suse2:~> sudo cat /proc/1990/stack [<0>] ptrace_stop+0x14a/0x260 [<0>] ptrace_do_notify+0x91/0xb0 [<0>] ptrace_notify+0x4e/0x70 [<0>] do_exit+0x910/0xb70 [<0>] do_group_exit+0x3a/0xa0 [<0>] get_signal+0x124/0x800 [<0>] arch_do_signal_or_restart+0xa9/0x290 [<0>] exit_to_user_mode_prepare+0xe7/0x1a0 [<0>] syscall_exit_to_user_mode+0x18/0x40 [<0>] entry_SYSCALL_64_after_hwframe+0x44/0xa9 izh@suse2:~> cat /proc/1891/status Name: strace Umask: 0022 State: t (tracing stop) Tgid: 1891 Ngid: 0 Pid: 1891 PPid: 1890 TracerPid: 1990 Uid: 1000 1000 1000 1000 Gid: 100 100 100 100 FDSize: 256 Groups: 100 NStgid: 1891 NSpid: 1891 NSpgid: 1891 NSsid: 1891 VmPeak: 24752 kB VmSize: 24752 kB VmLck: 0 kB VmPin: 0 kB VmHWM: 3828 kB VmRSS: 3828 kB RssAnon: 520 kB RssFile: 3308 kB RssShmem: 0 kB VmData: 284 kB VmStk: 132 kB VmExe: 1108 kB VmLib: 2828 kB VmPTE: 80 kB VmSwap: 0 kB HugetlbPages: 0 kB CoreDumping: 0 THP_enabled: 1 Threads: 1 SigQ: 4/15639 SigPnd: 0000000000000000 ShdPnd: 0000000000014100 SigBlk: 0000000000002000 SigIgn: 0000000000300000 SigCgt: 0000000180007007 CapInh: 0000000000000000 CapPrm: 0000000000000000 CapEff: 0000000000000000 CapBnd: 000001ffffffffff CapAmb: 0000000000000000 NoNewPrivs: 0 Seccomp: 0 Seccomp_filters: 0 Speculation_Store_Bypass: vulnerable SpeculationIndirectBranch: always enabled Cpus_allowed: 7 Cpus_allowed_list: 0-2 Mems_allowed: 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000001 Mems_allowed_list: 0 voluntary_ctxt_switches: 1561 nonvoluntary_ctxt_switches: 7 izh@suse2:~> cat /proc/1990/status Name: strace Umask: 0022 State: t (tracing stop) Tgid: 1990 Ngid: 0 Pid: 1990 PPid: 1847 TracerPid: 1891 Uid: 1000 1000 1000 1000 Gid: 100 100 100 100 FDSize: 256 Groups: 100 NStgid: 1990 NSpid: 1990 NSpgid: 1990 NSsid: 1847 VmPeak: 24752 kB VmSize: 24752 kB VmLck: 0 kB VmPin: 0 kB VmHWM: 3628 kB VmRSS: 3628 kB RssAnon: 520 kB RssFile: 3108 kB RssShmem: 0 kB VmData: 284 kB VmStk: 132 kB VmExe: 1108 kB VmLib: 2828 kB VmPTE: 88 kB VmSwap: 0 kB HugetlbPages: 0 kB CoreDumping: 0 THP_enabled: 1 Threads: 1 SigQ: 4/15639 SigPnd: 0000000000000000 ShdPnd: 0000000000014100 SigBlk: 0000000000002000 SigIgn: 0000000000300000 SigCgt: 0000000180007007 CapInh: 0000000000000000 CapPrm: 0000000000000000 CapEff: 0000000000000000 CapBnd: 000001ffffffffff CapAmb: 0000000000000000 NoNewPrivs: 0 Seccomp: 0 Seccomp_filters: 0 Speculation_Store_Bypass: vulnerable SpeculationIndirectBranch: always enabled Cpus_allowed: 7 Cpus_allowed_list: 0-2 Mems_allowed: 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000001 Mems_allowed_list: 0 voluntary_ctxt_switches: 180 nonvoluntary_ctxt_switches: 848 On 29.03.2021 19:49, Oleg Nesterov wrote:
On 03/29, Igor Zhbanov wrote:Mutual debugging of 2 processes can stuck in unkillable stopped statecan't reproduce and can't understand...Hi! When one process, let's say "A", is tracing the another process "B", and the process "B" is trying to attach to the process "A", then both of them are getting stuck in the "t+" state. And they are ignoring all of the signals including the SIGKILL,Why do you think so? What is your kernel version? "t" means TASK_TRACED, SIGKILL should wake it up and terminate.so it is not possible to terminate them without a reboot. To reproduce: 1) Run two terminals 2) Attach with "strace -p ..." from the first terminal to the shell (bash) of the second terminal. 3) In the second terminal run "exec strace -p ..." to attach to the PID of the first strace. Then you'll see that the second strace is hanging without any output. And the first strace will output following and hang too: ptrace(PTRACE_SEIZE, 11795, NULL, PTRACE_O_TRACESYSGOOD|PTRACE_O_TRACEEXEC|PTRACE_O_TRACEEXIT (The 11795 is the PID of the first strace itself.) And in the process list you will see following: ps awux | grep strace user 11776 0.0 0.0 24752 2248 pts/3 t+ 13:53 0:00 strace -p 11795 user 11795 0.0 0.0 24752 3888 pts/1 t+ 13:54 0:00 strace -p 11776OK, may be they sleep in PTRACE_EVENT_EXIT? After you tried to send SIGKILL? please show us the output from "cat /proc/{11795,11776}/stack". And "cat /proc/{11795,11776}/status" just in case.
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