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Re: [PATCH] linux-user: un-register threads from RCU before exit
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From: |
Alex Bennée |
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Subject: |
Re: [PATCH] linux-user: un-register threads from RCU before exit |
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Date: |
Tue, 11 Feb 2020 21:58:18 +0000 |
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User-agent: |
mu4e 1.3.8; emacs 26.1 |
Peter Maydell <address@hidden> writes:
> On Tue, 11 Feb 2020 at 17:36, Alex Bennée <address@hidden> wrote:
>>
>> Through a mechanism I don't quite yet understand we can find ourselves
>> with a left over RCU thread when we exit group. This is a racy failure
>> that occurs for example with:
>>
>> alpha-linux-user running testthread
>> with libhowvec.so plugin
>> but only when run from make
>>
>> This may not be the correct fix but it seems to alleviate the
>> symptoms.
>
> This is weird. The only time we call preexit_cleanup()
> is when the next thing we do is to terminate the entire
> process all at once. (For some reason in one place
> we do that by calling _exit() and in another place
> by calling exit_group() -- I don't see why we need that
> inconsistency).
>
> I'm pretty sure the system emulation threads don't
> call rcu_unregister_thread() for the "whole process
> is going away" case, so something odd is happening here...
So what I see is (although possibly confused further by rr's capture):
End of pthread test.
[New Thread 7966.7967]
Thread 3 received signal SIGKILL, Killed.
[Switching to Thread 7966.7967]
0x0000000070000002 in ?? ()
(rr) bt
#0 0x0000000070000002 in ?? ()
#1 0x00007f36981a490e in _raw_syscall () at
/build/rr-79viaC/rr-5.2.0/src/preload/raw_syscall.S:120
#2 0x00007f36981a13fe in traced_raw_syscall (call=call@entry=0x7f369656ffa0)
at ./src/preload/syscallbuf.c:222
#3 0x00007f36981a271a in sys_xstat64 (call=<optimized out>) at
./src/preload/syscallbuf.c:2439
#4 syscall_hook_internal (call=0x7f369656ffa0) at
./src/preload/syscallbuf.c:2651
#5 syscall_hook (call=0x7f369656ffa0) at ./src/preload/syscallbuf.c:2687
#6 0x00007f36981a12da in _syscall_hook_trampoline () at
/build/rr-79viaC/rr-5.2.0/src/preload/syscall_hook.S:282
#7 0x00007f36981a130a in __morestack () at
/build/rr-79viaC/rr-5.2.0/src/preload/syscall_hook.S:417
#8 0x00007f36981a1310 in _syscall_hook_trampoline_48_3d_01_f0_ff_ff () at
/build/rr-79viaC/rr-5.2.0/src/preload/syscall_hook.S:423
#9 0x00007f369758bf5f in syscall () at
../sysdeps/unix/sysv/linux/x86_64/syscall.S:38
#10 0x0000556b768b764b in qemu_futex_wait (val=<optimized out>, f=<optimized
out>) at /home/alex/lsrc/qemu.git/util/qemu-thread-posix.c:455
#11 qemu_event_wait (ev=ev@entry=0x556b7897a608 <rcu_call_ready_event>) at
/home/alex/lsrc/qemu.git/util/qemu-thread-posix.c:459
#12 0x0000556b768be29a in call_rcu_thread (opaque=opaque@entry=0x0) at
/home/alex/lsrc/qemu.git/util/rcu.c:260
#13 0x0000556b768b689a in qemu_thread_start (args=<optimized out>) at
/home/alex/lsrc/qemu.git/util/qemu-thread-posix.c:519
#14 0x00007f3697660fa3 in start_thread (arg=<optimized out>) at
pthread_create.c:486
#15 0x00007f36975914cf in clone () at
../sysdeps/unix/sysv/linux/x86_64/clone.S:95
(rr) info threads
Id Target Id Frame
* 3 Thread 7966.7967 (mmap_hardlink_3_qemu-alpha) 0x0000000070000002 in ??
()
(rr)
Although possibly it hasn't moved on from where it was during exit:
(rr) b preexit_cleanup
Breakpoint 1 at 0x556b768646d0: file
/home/alex/lsrc/qemu.git/linux-user/exit.c, line 31.
(rr) rc
Continuing.
[New Thread 7966.7966]
[Switching to Thread 7966.7966]
Thread 4 hit Breakpoint 1, preexit_cleanup (env=0x556b797aac40,
code=code@entry=0) at /home/alex/lsrc/qemu.git/linux-user/exit.c:31
31 rcu_unregister_thread();
(rr) bt
#0 preexit_cleanup (env=0x556b797aac40, code=code@entry=0) at
/home/alex/lsrc/qemu.git/linux-user/exit.c:31
#1 0x0000556b76850a63 in do_syscall1 (cpu_env=cpu_env@entry=0x556b797aac40,
num=num@entry=405, arg1=arg1@entry=0, arg2=arg2@entry=0,
arg3=arg3@entry=4832687680, arg4=arg4@entry=0, arg5=4095, arg6=4832686256,
arg8=0, arg7=0) at /home/alex/lsrc/qemu.git/linux-user/syscall.c:9373
#2 0x0000556b76859b88 in do_syscall (cpu_env=cpu_env@entry=0x556b797aac40,
num=405, arg1=0, arg2=0, arg3=<optimized out>, arg4=<optimized out>, arg5=4095,
arg6=4832686256,
arg7=0, arg8=0) at /home/alex/lsrc/qemu.git/linux-user/syscall.c:12110
#3 0x0000556b768645c6 in cpu_loop (env=0x556b797aac40) at
/home/alex/lsrc/qemu.git/linux-user/alpha/cpu_loop.c:109
#4 0x0000556b767e13de in main (argc=<optimized out>, argv=0x7ffe9d8f5ca8,
envp=<optimized out>) at /home/alex/lsrc/qemu.git/linux-user/main.c:865
(rr) info threads
Id Target Id Frame
3 Thread 7966.7967 (mmap_hardlink_3) 0x0000000070000002 in ?? ()
* 4 Thread 7966.7966 (mmap_hardlink_3) preexit_cleanup
(env=0x556b797aac40, code=code@entry=0) at
/home/alex/lsrc/qemu.git/linux-user/exit.c:31
(rr) thread 3
[Switching to thread 3 (Thread 7966.7967)]
#0 0x0000000070000002 in ?? ()
(rr) bt
#0 0x0000000070000002 in ?? ()
#1 0x00007f36981a490e in _raw_syscall () at
/build/rr-79viaC/rr-5.2.0/src/preload/raw_syscall.S:120
#2 0x00007f36981a13fe in traced_raw_syscall (call=call@entry=0x7f369656ffa0)
at ./src/preload/syscallbuf.c:222
#3 0x00007f36981a271a in sys_xstat64 (call=<optimized out>) at
./src/preload/syscallbuf.c:2439
#4 syscall_hook_internal (call=0x7f369656ffa0) at
./src/preload/syscallbuf.c:2651
#5 syscall_hook (call=0x7f369656ffa0) at ./src/preload/syscallbuf.c:2687
#6 0x00007f36981a12da in _syscall_hook_trampoline () at
/build/rr-79viaC/rr-5.2.0/src/preload/syscall_hook.S:282
#7 0x00007f36981a130a in __morestack () at
/build/rr-79viaC/rr-5.2.0/src/preload/syscall_hook.S:417
#8 0x00007f36981a1310 in _syscall_hook_trampoline_48_3d_01_f0_ff_ff () at
/build/rr-79viaC/rr-5.2.0/src/preload/syscall_hook.S:423
#9 0x00007f369758bf5f in syscall () at
../sysdeps/unix/sysv/linux/x86_64/syscall.S:38
#10 0x0000556b768b764b in qemu_futex_wait (val=<optimized out>, f=<optimized
out>) at /home/alex/lsrc/qemu.git/util/qemu-thread-posix.c:455
#11 qemu_event_wait (ev=ev@entry=0x556b7897a608 <rcu_call_ready_event>) at
/home/alex/lsrc/qemu.git/util/qemu-thread-posix.c:459
#12 0x0000556b768be29a in call_rcu_thread (opaque=opaque@entry=0x0) at
/home/alex/lsrc/qemu.git/util/rcu.c:260
#13 0x0000556b768b689a in qemu_thread_start (args=<optimized out>) at
/home/alex/lsrc/qemu.git/util/qemu-thread-posix.c:519
#14 0x00007f3697660fa3 in start_thread (arg=<optimized out>) at
pthread_create.c:486
#15 0x00007f36975914cf in clone () at
../sysdeps/unix/sysv/linux/x86_64/clone.S:95
(rr)
Of course it is occurring on my patched tree so I guess the unregister
approach doesn't actually help.
>
> thanks
> -- PMM
--
Alex Bennée