Re: closefrom blocking, wchan urdlck

On Mon, Dec 27, 2021 at 01:39:11PM +0100, Jan Mikkelsen wrote:
> Hi,
> 
> (On 11.2)
> 
> I am occasionally seeing closefrom() block in a child process created by a call to pdfork().
> 
> When this does happen, it is very early after the process has started, while other threads are being created elsewhere in the process. I cannot reproduce it after the thread creation is complete. According to the sigaction man page, this should be async signal safe.
> 
> Stack trace from the call to closefrom():
> 
>   * frame #0: 0x000000080090276c libthr.so.3`_umtx_op_err at _umtx_op_err.S:37
>     frame #1: 0x00000008008f6121 libthr.so.3`__thr_rwlock_rdlock(rwlock=<unavailable>, flags=<unavailable>, tsp=<unavailable>) at thr_umtx.c:307:10
>     frame #2: 0x00000008008ff1ac libthr.so.3`_thr_rtld_rlock_acquire [inlined] _thr_rwlock_rdlock(rwlock=0x0000000800911600, flags=0, tsp=0x0000000000000000) at thr_umtx.h:232:10
>     frame #3: 0x00000008008ff19b libthr.so.3`_thr_rtld_rlock_acquire(lock=0x0000000800911600) at thr_rtld.c:125
>     frame #4: 0x000000080075332b ld-elf.so.1`rlock_acquire(lock=0x0000000800765270, lockstate=0x00007fffdfbfb8d0) at rtld_lock.c:208:2
>     frame #5: 0x000000080074ba20 ld-elf.so.1`_rtld_bind(obj=0x0000000800769000, reloff=6072) at rtld.c:861:5
>     frame #6: 0x0000000800747c7d ld-elf.so.1`_rtld_bind_start at rtld_start.S:121
>     frame #7: 0x00000000006562d3 prog`Twio::ProcHandle::spawn(this=<unavailable>, command="/bin/echo", args=0x0000000800d7e000, descriptor_mapping=<unavailable>, descriptor_end=3) at prochandle_pdfork.cpp:308:2
And where is the closefrom() call in the demonstrated trace?

What version of the system do you use?
You need at least cbdec8db18b533f6d7be (on HEAD) or a5659943e37a74c96e
(stable/13) for pdfork() to behave sanely.  But you still not allowed to
call non-async signal safe functions in the child before exec.