Re: closefrom blocking, wchan urdlck

> On 27 Dec 2021, at 16:03, Konstantin Belousov <kostikbel@gmail.com> wrote:
> 
> On Mon, Dec 27, 2021 at 03:54:57PM +0100, Jan Mikkelsen wrote:
>> 
>>> On 27 Dec 2021, at 14:52, Konstantin Belousov <kostikbel@gmail.com> wrote:
>>> 
>>> 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.
>> 
>> 
>> This is 12.2-p11. I just noticed that I wrote 11.2 above, that is incorrect.
>> 
>> Frame 7 is a call to closefrom(). The child process calls dup2(), closefrom(), signal() and then execv(). No other calls are made, and I believe closefrom() is meant to be async signal safe.
>> 
> Frame 7 cannot be a call to closefrom(), it would be resolved to closefrom()
> symbol would it be.

From lldb, attached to the hung process:

(lldb) 
frame #6: 0x0000000800748c7d ld-elf.so.1`_rtld_bind_start at rtld_start.S:121
   118 		leaq	(%rsi,%rsi,2),%rsi	# multiply by 3
   119 		leaq	(,%rsi,8),%rsi		# now 8, for 24 (sizeof Elf_Rela)
   120 	
-> 121 		call	_rtld_bind		# Transfer control to the binder
   122 		/* Now %rax contains the entry point of the function being called. */
   123 	
   124 		movq	%rax,0x60(%rsp)		# Store target over reloff argument
(lldb) 
frame #7: 0x0000000000656813 amt5-chefd`Twio::ProcHandle::spawn(this=<unavailable>, command="/bin/date", args=0x0000000800d5f010, descriptor_mapping=<unavailable>, descriptor_end=3) at prochandle_pdfork.cpp:304:2
   301 			_exit(127);
   302 		}
   303 	
-> 304 		closefrom(descriptor_end);
   305 	
   306 		signal(SIGPIPE, SIG_DFL);
   307 		signal(SIGALRM, SIG_DFL);
(lldb) 


>> The commit you can apply cleanly to 12.2, I’m running a build now. Are there other issues with pdfork in 12.2?
> 
> pdfork() with threading processes requires 21f749da82e755aafab1276 and the
> followup cbdec8db18b533f6d7be.  I do not believe any of this is in 12.3,
> and definitely not in 12.2.

Thanks, will check and apply.

Regards,

Jan M.