Re: [PING] Re: [PATCH v2 2/3] PR remote/19496, interrupted syscall in forking-threads-plus-bkpt

[Don Breazeal <donb at codesourcery dot com>]

Hi Pedro,
Did you have any more suggestions about handling the interrupted system
call, or shall we go with the "loop until we don't get -1 and
errno===EINTR" approach?
Thanks,
--Don

On 3/3/2016 10:19 AM, Don Breazeal wrote:
> Ping.
> I checked, the patch still applies cleanly to mainline.
> Thanks
> --Don
> 
> On 2/25/2016 9:28 AM, Don Breazeal wrote:
>> Ping
>> Thanks,
>> --Don
>>
>> On 2/10/2016 4:28 PM, Don Breazeal wrote:
>>> Hi Pedro,
>>>
>>> On 2/1/2016 11:38 AM, Pedro Alves wrote:
>>>> On 01/28/2016 12:48 AM, Don Breazeal wrote:
>>>>> This patch addresses "fork:Interrupted system call" (or wait:) failures
>>>>> in gdb.threads/forking-threads-plus-breakpoint.exp.
>>>>>
>>>>> The test program spawns ten threads, each of which do ten fork/waitpid
>>>>> sequences.  The cause of the problem was that when one of the fork
>>>>> children exited before the corresponding fork parent could initiate its
>>>>> waitpid for that child, a SIGCHLD was delivered and interrupted a fork
>>>>> or waitpid in another thread.
>>>
>>> In fact, I think my diagnosis here was incorrect, or at least incorrect
>>> in some cases.  I believe at least some of the interruptions are caused
>>> by SIGSTOP, sent by GDB when stopping all the threads.  More below.
>>>
>>>>>
>>>>> The fix was to wrap the system calls in a loop to retry the call if
>>>>> it was interrupted, like:
>>>>>
>>>>> do
>>>>>   {
>>>>>     pid = fork ();
>>>>>   }
>>>>> while (pid == -1 && errno == EINTR);
>>>>>
>>>>> Since this is a Linux-only test I figure it is OK to use errno and EINTR.
>>>>>
>>>>> Tested on Nios II Linux target with x86 Linux host.
>>>>
>>>> I'd prefer to avoid this if possible.  These loops potentially hide
>>>> bugs like ERESTARTSYS escaping out of a syscall and mishandling of
>>>> signals.  See bc9540e842eb5639ca59cb133adef211d252843c for example:
>>>>    https://sourceware.org/ml/gdb-patches/2015-02/msg00654.html
>>>>
>>>> How about setting SIGCHLD to SIG_IGN, or making SIGCHLD be SA_RESTART?
>>>
>>> I spent a couple of days trying to find an alternate solution, but
>>> couldn't find one that was reliable.  Here is a snapshot of what I tried:
>>>
>>> 1) SIG_IGN: results in an ECHILD from waitpid.  The man page for waitpid
>>>    says "This can happen for one's own child if the action for SIGCHLD is
>>>    set to SIG_IGN."
>>>
>>> 2) SA_RESTART: While waitpid is listed as a system call that can be
>>>    restarted by SA_RESTART, fork is not.  Even if I leave the "EINTR loop"
>>>    in place for fork, using SA_RESTART I still see an interrupted system
>>>    call for waitpid.  Possibly because the problem is SIGSTOP and not
>>>    SIGCHLD.
>>>
>>> 3) pthread_sigblock: With this set for SIGCHLD in all the threads, I
>>>    still saw an interrupted system call.  You can't block SIGSTOP.
>>>
>>> 4) pthread_sigblock with sigwait: using pthread_sigblock on all the
>>>    blockable signals with a signal thread that called sigwait for all
>>>    the signals in a loop, the signal thread would see a bunch of SIGCHLDs,
>>>    but there would eventually be an interrupted system call.
>>>
>>> 5) bsd_signal: this function is supposed to automatically restart blocking
>>>    system calls.  fork is not a blocking system call, but it doesn't help
>>>    for waitpid either.
>>>
>>> I found this in the ptrace(2) man page: "Note that a suppressed signal
>>> still causes system calls to return prematurely.  In this case, system
>>> calls will be restarted: the tracer will observe the tracee to reexecute
>>> the interrupted system call (or restart_syscall(2) system call for a few
>>> system calls which use a different mechanism for restarting) if the tracer
>>> uses PTRACE_SYSCALL.  Even system calls (such as poll(2)) which are not
>>> restartable after signal are restarted after signal is suppressed; however,
>>> kernel bugs exist which cause some system calls to fail with EINTR even
>>> though no observable signal is injected to the tracee."
>>>
>>> The GDB manual mentions something similar about interrupted system calls.
>>>
>>> So, the bottom line is that I haven't changed the fix for the interrupted
>>> system calls, because I can't find anything that works as well as the
>>> original fix.  Perhaps this test puts enough stress on the kernel that the
>>> kernel bugs mentioned above are exposed.
>>>
>>> One change I did make from the previous version was to increase the
>>> timeout to 90 seconds, which was necessary to get more reliable results
>>> on the Nios II target.
>>>
>>> Let me know what you think.
>>> Thanks!
>>> --Don
>>>
>>> ---
>>> This patch addresses "fork:Interrupted system call" (or wait:) failures
>>> in gdb.threads/forking-threads-plus-breakpoint.exp.
>>>
>>> The test program spawns ten threads, each of which do ten fork/waitpid
>>> sequences.  The cause of the problem was that when one of the fork
>>> children exited before the corresponding fork parent could initiate its
>>> waitpid for that child, a SIGCHLD was delivered and interrupted a fork
>>> or waitpid in another thread.
>>>
>>> The fix was to wrap the system calls in a loop to retry the call if
>>> it was interrupted, like:
>>>
>>> do
>>>   {
>>>     pid = fork ();
>>>   }
>>> while (pid == -1 && errno == EINTR);
>>>
>>> Since this is a Linux-only test I figure it is OK to use errno and EINTR.
>>> I tried a number of alternative fixes using SIG_IGN, SA_RESTART,
>>> pthread_sigblock, and bsd_signal, but none of these worked as well.
>>>
>>> Tested on Nios II Linux target with x86 Linux host.
>>>
>>> gdb/testsuite/ChangeLog:
>>> 2016-02-10  Don Breazeal  <donb@codesourcery.com>
>>>
>>> 	* gdb.threads/forking-threads-plus-breakpoint.c (thread_forks):
>>> 	Retry fork and waitpid on interrupted system call errors.
>>> 	* gdb.threads/forking-threads-plus-breakpoint.exp: (do_test):
>>> 	Increase timeout to 90.
>>>
>>> ---
>>>  .../gdb.threads/forking-threads-plus-breakpoint.c          | 14 ++++++++++++--
>>>  .../gdb.threads/forking-threads-plus-breakpoint.exp        |  3 +++
>>>  2 files changed, 15 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.c b/gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.c
>>> index fc64d93..c169e18 100644
>>> --- a/gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.c
>>> +++ b/gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.c
>>> @@ -22,6 +22,7 @@
>>>  #include <sys/types.h>
>>>  #include <sys/wait.h>
>>>  #include <stdlib.h>
>>> +#include <errno.h>
>>>  
>>>  /* Number of threads.  Each thread continuously spawns a fork and wait
>>>     for it.  If we have another thread continuously start a step over,
>>> @@ -49,14 +50,23 @@ thread_forks (void *arg)
>>>      {
>>>        pid_t pid;
>>>  
>>> -      pid = fork ();
>>> +      do
>>> +	{
>>> +	  pid = fork ();
>>> +	}
>>> +      while (pid == -1 && errno == EINTR);
>>>  
>>>        if (pid > 0)
>>>  	{
>>>  	  int status;
>>>  
>>>  	  /* Parent.  */
>>> -	  pid = waitpid (pid, &status, 0);
>>> +	  do
>>> +	    {
>>> +	      pid = waitpid (pid, &status, 0);
>>> +	    }
>>> +	  while (pid == -1 && errno == EINTR);
>>> +
>>>  	  if (pid == -1)
>>>  	    {
>>>  	      perror ("wait");
>>> diff --git a/gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.exp b/gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.exp
>>> index ff3ca9a..6889c2b 100644
>>> --- a/gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.exp
>>> +++ b/gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.exp
>>> @@ -73,6 +73,9 @@ proc do_test { cond_bp_target detach_on_fork displaced } {
>>>      global linenum
>>>      global is_remote_target
>>>  
>>> +    global timeout
>>> +    set timeout 90
>>> +
>>>      set saved_gdbflags $GDBFLAGS
>>>      set GDBFLAGS [concat $GDBFLAGS " -ex \"set non-stop on\""]
>>>      clean_restart $binfile
>>>
>>
>