[AGENT++] Crash in function MibLeaf::get_value where object value calls a pure virtual function

[Frank Fock]

Hi,

The crash is most likely caused by a missing synchronization in the 
instrumentation code
(your code).

When accessing Mib data always lock in this order

myMib->lock_mib();

<search/lookup mibEntry>

mibEntry->start_synch();
myMib->unlock_mib();

<do something with mibEntry>

mibEntry->end_synch();

The unlock is done early (right after mibEntry is locked)
to allow concurrent operations in the agent (safely).

Best regards,
Frank

Am 07.02.2013 11:30, schrieb Jacquemin, Jean-Philippe:
> Dear All,
>
>   
>
> I am having a crash when porting my snmp agent to another CPU.
>
> This is due to an access to a method that is at that time a pure
> virtual.
>
> Here is the back trace:
>
>   
>
> Program received signal SIGABRT, Aborted.
>
> [Switching to LWP 1706]
>
> 0x4825319c in raise () from /lib/libc.so.0
>
> (gdb) where
>
> #0  0x4825319c in raise () from /lib/libc.so.0
>
> #1  0x4824da80 in abort () from /lib/libc.so.0
>
> #2  0x4815f740 in __gnu_cxx::__verbose_terminate_handler() ()
>
>     from /usr/lib/libstdc++.so.6
>
> #3  0x4815b510 in ?? () from /usr/lib/libstdc++.so.6
>
> #4  0x4815b564 in std::terminate() () from /usr/lib/libstdc++.so.6
>
> #5  0x4815d0b8 in __cxa_pure_virtual () from /usr/lib/libstdc++.so.6
>
> #6  0x10033a04 in Vb::set_value (this=0x48293be8, val=...)
>
>      at ../../../snmp++/include/snmp_pp/vb.h:161
>
> #7  0x100460e8 in Agentpp::MibLeaf::get_value (this=0x10183110) at
> mib.cpp:342
>
> #8  0x1004665c in Agentpp::MibLeaf::get_request (this=0x10183110, req=
>
>      0x101dee08, ind=5) at mib.cpp:396
>
> #9  0x1004f2d8 in Agentpp::MibTable::get_next_request (this=0x1017edb0,
>
>      req=0x101dee08, ind=5) at mib.cpp:2075
>
> #10 0x10056070 in Agentpp::Mib::process_request (this=0x10150d10,
>
>      req=0x101dee08, reqind=5) at mib.cpp:3355
>
> #11 0x10056a8c in Agentpp::Mib::do_process_request (this=0x10150d10,
>
>      req=0x101dee08) at mib.cpp:3542
>
> #12 0x1005e50c in Agentpp::MibTask::run (this=0x101722a8) at
> threads.cpp:957
>
> #13 0x1005d660 in Agentpp::TaskManager::run (this=0x101522e0)
>
>      at threads.cpp:779
>
> #14 0x1005cd80 in Agentpp::thread_starter (t=0x10152330) at
> threads.cpp:488
>
> #15 0x480223dc in ?? () from /lib/libpthread.so.0
>
> ---Type <return> to continue, or q <return> to quit---
>
> #16 0x482529ec in clone () from /lib/libc.so.0
>
>   
>
>   
>
> Since the code is running fine on another CPU (same CPU family but
> difference ressources and obviously speed) I am thinking of a race
> condition, that would show now an issue that has always been present.
>
> It seems here that the variable binding objet which holds the mibleaf
> value is being destroyed while accessed here. Which would result in that
> call to the pure virtural function.
>
>   
>
> When looking at the code of function get_request, we noticed that the
> mib is unlocked in the middle of the synchronization of "entry".
>
>   
>
>                          entry->start_synch();
>
>                          unlock_mib();
>
>                          entry->get_request(req, reqind);
>
>                          entry->end_synch();
>
>   
>
> This is probably made to avoid a deadlock. So we started to look in the
> code what could cause the deadlock and could not figure it out.
>
> So we moved the unlock at the end of this sequence, and since then it
> works fine.
>
>   
>
> However it seems that the unlock was put there specially for a reason.
>
> Can anyone elaborate on that ?
>
> Besides, if there is a possible deadlock, what would be the impact of
> changing the mutex which is behind the mib lock function to recursive?
>
>   
>
> Another additional information, the crash is happening with the agent
> running on the new CPU but running a 3.x kernel now, while on the old
> CPU is was using a 2.6.x kernel.
>
>   
>
> Thanks a lot,
>
>   
>
> Regards,
>
> Jean-Philippe
>
>   
>
>   
>
>
>
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