On Tue, May 14, 2013 at 12:14:53PM -0700, Jerry Cain wrote: > I am relatively new to libxml2, and I've stumbled across an issue around > xmlReadFile and threading. I am running on x86_64-linux, and I built and > installed libxml2 myself, ensuring that the build was configured with > --with-threads. I am coding in C++11, using the C++11 thread package, which > is itself implemented in terms of pthreads in g++4.6, which I'm currently > using. I've examined the bug database, and I've seen nothing to imply this > general of a problem with threads and libxml2 exists. > > I am seeing very occasional deadlock in an application with four threads—one > main thread, which spawns three child threads, each of which attempts to > parse a remote RSS document, as with: [...] > (gdb) bt > #0 pthread_cond_wait@@GLIBC_2.3.2 () at > ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:162 > #1 0x00007f0afc1e5d23 in xmlRMutexLock () from > /usr/lib/x86_64-linux-gnu/libxml2.so.2 > #2 0x00007f0afc1e20b9 in ?? () from /usr/lib/x86_64-linux-gnu/libxml2.so.2 > #3 0x00007f0afc1e2648 in ?? () from /usr/lib/x86_64-linux-gnu/libxml2.so.2 > #4 0x00007f0afc1e35ff in xmlACatalogResolve () from > /usr/lib/x86_64-linux-gnu/libxml2.so.2
Can you recompile without optimization? Because without the trace for the 2 functions called from xmlACatalogResolve() there is way too much guessing. Somehow it's the catalog resolution code which seems to deadlock. Are you using the latest version ? Could you configure without catalog support and see ? it is a bit surprizing as testThreads.c does this kind of heavy parallel testing and catalog access without raising any issue. > #5 0x00007f0afc19d7e3 in ?? () from /usr/lib/x86_64-linux-gnu/libxml2.so.2 > #6 0x00007f0afc1a0354 in ?? () from /usr/lib/x86_64-linux-gnu/libxml2.so.2 > #7 0x00007f0afc1a01df in xmlLoadExternalEntity () from > /usr/lib/x86_64-linux-gnu/libxml2.so.2 > #8 0x00007f0afc186b36 in xmlCreateURLParserCtxt () from > /usr/lib/x86_64-linux-gnu/libxml2.so.2 > #9 0x00007f0afc18cdaa in xmlReadFile () from > /usr/lib/x86_64-linux-gnu/libxml2.so.2 [...] > > Frame 7's rbx register can be used to confirm the URL strings are different > for each thread, so no two threads are unintentionally racing to parse the > same remote RSS feed. Actually as long as the parser context are different you can test > (gdb) frame 7 > #7 0x00007f0afc1a01df in xmlLoadExternalEntity () from > /usr/lib/x86_64-linux-gnu/libxml2.so.2 > (gdb) print (char *)$rbx > $22 = 0x7f0af00022d0 "http://feeds.washingtonpost.com/rss/world"; > (gdb) thread 3 > [Switching to thread 3 (Thread 0x7f0afa954700 (LWP 25902))] > #0 pthread_cond_wait@@GLIBC_2.3.2 () at > ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:162 > 162 in ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S > (gdb) frame 7 > #7 0x00007f0afc1a01df in xmlLoadExternalEntity () from > /usr/lib/x86_64-linux-gnu/libxml2.so.2 > (gdb) print (char *)$rbx > $23 = 0x7f0aec0022c0 "http://feeds.latimes.com/latimes/news"; > (gdb) thread 4 > [Switching to thread 4 (Thread 0x7f0afb155700 (LWP 25901))] > #0 pthread_cond_wait@@GLIBC_2.3.2 () at > ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:162 > 162 in ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S > (gdb) frame 7 > #7 0x00007f0afc1a01df in xmlLoadExternalEntity () from > /usr/lib/x86_64-linux-gnu/libxml2.so.2 > (gdb) print (char *)$rbx > $24 = 0x7f0af4002350 > "http://feeds.chicagotribune.com/chicagotribune/news/nationworld"; > > Of course, xmlRMutexLock and pthread_cond_wait are each at the bottom of the > three thread stacks, so my (hopefully not incorrect) assumption is that they > are all waiting to acquire the same mutex. > > This happens very rarely, but I'm assuming it's happening because of a race > I'm not protecting against. I assume you called xmlInitParser(), but the problem doesn't look there. I need to know which mutext they are freezing on, Daniel -- Daniel Veillard | Open Source and Standards, Red Hat [email protected] | libxml Gnome XML XSLT toolkit http://xmlsoft.org/ http://veillard.com/ | virtualization library http://libvirt.org/ _______________________________________________ xml mailing list, project page http://xmlsoft.org/ [email protected] https://mail.gnome.org/mailman/listinfo/xml
