[Bug libc/6420] New: Mtrace deadlock

["mjambor at suse dot cz" <sourceware-bugzilla at sourceware dot org>]

Different  locking order  of mtrace  lock and  dl_load_lock  can cause
deadlocks when mtrace is enabled. 

The reason is that at the moment mtrace hooks acquire both locks, they
lock their own lock (called lock) first and then call _dl_addr() which
in  turn locks  dl_load_lock.  However,  there is  at least  one place
where such a hook is triggered with dl_load_lock already locked.  That
place  is   a  call  to  malloc()  in   function  add_dependency()  in
dl-lookup.c and  there may be  others.  This results into  a different
lock ordering and therefore a potential deadlock.

Unfortunately  I cannot  reproduce  this deadlock  myself  but it  did
happen to one of  our customers and it also did go  away when we fixed
the  problem.  Although  they were  using  glibc 2.3.3  I noticed  the
problem is present in the CVS version too and so decided to file a bug
here.

The fix is in fact simple.   Mtrace hooks do not need to call _dl_addr
while also  holding its own lock and  so we can avoid  this problem by
calling  _dl_addr before locking  lock and  print it  afterwards (when
holding the lock).  The only slight  problem is one of taste. The fact
that  _dl_addr and the  structure it  returns, Dl_info,  are available
only when HAVE_ELF  macro is defined, makes direct  change of the code
very ugly.   Therefore I am about  to attach two patches  that fix the
problem.   The first  merely  splits the  function  tr_where to  three
smaller  ones.   The second  then  actually  changes  the behavior  as
described above.

-- 
           Summary: Mtrace deadlock
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
        AssignedTo: drepper at redhat dot com
        ReportedBy: mjambor at suse dot cz
                CC: glibc-bugs at sources dot redhat dot com


http://sourceware.org/bugzilla/show_bug.cgi?id=6420

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