[GHC] #10296: Segfaults when using dynamic wrappers and concurrency

[GHC]

#10296: Segfaults when using dynamic wrappers and concurrency
-------------------------------------+-------------------------------------
        Reporter:  bitonic           |                Owner:  jme
            Type:  bug               |               Status:  patch
        Priority:  high              |            Milestone:  8.2.1
       Component:  Runtime System    |              Version:  7.11
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Runtime crash     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2031
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by jme):

 * status:  new => patch
 * differential:   => Phab:D2031


Comment:

 In the example above, each call to `wrap` allocates a new stable
 pointer, which is then dereferenced when `call_fun` calls back into
 Haskell.  Since the stable pointers are not freed, the table holding
 them is periodically enlarged (using `realloc()`).  When one of these
 reallocations moves the table just as it is being read by another thread
 (to dereference a stable pointer), a segfault can occur.

 The fix provided by Phab:D2031 is rather simplistic: it eliminates the
 freeing of the old table during reallocation, thus ensuring the table
 can continue to be safely read.  While this approach allows dereferences
 to remain lock-free, it clearly wastes space (roughly twice the memory
 is now required to hold the stable pointer table).  If this is an issue,
 it should be relatively straightforward to eliminate the extra memory
 consumption without adversely affecting performance (albeit with some
 added complexity).  Let me know if such an implementation would be
 preferable.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10296#comment:7>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler