Potential deadlock in psp cache
-------------------------------
Key: MODPYTHON-69
URL: http://issues.apache.org/jira/browse/MODPYTHON-69
Project: mod_python
Type: Bug
Components: publisher
Versions: 3.2.0
Environment: All
Reporter: Jim Gallacher
This issue was discussed on the python-dev mailing list but not followed up on.
Fixing that now.
In psp.py
def dbm_cache_store(srv, dbmfile, filename, mtime, val):
dbm_type = dbm_cache_type(dbmfile)
### potential deadlock here! ###
_apache._global_lock(srv, "pspcache")
try:
dbm = dbm_type.open(dbmfile, 'c')
dbm[filename] = "%d %s" % (mtime, code2str(val))
finally:
try: dbm.close()
except: pass
_apache._global_unlock(srv, "pspcache")
"pspcache" will hash to one of 31 mutexes. Therefore there is a 1 in 31 chance
for a hash collision if a session is used in the same request, which would
result in a deadlock. (This has been confirmed by testing.)
Most obvious solution is to use the global lock 0, which will serialize all
accesses to either pspcache.dbm. Global lock 0 is also used by DbmSession, but
since the lock is not held for the duration of the request there should not be
any additional deadlock issues.
The fix is to replace the _apache._global_lock(srv, "pspcache") with
_apache._global_lock(srv, None, 0)
I will commit the this fix shortly.
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