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[[!meta title="libpthread: pthread_key_create, reuse"]]

[[!tag open_issue_libpthread]]

IRC, FreeNode, #hurd, 2011-07-02:

    < pinotree> hm, maybe i found a libpthread bug
    * pinotree tries a testcase
    < pinotree> yesssss, found the bug :)
    < pinotree> youpi: it's a problem of the key reuse in pthread_key_create()
    < youpi> it doesn't reset it?
    < youpi> were you looking at the licq issue?
    < pinotree> no, gtest
    < youpi> k
    < youpi> licq has a failing threadspecific issue
    < youpi> [  FAILED  ] ThreadSpecificData.dataDeletedWhenThreadExits
    < pinotree> basically, pthread_key_delete() does not delete the key values
      from the "thread_specifics" ihash
    < pinotree> but those were new keys, so i'm not sure it is allowed to
      return values of previous keys?
    < pinotree> after all, the actual key value is an implementation detail,
      applications shouldn't care about it being reused
    < pinotree> (imho)
    < youpi> Upon key creation, the value NULL shall be associated with the new
      key in all active threads.
    < youpi> ok, so we have to clear it in all threads
    < youpi> that's a pity
    < pinotree> or just remove the entry from the hash on key removal
    < youpi> pinotree: from all the hashes, you mean?
    < pinotree> youpi: from how i see it, adding a snippet like
      http://paste.debian.net/121690/ in pthread_key_delete() should do the job
    < youpi> that only drops from the current thread
    < pinotree> ah hm, other threads
    < youpi> we need to drop from all  threads
    < youpi> that's the pity part
    < pinotree> youpi: the licq case could look like a similar issue, at a
      veeery quick glance

Test program: [[pthread_key_create_reuse.c]]


2011-11-01:

    <pinotree> youpi: about the bug with pthread keys (reuse): would be an
      acceptable solution having a mutex for the thread_specifics of each
      thread?
    <youpi> you mean one per thread, one global, or one per key, or ?
    <youpi> what is it supposed to protect?
    <pinotree> the thread_specifics of each thread
    <youpi> pinotree: but against what?
    <pinotree> the idea would be: when destroying a key, iterate over all the
      exiting threads and remove the key data from the thread_specifics of each
      thread
    <youpi> one of the issue is getting to browse through the whole list of
      threads
    <youpi> the other is concurrency between that, and a thread dying
    <pinotree> there's the __pthread_threads_lock rwlock
    <youpi> it should be enough to keep it locked during the iteration
    <pinotree> but that wouldn't be enough when one thread is destroying a key,
      and another one is doing {get,set}specific() on that key
    <youpi> that's not supposed to happen
    <pinotree> mmm
    <youpi> “The effect of calling pthread_getspecific() or
      pthread_setspecific() with a     key value not obtained from
      pthread_key_create() or after key has been     deleted with
      pthread_key_delete() is undefined.”
    <youpi> undefined -> you are allowed to just blow up
    <pinotree> but it's not been deleted yet... :)
    <youpi> it could be, just a matter of time
    <youpi> you're not supposed to rely on time-luckyness :)
    <pinotree> mmm
    <pinotree> bah, you've convinced me ( :) )