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Re^8: threads on Windows

by BrowserUk (Patriarch)
on Feb 22, 2009 at 06:27 UTC ( [id://745596]=note: print w/replies, xml ) Need Help??

in reply to Re^7: threads on Windows
in thread threads on Windows

It's not a "race condition"! (And it not "<> blocking thread creation".)

You cannot clone the current thread whilst another thread has a (Perl internal) lock on one of the resources to be cloned. This is WAD (Working As Designed). Perl protects it's internal data structures from concurrency corruption by serialising access to them through an internal mutex. If one of the process global resources (PGR) of a spawning thread is currently in use by another thread then the cloning of the current thread will be blocked until that mutex is released. In this case, the shared PGR is STDIN--but it could be any other PGR that blocks.

In the following, if you supply an argument, STDIN will be closed in the main thread, hence it does not need to be cloned in order for thread 2 through 11 to be spawned, and the program will complete immediately.

If you do not supply an argument, STDIN is not closed, and when the attempt is made to clone the main thread for thread 2, the internal mutex is being held by thread 1 because it is in a blocking read state on STDIN. Therefore, the main thread is blocked until that read state is satisfied.

If you hit enter, the read on STDIN in thread 1 returns, lifting the mutex and allowing the main thread to continue. A few threads will be created before thread 1 gets another timeslice, and again enters a blocking read state whilst holding the internal mutex. The main thread is once again blocked. Hit enter again and the cycle repeats.

See the two sample runs after the __END__ token:

#! perl -slw use strict; use threads; async { getc while 1; }->detach; close STDIN if @ARGV; for ( 1 .. 10 ) { async { printf "Thread: %d ran\n", threads->tid; }->detach; } sleep 1 while threads->list( threads::running ); __END__ C:\test>junk Thread: 2 ran Thread: 3 ran Thread: 4 ran Thread: 5 ran Thread: 6 ran Thread: 7 ran Thread: 8 ran Thread: 9 ran Thread: 10 ran C:\test>junk 1 Thread: 2 ran Thread: 3 ran Thread: 4 ran Thread: 5 ran Thread: 6 ran Thread: 7 ran Thread: 8 ran Thread: 9 ran

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

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Re^9: threads on Windows
by ikegami (Patriarch) on Feb 22, 2009 at 06:36 UTC

    Blocking while waiting for a lock is not blocking? You're not making sense.

        You said <> isn't blocking thread creation, and proceed to contradict yourself as you said <> uses a mutex and "the cloning of the current thread will be blocked until that mutex is released".

        Are you implying that we should have said

        <> obtains a lock on STDIN, one which is also required to clone STDIN, an activity that occurs when a new thread is created, thus preventing new threads from being created until the read is complete.

        instead of

        <> blocks thread creation

        If so, that would be ironic given the content of your home node.

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