[Haskell-cafe] Threads and critical sections (threadWaitRead)

[Takenobu Tani]

Hi Lars,

uh, order issues are difficult for me.
Although this is not the answer, I write a little.

At first:
  "critical section" ... is block that should protect the invariants.
  "atomic block"     ... is undivided code block.

Your point (1) and (2) are "critical section" for mfd and fd,
but not "atomic block".

readMVar can't create "atomic block". It's only read the content of MVar.

Of cource, as you have said.
If point (1) have no memory allocations,
async exception and timer-based thread context switch don't occur there.
But you might think many other case, such as implement dependent.

If you want to protect the critical section exactly,
it is better to use an explicit mechanism rather than implicit mechanism.


Libraries on hackage[1] might be helpful.

For performance pursuit, there may be a gimmick way;-)

[1]: https://hackage.haskell.org/

Cheers,
Takenobu



2015-05-22 20:55 GMT+09:00 Lars Petersen <info at lars-petersen.net>:

> Hello cafe,
>
> I'm currently trying to understand Haskell's threading model and GHC's
> implementation of it. Specifically I wondered about the safety of file
> IO. Consider this piece of code:
>
> > newtype Socket = Socket (MVar Fd)
> >
> > read :: Socket -> IO ByteString
> > read (Socket mfd) = do
> >   fd <- readMVar mfd
> >   -- (1)
> >   threadWaitRead fd
> >   -- (2)
> >   withMVar mfd $ \fd'-> do
> >     -- (3) the actual read...
>
> Context:
>   - The socket is configured non-blocking.
>   - No other operation will hold the MVar over a blocking call (because
> this would make it impossible to close it while it's blocking).
>   - My close operation will fill the MVar with -1 after it's closed and
> will hold the MVar during the close operation.
>
> Questions:
>   - Is it theoretically possible that my thread gets paused at (1) or
> within threadWaitRead, another thread closes the socket (or even worse
> opens another file with the same fd) _and_ my thread is resumed
> afterwards (I'm not asking about async exceptions)?
>   - What constructs contain safepoints and which are guaranteed to be
> executed without interruption?
>
> Considerations (so far):
>   - It is unlikely that (1) is a safepoint as no memory allocations
> occur, but is it guaranteed?
>   - If the socket were closed during (1), threadWaitRead throws an
> exception complaining about a bad file descriptor which is unexpected
> but not harmful.
>   - I found this thread
> (
> https://mail.haskell.org/pipermail/haskell-cafe/2014-September/115841.html
> )
> which is somewhat related, but I couldn't extract a clear answer from it.
>
> Best,
>   Lars
>
>
>
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