Sterling Clover wrote:
This email is inspired by the discussion here:
http://hackage.haskell.org/trac/ghc/ticket/2401
As the ticket discusses, unsafeIOToSTM is, unlike unsafePerformIO or
unsafeInterleaveIO, genuinely completely unsafe in that there is no way
to use it such that a segfault or deadlock is not at least somewhat
encouraged. The code attached to the ticket creates a deadlock solely
through using it to write to stdout. But, for the same reason that
unsafeIOToSTM is unstable, unsafeInterleaveIO now is very unstable as
well -- conceivably, data generated from functions with lazy IO
(including those in the prelude) could cause deadlocks within STM, and
even segfaults.
In summary, a "validation" step is performed on all threads inside
atomically blocks during garbage collection. This validation step will,
on encountering invalid threads (i.e. ones which should be rolled back)
immediately kill them dead and retry. This is different than the
implementation described in the STM paper, where rollbacks only occur on
commit. However, it does add a measure of efficiency.
Its not just an efficiency trick, in fact. The validation step is
absolutely necessary for correctness. The problem is that a transaction
may have seen an inconsistent view of memory, and as a result it may have
gone into an infinite loop; the only way to catch and recover from this
situation is to validate at regular intervals, say before a GC (this
suffers from the problem that the transaction has to be allocating in order
to be stopped, but that's another matter). e.g. the code might be
something like
atomically $ do
a <- readTVar ta
b <- readTVar tb
if a == b then loop else return ()
now we might know that a is never equal to b under normal conditions: all
the transactions in the program satisfy the invariant. However, since we
use optimistic concurrency, it might be the case that this thread sees an
inconsistent view of memory in which a==b. The case would normally be
caught at commit time, but this thread isn't going to commit: it goes into
an infinite loop instead.
As Simon M. notes, the obvious solution would be to turn rollbacks into
regular exceptions, but this would open a number of cans of worms.
A start, though not sufficient, would be for stm validation to respect
blocked status -- not to block on it, obviously, but simply to refuse to
rollback a transaction within it.
That wouldn't be correct, because the thread might be in an infinite loop
inside a block. However, it would probably work in the cases you're
interested in, so I wouldn't object to a patch that implemented this
workaround for the time being.
I do agree that we have a problem here, and I'll re-open the ticket (sorry
for leaving it closed). I think raising an (asynchronous) exception is the
right solution. We have to make sure the exception cannot be caught by an
STM catch, but I think that's do-able.
However, another problem we have is that when the IO system re-raises the
exception, it'll be raised as a synchronous exception rather than an
asynchronous exception. I've just spent an hour or so talking this over
here with Simon PJ and we have some ideas for fixing it, I'll try to write
it up in a ticket later.
Cheers,
Simon
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