Re: mask, catch, myThreadId, throwTo

Nice, thanks, Edward =). On Tue, Apr 16, 2013 at 6:20 PM, Edward Z. Yang wrote:

OK, I've updated the docus.

Excerpts from Felipe Almeida Lessa's message of Mon Apr 15 13:34:50 -0700 2013:

...

Thanks a lot, you're correct! The trouble is, I was misguided by the "Interruptible operations" note [1] which states that

The following operations are guaranteed not to be interruptible: ... * everything from Control.Exception ...

Well, it seems that not everything from Control.Exception fits the bill.

Thanks, =)

[1] http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Exception...

On Mon, Apr 15, 2013 at 5:25 PM, Bertram Felgenhauer wrote:

...

Felipe Almeida Lessa wrote:

...

I have some code that is not behaving the way I thought it should.

The gist of it is

sleeper = mask_ $ forkIOWithUnmask $ \restore -> forever $ restore sleep `catch` throwBack

throwBack (Ping tid) = myThreadId >>= throwTo tid . Pong throwBack (Pong tid) = myThreadId >>= throwTo tid . Ping

Since (a) throwBack is executed on a masked state, (b) myThreadId is uninterruptible, and (c) throwTo is uninterruptible, my understanding is that the sleeper thread should catch all PingPong exceptions and never let any one of them through.

(c) is wrong, throwTo may block, and blocking operations are interruptible.

http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Exception...

explains this in some more detail.

The simplest way that throwTo can actually block in your program, as far as I can see, and one that will only affect the threaded RTS, is if the sleeper thread and whichever thread is running the other throwBack are executing on different capabilities; this will always cause throwTo to block. (You could try looking at a ghc event log to find out more.)

I last ran into trouble like that with System.Timeout.timeout; for that function I finally convinced myself that uninterruptibleMask is the only way to avoid such problems; then throwTo will not be interrupted by exceptions even when it blocks. Maybe this is the solution for your problem, too.

Hope that helps,

Bertram

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-- Felipe.