#4514: IO manager can deadlock if a file descriptor is closed behind its back
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Reporter: adept | Owner:
Type: bug | Status: new
Priority: high | Milestone: 7.0.2
Component: Runtime System | Version: 7.0.1
Resolution: | Keywords:
Testcase: | Blockedby:
Difficulty: | Os: Linux
Blocking: | Architecture: Unknown/Multiple
Failure: Incorrect result at runtime |
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Comment(by bos):
I'm pasting in some comments from Ian in email:
Skimming through this patch I noticed some interface changes. We try not
to change the interfaces of libraries within a GHC branch (e.g. 7.0.1 and
7.0.2 should have the same library APIs). If it's really necessary then we
can make changes, although we should keep them as small as possible.
The particular changes also aren't obviously right to me. e.g. I
wouldn't expect {{{Control.Concurrent}}} to export something called
{{{closeFd}}}, and I wouldn't expect something called {{{closeFd}}} to
have that type.
I believe that it's not possible to avoid any of the submitted API changes
here. Specifically:
* The {{{fdWasClosed}}} function that we added during 6.13 development was
subject to a race condition: you could mark an fd as closed after another
thread had performed an {{{open}}} or similar that could reuse that fd.
* The new {{{closeFd}}} function is necessary to close that race. It
manages this by closing the fd while holding an {{{MVar}}}, so another
thread can't race in and register the fd.
* Why does {{{closeFd}}} accept an action as parameter? Because under
Windows, you have to use different functions to close a file descriptor
depending on whether it's owned by Winsock or the rest of win32.
* If the name should be something like {{{closeFdWith}}} to be more
descriptive, that's fine (please let me know if you have a preference), as
my purpose is to close the race :-)
* Why should {{{closeFdWith}}} be exported from {{{Control.Concurrent}}}?
Because {{{threadWaitRead}}} and {{{threadWaitWrite}}} already are, and
{{{closeFdWith}}} must be used with those functions now to guarantee race-
free closing.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4514#comment:32>
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