setnumcapabilities001 failure

Simon Marlow marlowsd at
Fri Oct 28 07:02:36 UTC 2016

Hi Ryan, I don't think that's the issue.  Those variables can only be
modified in setNumCapabilities, which acquires *all* the capabilities
before it makes any changes.  There should be no other threads running RTS
code(*) while we change the number of capabilities.  In particular we
shouldn't be in releaseGCThreads while enabled_capabilities is being

(*) well except for the parts at the boundary with the external world which
run without a capability, such as rts_lock() which acquires a capability.


On 27 Oct 2016 17:10, "Ryan Yates" <fryguybob at> wrote:

> Briefly looking at the code it seems like several global variables
> involved should be volatile: n_capabilities, enabled_capabilities, and
> capabilities.  Perhaps in a loop like in scheduleDoGC the compiler moves
> the reads of n_capabilites or capabilites outside the loop.  A failed
> requestSync in that loop would not get updated values for those global
> pointers.  That particular loop isn't doing that optimization for me, but I
> think it could happen without volatile.
> Ryan
> On Thu, Oct 27, 2016 at 9:18 AM, Ben Gamari <ben at> wrote:
>> Simon Marlow <marlowsd at> writes:
>> > I haven't been able to reproduce the failure yet. :(
>> >
>> Indeed I've also not seen it in my own local builds. It's quite an
>> fragile failure.
>> Cheers,
>> - Ben
>> _______________________________________________
>> ghc-devs mailing list
>> ghc-devs at
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <>

More information about the ghc-devs mailing list