Request for help / ideas to debug issue

Michael Hudson-Doyle michael.hudson at
Sun Mar 12 08:37:38 UTC 2017

Before we get into this, what is the actual problem here? Just the ugly

On 11 March 2017 at 02:58, Alfonso Sanchez-Beato <
alfonso.sanchez-beato at> wrote:

> On Fri, Mar 10, 2017 at 10:22 AM, John Lenton <john.lenton at>
> wrote:
> > Hello!
> >
> > We're seeing a weird issue with either go, pthreads, or the kernel. If
> > you're knowledgeable about one or more of those things, could you take
> > a look? Thank you.
> >
> > The issue manifests as nasty warnings from the "snap run" command,
> > which is also the first step into a snapped app or service. It looks
> > like
> >
> > runtime/cgo: pthread_create failed: Resource temporarily unavailable
> >
> > a very stripped-down reproducer is
> >
> > build that, run it in a loop, and you'll see a bunch of those messages
> > (and some weirder ones, but let's take it one step at a time)

Turns out this was fixed in Go 1.8:

> > if you comment out the 'import "C"' line the message will change but
> > still happen, which makes me think that at least in part this is a Go
> > issue (or that we're holding it wrong).

... but only in the non-cgo case, you can (occasionally) still get messages

runtime: failed to create new OS thread (have 5 already; errno=11)
runtime: may need to increase max user processes (ulimit -u)
fatal error: newosproc

if you comment out the import "C". I guess we should report that upstream.

> > Note that the exec does work; the warning seems to come from a
> > different thread than the one doing the Exec (the other clue that
> > points in this direction is that sometimes the message is truncated).
> > You can verify the fact that it does run by changing the /bin/true to
> > /bin/echo os.Args[1], but because this issue is obviously a race
> > somewhere, this change makes it less likely to happen (from ~10% down
> > to ~.5% of runs, in my machines).
> >
> > One thing that makes this harder to debug is that strace'ing the
> > process hangs (hard, kill -9 of strace to get out) before reproducing
> > the issue. This probably means we need to trace it at a lower level,
> > and I don't know enough about tracing a process group from inside the
> > kernel to be able to do that; what I can find about kernel-level
> > tracing is around syscalls or devices.
> >
> > Ideas?
> >
> I found this related thread:
> <<
> I believe this can happen on GNU/Linux if your program uses cgo and if
> thread A is in the Go runtime starting up a new thread B while thread
> C is execing a program.  The underlying cause is that while one thread
> is calling exec the Linux kernel will fail attempts by other threads
> to call clone by returning EAGAIN.  (Look for uses of the in_exec
> field in the kernel sources.)
> >>

Yeah, this seems to be very accurate. It's also why it seems this is a
cosmetic problem only, some thread not calling exec fails, but well, the
thread is about to die anyway.

> Something like adding a little sleep removes the traces, for instance:
> where the program run sleep for 1ms before calling Exec. For smaller units
> (say, 20 us) the issue still happens.
> It looks to me that right before running main(), go creates some threads,
> calling clone() and probably getting the race described in the thread. As
> anyway you are running Exec I guess the traces are harmless, you do not
> need the go threads. Nonetheless, I think that the go run time should retry
> instead of printing that trace.


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