If you have/find a clean example, certainly posting an issue will make sense. I
can't comment on whether the task switch during I/O is inevitable.
--Tim
On Sunday, September 21, 2014 10:25:11 AM Erik Schnetter wrote:
> I'm aware that Julia's threads are "green threads". The issue of
> thread safety still remains; if one thread is suspended in a critical
> region, another can enter that region. Storing handles in global data
> structures and incrementing global variables are such actions, and I'm
> not 100% sure that the respective region in serialize.jl are
> yield-free, even without my info output. I was surprised to see that
> I/O causes task switches -- maybe something else (hashing?
> dictionaries? creating new lambdas in C?) also causes task switches?
>
> gdb points to memory allocation routines in libc, called from gc.c or
> array.c. I assume that something overwrites memory, destroying libc
> malloc's data structures, leading to a crash later.
>
> -erik
>
> On Sun, Sep 21, 2014 at 5:26 AM, Tim Holy <tim.h...@gmail.com> wrote:
> > Hi Erik,
> >
> > First, one comment: tasks are not "true" (kernel) threads. Currently a
> > julia process is single-threaded. Tasks are better considered as a form
> > of cooperative multitasking.
> >
> > Yes, I've also found that I/O causes task switching. I don't personally
> > know a great way around this. One option would presumably be to have some
> > form of message queue; I am pretty sure that push!ing a new message on
> > it---as long as you don't need to touch I/O to create the message---would
> > not cause a switch. You can also use time() and other markers to indicate
> > the status of control flow.
> >
> > I haven't been reading things carefully enough to know whether there's any
> > history behind this, but if you haven't said so already...what does gdb
> > (or
> > equivalent) say about the segfault?
> >
> > --Tim
> >
> > On Saturday, September 20, 2014 08:24:59 PM Erik Schnetter wrote:
> >> I am trying to track down a segfault in a Julia application. Currently I
> >> am
> >> zooming in on "deserialize", as avoiding calling it seems to reliably
> >> cure
> >> the problem, while calling it (even if not using the result) seems to
> >> reliably trigger the segfault.
> >>
> >> I am using many threads (tasks), and deserialize is called concurrently.
> >> Is
> >> this safe? I've been bitten in the past by this; e.g. I've accidentally
> >> added an "info" statement into a sequence of statements that needs to be
> >> atomic, and I/O apparently switches threads. Is there a list of
> >> known-to-be-safe or known-to-be-unsafe functions? Is deserialization
> >> thread-safe in this respect?
> >>
> >> I am in particular deserializing function calls and lambda expressions,
> >> and
> >> I see global variables ("lambda_numbers", "known_lambda_data"). Are the
> >> respective data structures (WeakKeyDict and Dict) thread-safe?
> >>
> >> Is there a locking mechanism in Julia? This would temporarily only allow
> >> a
> >> single thread (task) to run, aborting with an error if this thread
> >> becomes
> >> unrunnable. In other words, calling "yield" when holding a lock would be
> >> a
> >> no-op.
> >>
> >> -erik
