swap! is implemented using Java's AtomicReference class and its compareAndSet method. I haven't dug into the Java source code implementing that class, but you can read the Java documentation for all Atomic* Java classes here:
https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/atomic/package-summary.html The summary there is that all accesses to Atomic* classes should have similar memory model guarantees as Java fields declared volatile. I don't have a good authoritative link handy for Java documentation on volatile, but the basic idea is that they should not be cached locally by individual threads, but be visible to all. Andy On Tue, Jun 9, 2015 at 11:33 AM, Michael Gardner <gardne...@gmail.com> wrote: > I'm talking about a scenario where a single thread does inc, followed > shortly by dec. Obviously from that thread's perspective, the value will > never be zero, but what about as seen by other threads? > > My understanding of Java's memory model is that instructions within a > single thread *can* get reordered, and Java only guarantees that they will > *appear* to have executed in order from the perspective of the executing > thread. Other threads might see those instructions as executing > out-of-order. What I'm wondering is whether atoms are subject to this as > well. > > > On Jun 9, 2015, at 10:16 AM, Andy Fingerhut <andy.finger...@gmail.com> > wrote: > > > > I am not sure why Atamert says "No". > > > > If the (swap! a inc) and (swap! a dec) are executed in different > threads, and they can occur in either order because there is no > synchronization between those threads that prevents one of the two possible > orders from occurring, then another thread *could* see a value of 0, if and > when the (swap! a dec) occurs first. > > > > If the (swap! a inc) and (swap! a dec) are executed sequentially in a > single thread, and no other thread is modifying a, then by normal > sequential execution the atom should change from 1 to 2, then from 2 back > to 1. > > > > Andy > > > > On Tue, Jun 9, 2015 at 9:38 AM, Atamert Ölçgen <mu...@muhuk.com> wrote: > > > > > > On Tue, Jun 9, 2015 at 7:30 PM, Michael Gardner <gardne...@gmail.com> > wrote: > > This might be blindingly obvious to some, but I can't find any > discussion about it. Let's say I have code like the following: > > > > (def a (atom 1)) > > ... > > (swap! a inc) > > (swap! a dec) > > > > Is there any possibility of another thread seeing a=0? If not, what > provides this guarantee? > > > > No. Not if those two swap! calls are made from different threads. > > > > > > -- > > You received this message because you are subscribed to the Google > > Groups "Clojure" group. > > To post to this group, send email to clojure@googlegroups.com > > Note that posts from new members are moderated - please be patient with > your first post. > > To unsubscribe from this group, send email to > > clojure+unsubscr...@googlegroups.com > > For more options, visit this group at > > http://groups.google.com/group/clojure?hl=en > > --- > > You received this message because you are subscribed to the Google > Groups "Clojure" group. > > To unsubscribe from this group and stop receiving emails from it, send > an email to clojure+unsubscr...@googlegroups.com. > > For more options, visit https://groups.google.com/d/optout. > > > > > > > > -- > > Kind Regards, > > Atamert Ölçgen > > > > ◻◼◻ > > ◻◻◼ > > ◼◼◼ > > > > www.muhuk.com > > www.olcgen.com > > > > -- > > You received this message because you are subscribed to the Google > > Groups "Clojure" group. > > To post to this group, send email to clojure@googlegroups.com > > Note that posts from new members are moderated - please be patient with > your first post. > > To unsubscribe from this group, send email to > > clojure+unsubscr...@googlegroups.com > > For more options, visit this group at > > http://groups.google.com/group/clojure?hl=en > > --- > > You received this message because you are subscribed to the Google > Groups "Clojure" group. > > To unsubscribe from this group and stop receiving emails from it, send > an email to clojure+unsubscr...@googlegroups.com. > > For more options, visit https://groups.google.com/d/optout. > > > > > > -- > > You received this message because you are subscribed to the Google > > Groups "Clojure" group. > > To post to this group, send email to clojure@googlegroups.com > > Note that posts from new members are moderated - please be patient with > your first post. > > To unsubscribe from this group, send email to > > clojure+unsubscr...@googlegroups.com > > For more options, visit this group at > > http://groups.google.com/group/clojure?hl=en > > --- > > You received this message because you are subscribed to the Google > Groups "Clojure" group. > > To unsubscribe from this group and stop receiving emails from it, send > an email to clojure+unsubscr...@googlegroups.com. > > For more options, visit https://groups.google.com/d/optout. > > -- > You received this message because you are subscribed to the Google > Groups "Clojure" group. > To post to this group, send email to clojure@googlegroups.com > Note that posts from new members are moderated - please be patient with > your first post. > To unsubscribe from this group, send email to > clojure+unsubscr...@googlegroups.com > For more options, visit this group at > http://groups.google.com/group/clojure?hl=en > --- > You received this message because you are subscribed to the Google Groups > "Clojure" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to clojure+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en --- You received this message because you are subscribed to the Google Groups "Clojure" group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
