[Openmcl-devel] [openmcl-devel] atomic-incf
gb at clozure.com
Thu Oct 15 18:38:30 EDT 2009
I'm not sure that I understand the question fully. To the extent
that I do: yes, ATOMIC-INCF is thread-safe in that:
- it behaves the same way, regardless of how many threads are
trying to modify the same memory location.
- it can be used to achieve the same effects that might otherwise
require more heavyweight mechanisms to ensure thread-safety.
For instance: suppose that a number of threads were processing "requests"
(it doesn't matter what "requests" are) and we want to know how many
requests have been completed and maintain a global *REQUESTS-COMPLETED*
variable to track that information. If each thread does something like:
then *REQUESTS-COMPLETED* may be at least slightly inaccurate: multiple
threads can't safely update a shared memory location concurrently in this
way because the read/modify/write sequences that they'd use aren't atomic.
We -could- use a lock to ensure that that doesn't happen:
and that'd be thread-safe, but grabbing a lock (and releasing it, and
using UNWIND-PROTECT to guarantee that that the lock is released) is
a fairly heavy price to pay to increment a variable safely.
We could also do:
and that'd also be thread-safe (we can say that each thread has
exclusive access to the global variable for the few instructions
it takes to increment its value) and less expensive than using
a lock would be.
So yes, what ATOMIC-INCF does is thread-safe and it can also
be used to do things in a thread-safe way (with less overhead
than other mechanisms incur.)
On Thu, 15 Oct 2009, David L. Rager wrote:
> Hi Gary,
> I'm pretty sure I understand the multi-threading issue. Even if it
> were a single CISC instruction, there would be no "atomicity"
> guarantee without the lock on the memory address (where the lock
> prohibits reading/writing of that address).
>> ATOMIC-INCF basically tries to guarantee that the
>> read-modify-write sequence happens atomically.
>> I think that we could actually implement it as
>> (addq ($ fixnumone) (@ loc))
>> (movq increment-by (% reg))
>> (addq (% reg) (@ loc))
>> on x86; we actually do a similar compare-and-swap loop on both
> So do we know whether the atomic-incf in CCL version 1.3 of
> x86-misc.lisp is thread-safe? I'm happy to go read more about the X86
> assembly "specification" in order to better understand the code myself
> if that's what's required.
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