On 04/25/2012 01:09 PM, Dalleau, Frederic wrote:
Hi David,
On Tue, Apr 24, 2012 at 3:01 PM, David Henningsson
<[email protected]> wrote:
On 04/23/2012 05:03 PM, Dalleau, Frederic wrote:
Hi David,
On Fri, Apr 20, 2012 at 4:44 PM, David Henningsson
<[email protected]> wrote:
While researching a bug I came across something that might be a bug in
the
pa_once logic, but this stuff is tricky, so I might also be missing
something.
Imagine this:
* Thread 1 runs pa_once_begin, succeeds and starts running the payload
(i e
the code that should only run once).
* Thread 2 starts running pa_once_begin, but only the first row. We're
now
right *before* pa_atomic_inc(&control->ref) but *after*
pa_atomic_load(&control->done).
* Thread 1 finishes the payload, runs pa_once_done which sets
control->done
and frees the mutex.
* Thread 2 continues, pa_once_begin succeeds and the payload is now run
a
second time!
After reading your mail, I made some experiments by adding a usleep() call
in Thread 1 between pa_atomic_load(&control->done) and
pa_atomic_inc(&control->ref)
and that failed once-test 100% of time.
I reverted the usleep and made another experiment using 50000 iterations
in
once-test and it just failed.
Good catch !
I tried to look up implementation/algorithm suggestions, and for the ones I
found [1], there was no freeing of the mutex. Without freeing, the code
becomes simpler. The attached patch is a version of that. I've just tried a
simple once-test (but do feel free to run it 50000 times :-) ).
But of course, now we leak a mutex. But that's what we already do with the
static mutexes we use in a few places already, so maybe it doesn't matter
much?
Your patch has passed the 50000 iteration test, but I'm reluctant in the idea of
leaking memory since solutions exists.
I have attached a version which uses reference counting to free the
mutex. I can't find
any significant difference in cpu usage, but it must be double checked
for correctness.
Right, so it's a trade-off between keeping a mutex allocated for my
version, and three extra atomic ops for your version. Maybe it doesn't
matter much and we're essentially bikeshedding?
libatomic_ops documentation suggest the following :
http://www.hpl.hp.com/research/linux/atomic_ops/example.php4
With a static mutex, it uses a few extra bytes (pthread_mutex_t is 24
bytes on my box), but it may also offer the best performances of all:
no allocation
and no busy loop waiting for mutex creation. I'm giving it a try but
it needs a bit
of plumbing.
That looks slightly more elegant than our solutions IMO. Perhaps we can
use pa_static_mutex_get function for the static mutex?
--
David Henningsson, Canonical Ltd.
https://launchpad.net/~diwic
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