On 24/10/19(Thu) 13:30, Stuart Henderson wrote:
> On 2019/10/21 04:06, Martin Pieuchot wrote:
> > CVSROOT:/cvs
> > Module name:src
> > Changes by: m...@cvs.openbsd.org2019/10/21 04:06:31
> >
> > Modified files:
> > lib/librthread : synch.h
> >
> > Log message:
> > Use process-private futexes to avoid the uvm_map lookup overhead.
> >
> > While here kill unused _wait() function.
> >
> > ok visa@
> >
>
> I'm going to back this one out for now - it causes hangs with Python
> (seems to be 100% reproducible). Easy/fast test case:
>
> cd /usr/ports/graphics/py-Pillow
> make
>
> With this commit, it hangs after
>
> writing manifest file 'src/Pillow.egg-info/SOURCES.txt'
ltrace shows that sempahore are used.
gdb says that sem_init() is always called with `pshared' set to 0.
However python forks then sleeps in futex(2), via sem_wait(3), when
private futexes are used.
When non-private futexes are used it it seems that threads from a
process wake up the sleeping thread from a parent process:
84294/104794 python2.7 CALL futex(0x1a49bbca3008,0x1,0,0,0)
84677/611464 python2.7 CALL futex(0x1a49bbca3008,0x2,1,0,0)
To me it seems that shared semaphore, that are advertised in librthread as
non-supported, are in fact used.
Python has a second lock implementation using mutex and condvar, I'd be
interested to know if that works.
