Re: __synccall: deadlock and reliance on racy /proc/self/task

[Rich Felker <dalias@libc.org>]

On Sat, Feb 09, 2019 at 10:40:45PM +0100, Szabolcs Nagy wrote:
> * Alexey Izbyshev <izbyshev@ispras.ru> [2019-02-09 21:33:32 +0300]:
> > On 2019-02-09 19:21, Szabolcs Nagy wrote:
> > > * Rich Felker <dalias@libc.org> [2019-02-08 13:33:57 -0500]:
> > > > On Fri, Feb 08, 2019 at 09:14:48PM +0300, Alexey Izbyshev wrote:
> > > > > On 2/7/19 9:36 PM, Rich Felker wrote:
> > > > > >Does it work if we force two iterations of the readdir loop with no
> > > > > >tasks missed, rather than just one, to catch the case of missed
> > > > > >concurrent additions? I'm not sure. But all this makes me really
> > > > > >uncomfortable with the current approach.
> > > > >
> > > > > I've tested with 0, 1, 2 and 3 retries of the main loop if miss_cnt
> > > > > == 0. The test eventually failed in all cases, with 0 retries
> > > > > requiring only a handful of iterations, 1 -- on the order of 100, 2
> > > > > -- on the order of 10000 and 3 -- on the order of 100000.
> > > > 
> > > > Do you have a theory on the mechanism of failure here? I'm guessing
> > > > it's something like this: there's a thread that goes unseen in the
> > > > first round, and during the second round, it creates a new thread and
> > > > exits itself. The exit gets seen (again, it doesn't show up in the
> > > > dirents) but the new thread it created still doesn't. Is that right?
> > > > 
> > > > In any case, it looks like the whole mechanism we're using is
> > > > unreliable, so something needs to be done. My leaning is to go with
> > > > the global thread list and atomicity of list-unlock with exit.
> > > 
> > > yes that sounds possible, i added some instrumentation to musl
> > > and the trace shows situations like that before the deadlock,
> > > exiting threads can even cause old (previously seen) entries to
> > > disappear from the dir.
> > > 
> > Thanks for the thorough instrumentation! Your traces confirm both my theory
> > about the deadlock and unreliability of /proc/self/task.
> > 
> > I'd also done a very light instrumentation just before I got your email, but
> > it took me a while to understand the output I got (see below).
> 
> the attached patch fixes the issue on my machine.
> i don't know if this is just luck.
> 
> the assumption is that if /proc/self/task is read twice such that
> all tids in it seem to be active and caught, then all the active
> threads of the process are caught (no new threads that are already
> started but not visible there yet)

I'm skeptical of whether this should work in principle. If the first
scan of /proc/self/task misses tid J, and during the next scan, tid J
creates tid K then exits, it seems like we could see the same set of
tids on both scans.

Maybe it's salvagable though. Since __block_new_threads is true, in
order for this to happen, tid J must have been between the
__block_new_threads check in pthread_create and the clone syscall at
the time __synccall started. The number of threads in such a state
seems to be bounded by some small constant (like 2) times
libc.threads_minus_1+1, computed at any point after
__block_new_threads is set to true, so sufficiently heavy presignaling
(heavier than we have now) might suffice to guarantee that all are
captured. 

Rich