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Date: Thu, 16 Aug 2018 08:34:58 +0200
From: tworaz <tworaz666@...il.com>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] Don't return new thread ID from pthread_create.
Hi Rich,
You're absolutely right, my fix is not correct. I should have probably
spent a bit more time analysing the code before making the change ;) I can
confirm your fix solves my original problem. Qt's QThread should be once
again usable on musl based systems :)
/ptw
On Thu, Aug 16, 2018 at 1:30 AM Rich Felker <dalias@...c.org> wrote:
> On Thu, Aug 16, 2018 at 01:09:33AM +0200, Piotr Tworek wrote:
> > From: Piotr Tworek <tworaz666@...il.com>
> >
> > According to documentation pthread_create is supposed to return 0 on
> > success or error code in case of failure. Unfortunately this is not
> > always the case in musl. In case the application has called
> > pthread_attr_setinheritsched(&attr, PTHREAD_EXPLICIT_SCHED),
> > pthread_create will return new thread ID. To make matters worse such non
> > zero value does not mean thread creation failed. Its quite the oposite.
> > Thread was created succesfully and start routine passed to pthread_create
> > will actually get invoked.
> >
> > To fix the problem simply drop the return statement from the last
> > if statement in musl's pthread_create. The value currently being returned
> > from there is a return code from __clone. Negative values indicating
> > failure have already been handled by previous if statement and positive
> > value indicates everything went correctly and pthread_create should
> return 0.
> > ---
> > src/thread/pthread_create.c | 1 -
> > 1 file changed, 1 deletion(-)
> >
> > diff --git a/src/thread/pthread_create.c b/src/thread/pthread_create.c
> > index 2df2e9f9..792cb42e 100644
> > --- a/src/thread/pthread_create.c
> > +++ b/src/thread/pthread_create.c
> > @@ -306,7 +306,6 @@ int __pthread_create(pthread_t *restrict res, const
> pthread_attr_t *restrict att
> >
> > if (do_sched) {
> > __futexwait(&ssa.futex, -1, 1);
> > - if (ret) return ret;
> > }
> >
> > *res = new;
>
> You're right that there's a bug, but the fix is not correct, and in
> some sense makes the problem worse -- pthread_create will wrongly
> return success with an invalid pthread_t as the result (since
> __start_sched detached itself and exited).
>
> The problem is just that the value of ret isn't being updated to match
> the error returned from __start_sched. Rather than:
>
> if (do_sched) {
> __futexwait(&ssa.futex, -1, 1);
> - if (ret) return ret;
> }
>
> it should look like (untested):
>
> if (do_sched) {
> __futexwait(&ssa.futex, -1, 1);
> + ret = ssa.futex;
> if (ret) return ret;
> }
>
> Note that this is a new regression and not in any released version
> yet. Let me know if you see any problems with the fix I just proposed
> and if you get a chance to test that it works, and I'll make sure the
> fix goes in for this release.
>
> Rich
>
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