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Message-ID: <887d459e35e771767465267e578a8490@ispras.ru>
Date: Wed, 05 Oct 2022 19:23:48 +0300
From: Alexey Izbyshev <izbyshev@...ras.ru>
To: musl@...ts.openwall.com
Subject: Re: Illegal killlock skipping when transitioning to
single-threaded state
On 2022-10-05 17:37, Rich Felker wrote:
> On Wed, Oct 05, 2022 at 10:03:03AM -0400, Rich Felker wrote:
>> On Wed, Oct 05, 2022 at 03:10:09PM +0300, Alexey Izbyshev wrote:
>> > On 2022-10-05 04:00, Rich Felker wrote:
>> > >On Wed, Sep 07, 2022 at 03:46:53AM +0300, Alexey Izbyshev wrote:
>> > >>Reordering the "libc.need_locks = -1" assignment and
>> > >>UNLOCK(E->killlock) and providing a store barrier between them
>> > >>should fix the issue.
>> > >
>> > >Back to this, because it's immediately actionable without resolving
>> > >the aarch64 atomics issue:
>> > >
>> > >Do you have something in mind for how this reordering is done, since
>> > >there are other intervening steps that are potentially ordered with
>> > >respect to either or both? I don't think there is actually any
>> > >ordering constraint at all on the unlocking of killlock (with the
>> > >accompanying assignment self->tid=0 kept with it) except that it be
>> > >past the point where we are committed to the thread terminating
>> > >without executing any more application code. So my leaning would be to
>> > >move this block from the end of pthread_exit up to right after the
>> > >point-of-no-return comment.
>> > >
>> > This was my conclusion as well back when I looked at it before
>> > sending the report.
>> >
>> > I was initially concerned about whether reordering with
>> > a_store(&self->detach_state, DT_EXITED) could cause an unwanted
>> > observable change (pthread_tryjoin_np() returning EBUSY after a
>> > pthread function acting on tid like pthread_getschedparam() returns
>> > ESRCH), but no, pthread_tryjoin_np() will block/trap if the thread
>> > is not DT_JOINABLE.
>> >
>> > >Unfortunately while reading this I found another bug, this time a lock
>> > >order one. __dl_thread_cleanup() takes a lock while the thread list
>> > >lock is already held, but fork takes these in the opposite order. I
>> > >think the lock here could be dropped and replaced with an atomic-cas
>> > >list head, but that's rather messy and I'm open to other ideas.
>> > >
>> > I'm not sure why using a lock-free list is messy, it seems like a
>> > perfect fit here to me.
>>
>> Just in general I've tried to reduce the direct use of atomics and use
>> high-level primitives, because (as this thread is evidence of) I find
>> the reasoning about direct use of atomics and their correctness to be
>> difficult and inaccessible to a lot of people who would otherwise be
>> successful readers of the code. But you're right that it's a "good
>> match" for the problem at hand.
>>
>> > However, doesn't __dl_vseterr() use the libc-internal allocator
>> > after 34952fe5de44a833370cbe87b63fb8eec61466d7? If so, the problem
>> > that freebuf_queue was originally solving doesn't exist anymore. We
>> > still can't call the allocator after __tl_lock(), but maybe this
>> > whole free deferral approach can be reconsidered?
>>
>> I almost made that change when the MT-fork changes were done, but
>> didn't because it was wrong. I'm not sure if I documented this
>> anywhere (it might be in mail threads related to that or IRC) but it
>> was probably because it would need to take malloc locks with the
>> thread list lock held, which isn't allowed.
>>
>> It would be nice if we could get rid of the deferred freeing here, but
>> I don't see a good way. The reason we can't free the buffer until
>> after the thread list lock is taken is that it's only freeable if this
>> isn't the last exiting thread. If it is the last exiting thread, the
>> buffer contents still need to be present for the atexit handlers to
>> see. And whether this is the last exiting thread is only
>> stable/determinate as long as the thread list lock is held.
>
> Proposed patch with atomic list attached, along with a stupid test
> program (to be run under a debugger to see anything happening).
>
The patch looks good to me, and the program does the expected thing for
me when linked with the patched musl.
Inclusion of "lock.h" and "fork_impl.h" can also be removed from
dlerror.c.
Alexey
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