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Re: [PATCH] malloc: Run fork handler as late as possible [BZ #19431]

On 04/12/2016 08:16 PM, Torvald Riegel wrote:
On Thu, 2016-02-18 at 17:15 +0100, Florian Weimer wrote:
On 02/15/2016 07:10 PM, Torvald Riegel wrote:

It explicitly encodes the lock order in the implementations of fork,
and does not rely on the registration order, thus avoiding the deadlock.

I couldn't test the Hurd bits, but the changes look straightforward enough.

Are those changes, and thus the new scheme, documented anywhere?

Fork handlers used by the implementation should be invisible to
applications.  The old one wasn't, and this was a bug.

But have you documented this and your understanding of the
synchronization scheme anywhere?  Your explanation in the email with the
patch seems more detailed than the comments in the code.

That's because I'm talking mainly about removed code and explaining a bug. I'm not sure how this information will be useful to future developers once the bug is gone. We have a regression test, which should avoid reintroducing precisely the same bug. For similar issues, we need to rely on code review and collective memory.

I think it's really worthwhile to always update and improve the
documentation, especially if we're kind of (re)discovering knowledge
about the current implementation such as in this case.  I know this is
extra work, but if this isn't documented, it's less likely anybody but
you will be aware of how this all works.

Do you mean something like this inside the fork implementation?

   /* Acquire malloc locks.  This needs to come last because fork
      handlers may use malloc, and the libio list lock has an indirect
      malloc dependency as well (via the getdelim function).  */
   __malloc_fork_lock_parent ();

Not quite.  IIRC this is one comment in one part of the code, but it's
not easy to find it from the other parts of code that are related.

One of the main motivations to document the synchronization scheme is to
avoid requiring future developers that want to modify the code to
closely review all the code (eg, to look for comments such as the one

The cycle isn't there anymore, so it would not particularly helpful to mention fork handlers in a comment on _IO_flush_all_lockp.

I would even go one step further: It would be misleading because it implies that we are confident about the libio locking behavior and lack of cycles. But libio calls user-supplied callbacks (set by fopencookie) while implementation locks are acquired (again in _IO_flush_all_lockp). Diagnostic functions in the malloc subsystem call into libio (via stdio streams) while malloc locks are acquired, and libio may in turn use malloc. I'm sure I can find more examples with more poking.

(The thinking is that, simplified, in sequential code it's
sufficient to look at the callers or the contract of your function,
whereas in concurrent code you don't easily know who is actually
affected by a particiular lock acquisition order.)

With callbacks, you can't be sure even with sequential code.

This is made easier if there's an overview of the synchronization
somewhere in the code, and it is referenced in affected parts of the

I can add some comments to libio with what I have found, but this doesn't really related to the locking cycle I removed (because again, it's no longer there).

diff --git a/malloc/malloc-private.h b/malloc/malloc-private.h
new file mode 100644
index 0000000..2fef840
--- /dev/null
+++ b/malloc/malloc-private.h

Should this perhaps be malloc-internal.h to match libc-internal.h?

We also have libio/libioP.h, and include/malloc.h.  But I think a
separate header makes sense if its contents is truly internal.

What are the preferences here?

I think having an internal header is good, but I can't give a definitive
reply regarding what the project prefers.

See my reply to Roland. I was under the impression you objected to the name.


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