Re: futex_cmpxchg_enabled breakage

On Sun, Sep 16, 2018 at 03:38:44PM +0200, Florian Weimer wrote:

* Rich Felker:

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I believe the expected userspace interface is that you probe support
with set_robust_list first, and then start using the relevant futex
interfaces only if that call succeeded.

In order for it to work, set_robust_list needs to succeed for all
threads, present and future, so there's an implicit contract needed
here that, if it succeeds once, it needs to always succeed. This is
satisfied by the kernel implementation.

It certainly makes simpler if set_robust_list cannot fail due to
resource allocation issues.

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Presumably a similar probing should happen in
pthread_mutexattr_setprotocol for PI mutex support. Does glibc do
this? musl still lacks PI mutex support so I'll save this as a note
for when it's added.

glibc currently implements checking for support in pthread_mutex_init,
presumably due to the fact that some invalid attribute/flag
combinations can only reasonably detected at that point.  It makes
probing for support slightly more difficult, of course.

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If you do that, most parts of
a typical system will work as expected even if the kernel support is
not there, which is a bit surprising.  It definitely makes the root
cause harder to spot.

I don't follow here. "most parts of a typical system will work as
expected" seems to be the case whether you do or don't correctly
probe. The only difference is whether a program that carefully checks
for errors will see and report that pthread_mutexattr_setrobust
failed.

This may be the case.  We only ever had the glibc test failures as
evidence that something was quite wrong, despite ongoing validation of
the system.  But this could have been accident due to an invalid test
environment.  (The product in question is only supposed to support the
radix MMU, but when running under KVM, the kernel switches to the hash
MMU instead, which masks the presence of the bug—set_robust_list is
magically available again.)

BTW here's a horrible thought: can the availability of set_robust_list
change across checkpoint/restore? If so that's fundamental breakage in
the checkpoint/restore functionality, and a good reason to make it so
this functionality is not runtime-variable for a given kernel.

Rich