Re: [PATCH] [BZ 18463] ARM - fix PI mutex breakge

[Joseph Myers <joseph at codesourcery dot com>]

On Mon, 20 Jul 2015, Marc Kleine-Budde wrote:

> This patch removes the "undef __ASSUME_REQUEUE_PI" to fix the issue.

Nothing in your explanation says why this should be ARM-specific.

You seem to be asserting that the semantics of __ASSUME_REQUEUE_PI are 
something other than "PI futexes work with FUTEX_*_REQUEUE_PI support in 
the kernel" - or at least, something other than (given the 2.6.32 minimum 
kernel version) "futex_atomic_cmpxchg_inatomic is supported".

What are the semantics of __ASSUME_REQUEUE_PI?  Your analysis only refers 
to one use of that macro; you need to examine all such uses.  Then, if the 
reference to futex_atomic_cmpxchg_inatomic in 
sysdeps/unix/sysv/linux/kernel-features.h is incorrect, you need to update 
the comment on the definition there of __ASSUME_REQUEUE_PI so that it 
accurately reflects the circumstances under which it is correct, or 
incorrect, for that macro to be defined on particular architectures.  
(That might mean adding an explanation of the semantics of the macro to 
that comment that makes clear why it's desirable to define it even if PI 
futexes might not be supported at runtime, for example.)  Then, each 
architecture that may undefine __ASSUME_REQUEUE_PI needs to be updated to 
be consistent with the clarified (and documented) understanding of the 
semantics of that macro - rather than just changing one architecture in 
isolation without justification for the undefining being correct for the 
other architectures.

-- 
Joseph S. Myers
joseph@codesourcery.com