RE: [Patch] ARM define atomic_exchange_acq/atomic_exchange_rel to __atomic_exchange_n

[Abhishek Deb <adeb at nvidia dot com>]

From the functionality point of view, the changes look ok.

An objdump of the atomic_compare_and_exchange_bool_acq shows the following:

  60:   e1903f9f        ldrex   r3, [r0]
  64:   e1530002        cmp     r3, r2
  68:   1a000003        bne     7c <local__lll_lock_wait+0x34>
  6c:   e1804f91        strex   r4, r1, [r0]
  70:   e3540000        cmp     r4, #0
  74:   1afffff9        bne     60 <local__lll_lock_wait+0x18>
  78:   f57ff05f        dmb     sy

As you can see it contains only one dmb in the end and not in the beginning, which is in-line with acquire semantics to not allow any code hoisting.

Abhishek

-----Original Message-----
From: Joseph Myers [mailto:joseph@codesourcery.com] 
Sent: Monday, September 02, 2013 8:08 AM
To: Dinar Temirbulatov
Cc: libc-ports@sourceware.org; Abhishek Deb
Subject: Re: [Patch] ARM define atomic_exchange_acq/atomic_exchange_rel to __atomic_exchange_n

On Mon, 2 Sep 2013, Dinar Temirbulatov wrote:

> Hi,
> Another version of change, I added
> atomic_compare_and_exchange_val_acq/atomic_compare_and_exchange_val_re
> l and 
> atomic_compare_and_exchange_bool_acq/atomic_compare_and_exchange_boot_
> rel defenitions and also for gcc-4.7 and higher in the case of 
> unsupported atomic compare and swap operation, it uses the kernel 
> helper inlines.
> Tested on arm a9 with no new regressions. Ok to commit?
> Oh, sorry. I missed to attach the change. Here it is.

For subsequent patch revisions, please note there should be an extra space after "#" for preprocessor directives inside #if conditionals, one per level of #if nesting (other than toplevel multiple-inclusion guards) - which means that if conditioning existing code, you need to adjust directives inside that code.

This patch appears to have too much duplication.  For example, you duplicate the definition of __arm_assisted_compare_and_exchange_val_32_acq
- but that should not need any extra conditionals at all (beyond the existing #ifndef __arm_assisted_compare_and_exchange_val_32_acq), there's no reason ever not to define it.  Similarly, you duplicate __arch_compare_and_exchange_val_64_acq, but with proper #if structure there should only need to be one copy of the version that uses __arm_link_error.

What I think you should aim for is that each definition, or small group of definitions, uses conditionals in the form

#if __GNUC_PREREQ (4, 7) && defined __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4
/* Version using __atomic_*.  */
#elif defined __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4
/* Version using __sync_*.  */
#else
/* Version using __arm_assisted_*.  */
#endif

with cases omitted if not useful for that particular macro (this may include some macros not being defined at all in some cases).  So don't insert any global conditionals affecting all the existing definitions at all - look at each block of conditionals and add a third case as needed, along with any new macros (again with conditionals in that form) that are appropriate.

Where you use abort () in some definitions, use __arm_link_error () instead, like for the existing definitions.

--
Joseph S. Myers
joseph@codesourcery.com