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Re: [Xen-devel] [PATCH for-4.9] x86/mm: Placate DEADCODE Coverity warning



CC Ian + Wei for the testing

On 01/06/17 12:21, Jan Beulich wrote:
On 01.06.17 at 13:18, <julien.grall@xxxxxxx> wrote:
Hi Jan,

On 01/06/17 12:15, Jan Beulich wrote:
On 01.06.17 at 13:09, <julien.grall@xxxxxxx> wrote:
Hi Andrew,

On 31/05/17 14:23, Andrew Cooper wrote:
On 31/05/17 09:52, Julien Grall wrote:
Hi,

On 05/22/2017 02:32 PM, Jan Beulich wrote:
On 22.05.17 at 15:12, <andrew.cooper3@xxxxxxxxxx> wrote:
_PAGE_GNTTAB is only used in debug builds of Xen; in release builds,
it has
the value 0.  Coverity complains that "l1e_get_flags(l1e) & 0" is
logically
dead.

Add an extra condition into the logic to skip the flag check if
_PAGE_GNTTAB
is 0.

And this helps? To me "if ( 0 && ... )" and "if ( (x & 0) && ... )" look
logically the same (i.e. I'd expect the same warnings to be triggered
[or not]).

I haven't seen any answer on this question. Andrew, does this patch
still hold for Xen 4.9?

Sorry - it fell through the cracks, but yes, it does stand for 4.9.

As to the "if ( 0 && ... )" and "if ( (x & 0) && ... )", one is very
clearly a "short circuit every thing else if this value is zero", while
the other looks like a programming mistake, which is also why I expect
this to resolve Coverity's complaint.

Unfortunately, I can't be certain that this will resolve the issue until
it gets committed, as I don't have a useful way to run Coverity on
arbitrary non-debug builds.

Are we running staging-4.* branch on Coverity? Looking at the git, I see
*coverity* branch only on unstable.

Yes, so my suggestion would be to commit the patch on master, see
if it helps, and if so consider backporting for 4.9. If it doesn't help,
it should be reverted or replaced by something "better".

Well master has now debug enabled and AFAIU the warning can only
occurred on non-debug build. So I am not sure how this would help to be
in master.

Oh, good point. I think it would really be a good idea to have
both debug and non-debug builds tested, which at once would
avoid sudden bursts of issues when switching the default.

Jan


--
Julien Grall

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