[PATCH][ARM] Fix _dl_tlsdesc_resolve_hold to save r0

[Ramana Radhakrishnan]

On Wed, Oct 14, 2015 at 2:51 PM, Szabolcs Nagy <szabolcs.nagy@arm.com> wrote:
> On 14/10/15 13:17, Joseph Myers wrote:
>>
>> On Wed, 14 Oct 2015, Szabolcs Nagy wrote:
>>
>>> _dl_tlsdesc_resolve_hold calls into a C function that clobbers r0,
>>> but it assumes the original argument is still in r0 after the call.
>>> This can cause crash in case of concurrent TLS access when TLSDESC
>>> is in use (-mtls-dialect=gnu2).
>>
>>
>> I presume this issue was user-visible in a release, in which case a bug
>> should be filed in Bugzilla for it.  Should I take it that it's hard to
>> write a testcase for this bug that fails reliably without the patch?
>>
>
> ok it's BZ 19129
>
> i have a test that triggers this or the other tlsdesc race
> i was about to fix, but it is a bit expensive.
>
> (666 tls objects accessed from 66 threads, 2 versions to
> get tlsdesc entries 8byte-unaligned, running 10x triggers
> the failure, a smaller test triggers less frequently.)
>
> i'm not sure what's the glibc policy for expensive tests.
>
> and i think -mtls-dialect=gnu2 is not available in all
> supported gcc versions.

Support for this appeared in FSF GCC 4.7


Ramana

>
> (the test makes sense on all archs where lazy tls
> initialization may be racy).
>