[PATCH] Async signal safe TLS accesses

[Alexandre Oliva]

On Dec  5, 2013, Andrew Hunter <ahh@google.com> wrote:

> On Thu, Nov 28, 2013 at 5:03 PM, Alexandre Oliva <aoliva@redhat.com> wrote:

>> The lock is recursive, but I don't see anything in the patch besides
>> the CAS loops that avoids inconsistent state, particularly in the
>> case of lazy TLS relocation (introduced with GNU2 TLS).  Was that not
>> taken into account?

> glibc recursive locks aren't async-signal-safe.  We avoid problems
> with reentrancy via dl_mask_all_signals.

But that doesn't avoid *concurrency* problems, does it?

Besides, GNU2 TLS provides for lazy TLS relocation (as mentioned above),
and that most certainly takes the DL lock.  I don't see anything in the
patch that would avoid signal-safety problems in that path.

GNU2 will probably be made default on x86 and ARM eventually, and it
already is the default on a number of other recent architectures;
AArch64 is one of them IIRC.  So, if this issue was not addressed by
this patch, I would recommend being careful with the messaging: it may
fail to make TLS AS-safe in general.  Some qualifier would be necessary
to make the statement correct.

-- 
Alexandre Oliva, freedom fighter    http://FSFLA.org/~lxoliva/
You must be the change you wish to see in the world. -- Gandhi
Be Free! -- http://FSFLA.org/   FSF Latin America board member
Free Software Evangelist      Red Hat Brazil Compiler Engineer