Re: libthr and main thread stack size

On Sun, 21 Sep 2014, Julian Elischer wrote:

> On 9/20/14, 3:27 AM, John Baldwin wrote:
>> On Tuesday, September 16, 2014 11:13:24 AM Konstantin Belousov wrote:
>>> On Mon, Sep 15, 2014 at 03:47:41PM -0600, Justin T. Gibbs wrote:
>>>> On Aug 8, 2014, at 5:22 AM, Konstantin Belousov <kostikbel_at_gmail.com>
>>>> wrote:
>>>> 
>>>> ?
>>>> 
>>>>> Below is the patch which adds environment variable
>>>>> LIBPTHREAD_BIGSTACK_MAIN. Setting it to any value results in the
>>>>> main thread stack left as is, and other threads allocate stack
>>>>> below the area of RLIMIT_STACK. Try it. I do not want to set this
>>>>> behaviour as default.
>>>> Is there a reason this should not be the default? Looking at the
>>>> getrlimit() page on the OpenGroup?s site they say:
>>>> 
>>>> RLIMIT_STACK This is the maximum size of the initial thread's stack,
>>>> in bytes. The implementation does not automatically grow the stack
>>>> beyond this limit. If this limit is exceeded, SIGSEGV shall be
>>>> generated for the thread. If the thread is blocking SIGSEGV, or the
>>>> process is ignoring or catching SIGSEGV and has not made arrangements
>>>> to use an alternate stack, the disposition of SIGSEGV shall be set to
>>>> SIG_DFL before it is generated.
>>>> 
>>>> Does posix say something different?
>>>> 
>>>> I ran into this issue when debugging a segfault on Postgres when
>>>> running an (arguably quite bogus) query that should have fit within
>>>> both the configured stack rlimit and Postgres? configured stack limit.
>>>> The Postgres backend is really just single threaded, but happens
>>>> to pull in libpthread due to the threading support in some of the
>>>> libraries it uses. The segfault definitely violates POLA.
>>>> 
>>>> ? Justin
>>> I am conservative to not disturb the address space layout in single go.
>>> If enough people test this setting, I can consider flipping the default
>>> to the reverse.
>>> 
>>> I am still curious why the things were done in this way, but nobody
>>> replied.
>> I suspect it was done out of reasons of being overly conservative in
>> interpreting RLIMIT_STACK.  I think it is quite surprising behavior though 
>> and
>> would rather we make your option the default and implement what the Open 
>> Group
>> says above.
>> 
> that is my memory..
> The transition from a non threaded app to a threaded app with one thread is 
> sort of an undefined area.
> Feel free to change it if Dan agrees..

I'm all for adopting what POSIX specifies as the default.  I
would shy away from adding another knob (LIBPTHREAD_BIGSTACK_MAIN)
if possible.

-- 
DE