[Xenomai] Dealing with too small thread stacks

[Xenomai] Dealing with too small thread stacks

[Jan Kiszka]

Hi,

analyzing page faults of an application that prefers to set its own
stacks, I noticed a problem in Xenomai (2 and 3), at least from the
usability POV: We document the minimum stack stack as PTHREAD_STACK_MIN
+ 1 page, at least in Xenomai 3, and we enforce that on thread creation.
However, enforcement is doomed to fail if the stack is preallocated (and
that too small).

As we cannot detect if the user set a stack address in pthread_attr_t, I
would suggest to fail thread creation instead of performing it with
improper parameters. Other suggestions? If not, I would prepare a patch
for Xenomai 3 (for 2 only if desired).

Jan

-- 
Siemens AG, Corporate Technology, CT RTC ITP SES-DE
Corporate Competence Center Embedded Linux

Re: [Xenomai] Dealing with too small thread stacks

[Gilles Chanteperdrix]

On Fri, Apr 17, 2015 at 07:34:30PM +0200, Jan Kiszka wrote:
> Hi,
> 
> analyzing page faults of an application that prefers to set its own
> stacks, I noticed a problem in Xenomai (2 and 3), at least from the
> usability POV: We document the minimum stack stack as PTHREAD_STACK_MIN
> + 1 page, at least in Xenomai 3, and we enforce that on thread creation.
> However, enforcement is doomed to fail if the stack is preallocated (and
> that too small).
> 
> As we cannot detect if the user set a stack address in pthread_attr_t, I
> would suggest to fail thread creation instead of performing it with
> improper parameters. Other suggestions? If not, I would prepare a patch
> for Xenomai 3 (for 2 only if desired).

It seems to me we can detect the parameters in the pthread_attr_t
using pthread_attr_getstack. So, we can get __wrap_pthread_create to
fail if the size is not sufficient.

-- 
					    Gilles.

Re: [Xenomai] Dealing with too small thread stacks

[Jan Kiszka]

On 2015-04-17 19:50, Gilles Chanteperdrix wrote:
> On Fri, Apr 17, 2015 at 07:34:30PM +0200, Jan Kiszka wrote:
>> Hi,
>>
>> analyzing page faults of an application that prefers to set its own
>> stacks, I noticed a problem in Xenomai (2 and 3), at least from the
>> usability POV: We document the minimum stack stack as PTHREAD_STACK_MIN
>> + 1 page, at least in Xenomai 3, and we enforce that on thread creation.
>> However, enforcement is doomed to fail if the stack is preallocated (and
>> that too small).
>>
>> As we cannot detect if the user set a stack address in pthread_attr_t, I
>> would suggest to fail thread creation instead of performing it with
>> improper parameters. Other suggestions? If not, I would prepare a patch
>> for Xenomai 3 (for 2 only if desired).
> 
> It seems to me we can detect the parameters in the pthread_attr_t
> using pthread_attr_getstack. So, we can get __wrap_pthread_create to
> fail if the size is not sufficient.

Nope, unfortunately not:

"If the pthread_attr_getstack() function is called before the stackaddr
attribute has been set, the behavior is unspecified."

Jan

-- 
Siemens AG, Corporate Technology, CT RTC ITP SES-DE
Corporate Competence Center Embedded Linux

Re: [Xenomai] Dealing with too small thread stacks

[Gilles Chanteperdrix]

On Fri, Apr 17, 2015 at 08:05:57PM +0200, Jan Kiszka wrote:
> On 2015-04-17 19:50, Gilles Chanteperdrix wrote:
> > On Fri, Apr 17, 2015 at 07:34:30PM +0200, Jan Kiszka wrote:
> >> Hi,
> >>
> >> analyzing page faults of an application that prefers to set its own
> >> stacks, I noticed a problem in Xenomai (2 and 3), at least from the
> >> usability POV: We document the minimum stack stack as PTHREAD_STACK_MIN
> >> + 1 page, at least in Xenomai 3, and we enforce that on thread creation.
> >> However, enforcement is doomed to fail if the stack is preallocated (and
> >> that too small).
> >>
> >> As we cannot detect if the user set a stack address in pthread_attr_t, I
> >> would suggest to fail thread creation instead of performing it with
> >> improper parameters. Other suggestions? If not, I would prepare a patch
> >> for Xenomai 3 (for 2 only if desired).
> > 
> > It seems to me we can detect the parameters in the pthread_attr_t
> > using pthread_attr_getstack. So, we can get __wrap_pthread_create to
> > fail if the size is not sufficient.
> 
> Nope, unfortunately not:
> 
> "If the pthread_attr_getstack() function is called before the stackaddr
> attribute has been set, the behavior is unspecified."

It is unspecified by POSIX, but Xenomai supports only two
implementations, glibc and uClibc, so, we can look at what these two
libraries do. I would bet they return you a NULL stack pointer or
something.

-- 
					    Gilles.

Re: [Xenomai] Dealing with too small thread stacks

[Jan Kiszka]

On 2015-04-17 20:08, Gilles Chanteperdrix wrote:
> On Fri, Apr 17, 2015 at 08:05:57PM +0200, Jan Kiszka wrote:
>> On 2015-04-17 19:50, Gilles Chanteperdrix wrote:
>>> On Fri, Apr 17, 2015 at 07:34:30PM +0200, Jan Kiszka wrote:
>>>> Hi,
>>>>
>>>> analyzing page faults of an application that prefers to set its own
>>>> stacks, I noticed a problem in Xenomai (2 and 3), at least from the
>>>> usability POV: We document the minimum stack stack as PTHREAD_STACK_MIN
>>>> + 1 page, at least in Xenomai 3, and we enforce that on thread creation.
>>>> However, enforcement is doomed to fail if the stack is preallocated (and
>>>> that too small).
>>>>
>>>> As we cannot detect if the user set a stack address in pthread_attr_t, I
>>>> would suggest to fail thread creation instead of performing it with
>>>> improper parameters. Other suggestions? If not, I would prepare a patch
>>>> for Xenomai 3 (for 2 only if desired).
>>>
>>> It seems to me we can detect the parameters in the pthread_attr_t
>>> using pthread_attr_getstack. So, we can get __wrap_pthread_create to
>>> fail if the size is not sufficient.
>>
>> Nope, unfortunately not:
>>
>> "If the pthread_attr_getstack() function is called before the stackaddr
>> attribute has been set, the behavior is unspecified."
> 
> It is unspecified by POSIX, but Xenomai supports only two
> implementations, glibc and uClibc, so, we can look at what these two
> libraries do. I would bet they return you a NULL stack pointer or
> something.

I would have expected that, too, but the results for glibc seem random.
Plus there is the risk that something changes, thus we become
version-dependent.

Jan

-- 
Siemens AG, Corporate Technology, CT RTC ITP SES-DE
Corporate Competence Center Embedded Linux

Re: [Xenomai] Dealing with too small thread stacks

[Gilles Chanteperdrix]

On Fri, Apr 17, 2015 at 08:10:40PM +0200, Jan Kiszka wrote:
> On 2015-04-17 20:08, Gilles Chanteperdrix wrote:
> > On Fri, Apr 17, 2015 at 08:05:57PM +0200, Jan Kiszka wrote:
> >> On 2015-04-17 19:50, Gilles Chanteperdrix wrote:
> >>> On Fri, Apr 17, 2015 at 07:34:30PM +0200, Jan Kiszka wrote:
> >>>> Hi,
> >>>>
> >>>> analyzing page faults of an application that prefers to set its own
> >>>> stacks, I noticed a problem in Xenomai (2 and 3), at least from the
> >>>> usability POV: We document the minimum stack stack as PTHREAD_STACK_MIN
> >>>> + 1 page, at least in Xenomai 3, and we enforce that on thread creation.
> >>>> However, enforcement is doomed to fail if the stack is preallocated (and
> >>>> that too small).
> >>>>
> >>>> As we cannot detect if the user set a stack address in pthread_attr_t, I
> >>>> would suggest to fail thread creation instead of performing it with
> >>>> improper parameters. Other suggestions? If not, I would prepare a patch
> >>>> for Xenomai 3 (for 2 only if desired).
> >>>
> >>> It seems to me we can detect the parameters in the pthread_attr_t
> >>> using pthread_attr_getstack. So, we can get __wrap_pthread_create to
> >>> fail if the size is not sufficient.
> >>
> >> Nope, unfortunately not:
> >>
> >> "If the pthread_attr_getstack() function is called before the stackaddr
> >> attribute has been set, the behavior is unspecified."
> > 
> > It is unspecified by POSIX, but Xenomai supports only two
> > implementations, glibc and uClibc, so, we can look at what these two
> > libraries do. I would bet they return you a NULL stack pointer or
> > something.
> 
> I would have expected that, too, but the results for glibc seem random.
> Plus there is the risk that something changes, thus we become
> version-dependent.

Ok then, what about the influence of pthread_attr_setstack() on
pthread_attr_getstacksize(), maybe more luck there?

-- 
					    Gilles.

Re: [Xenomai] Dealing with too small thread stacks

[Jan Kiszka]

On 2015-04-17 20:12, Gilles Chanteperdrix wrote:
> On Fri, Apr 17, 2015 at 08:10:40PM +0200, Jan Kiszka wrote:
>> On 2015-04-17 20:08, Gilles Chanteperdrix wrote:
>>> On Fri, Apr 17, 2015 at 08:05:57PM +0200, Jan Kiszka wrote:
>>>> On 2015-04-17 19:50, Gilles Chanteperdrix wrote:
>>>>> On Fri, Apr 17, 2015 at 07:34:30PM +0200, Jan Kiszka wrote:
>>>>>> Hi,
>>>>>>
>>>>>> analyzing page faults of an application that prefers to set its own
>>>>>> stacks, I noticed a problem in Xenomai (2 and 3), at least from the
>>>>>> usability POV: We document the minimum stack stack as PTHREAD_STACK_MIN
>>>>>> + 1 page, at least in Xenomai 3, and we enforce that on thread creation.
>>>>>> However, enforcement is doomed to fail if the stack is preallocated (and
>>>>>> that too small).
>>>>>>
>>>>>> As we cannot detect if the user set a stack address in pthread_attr_t, I
>>>>>> would suggest to fail thread creation instead of performing it with
>>>>>> improper parameters. Other suggestions? If not, I would prepare a patch
>>>>>> for Xenomai 3 (for 2 only if desired).
>>>>>
>>>>> It seems to me we can detect the parameters in the pthread_attr_t
>>>>> using pthread_attr_getstack. So, we can get __wrap_pthread_create to
>>>>> fail if the size is not sufficient.
>>>>
>>>> Nope, unfortunately not:
>>>>
>>>> "If the pthread_attr_getstack() function is called before the stackaddr
>>>> attribute has been set, the behavior is unspecified."
>>>
>>> It is unspecified by POSIX, but Xenomai supports only two
>>> implementations, glibc and uClibc, so, we can look at what these two
>>> libraries do. I would bet they return you a NULL stack pointer or
>>> something.
>>
>> I would have expected that, too, but the results for glibc seem random.
>> Plus there is the risk that something changes, thus we become
>> version-dependent.
> 
> Ok then, what about the influence of pthread_attr_setstack() on
> pthread_attr_getstacksize(), maybe more luck there?

setstack defines the size getstacksize returns. So does setstacksize.

What happens during setstacksize is apparently that the address is set
to NULL - size. But, again, that is just the current glibc behaviour.

Jan

-- 
Siemens AG, Corporate Technology, CT RTC ITP SES-DE
Corporate Competence Center Embedded Linux

Re: [Xenomai] Dealing with too small thread stacks

[Gilles Chanteperdrix]

On Fri, Apr 17, 2015 at 08:17:26PM +0200, Jan Kiszka wrote:
> On 2015-04-17 20:12, Gilles Chanteperdrix wrote:
> > On Fri, Apr 17, 2015 at 08:10:40PM +0200, Jan Kiszka wrote:
> >> On 2015-04-17 20:08, Gilles Chanteperdrix wrote:
> >>> On Fri, Apr 17, 2015 at 08:05:57PM +0200, Jan Kiszka wrote:
> >>>> On 2015-04-17 19:50, Gilles Chanteperdrix wrote:
> >>>>> On Fri, Apr 17, 2015 at 07:34:30PM +0200, Jan Kiszka wrote:
> >>>>>> Hi,
> >>>>>>
> >>>>>> analyzing page faults of an application that prefers to set its own
> >>>>>> stacks, I noticed a problem in Xenomai (2 and 3), at least from the
> >>>>>> usability POV: We document the minimum stack stack as PTHREAD_STACK_MIN
> >>>>>> + 1 page, at least in Xenomai 3, and we enforce that on thread creation.
> >>>>>> However, enforcement is doomed to fail if the stack is preallocated (and
> >>>>>> that too small).
> >>>>>>
> >>>>>> As we cannot detect if the user set a stack address in pthread_attr_t, I
> >>>>>> would suggest to fail thread creation instead of performing it with
> >>>>>> improper parameters. Other suggestions? If not, I would prepare a patch
> >>>>>> for Xenomai 3 (for 2 only if desired).
> >>>>>
> >>>>> It seems to me we can detect the parameters in the pthread_attr_t
> >>>>> using pthread_attr_getstack. So, we can get __wrap_pthread_create to
> >>>>> fail if the size is not sufficient.
> >>>>
> >>>> Nope, unfortunately not:
> >>>>
> >>>> "If the pthread_attr_getstack() function is called before the stackaddr
> >>>> attribute has been set, the behavior is unspecified."
> >>>
> >>> It is unspecified by POSIX, but Xenomai supports only two
> >>> implementations, glibc and uClibc, so, we can look at what these two
> >>> libraries do. I would bet they return you a NULL stack pointer or
> >>> something.
> >>
> >> I would have expected that, too, but the results for glibc seem random.
> >> Plus there is the risk that something changes, thus we become
> >> version-dependent.
> > 
> > Ok then, what about the influence of pthread_attr_setstack() on
> > pthread_attr_getstacksize(), maybe more luck there?
> 
> setstack defines the size getstacksize returns. So does setstacksize.
> 
> What happens during setstacksize is apparently that the address is set
> to NULL - size. But, again, that is just the current glibc behaviour.

Yes, ok, but in order to test the stack size passed by the user, it
simply means we can use getstacksize and bail out if not sufficient.

-- 
					    Gilles.

Re: [Xenomai] Dealing with too small thread stacks

[Jan Kiszka]

On 2015-04-17 20:24, Gilles Chanteperdrix wrote:
> On Fri, Apr 17, 2015 at 08:17:26PM +0200, Jan Kiszka wrote:
>> On 2015-04-17 20:12, Gilles Chanteperdrix wrote:
>>> On Fri, Apr 17, 2015 at 08:10:40PM +0200, Jan Kiszka wrote:
>>>> On 2015-04-17 20:08, Gilles Chanteperdrix wrote:
>>>>> On Fri, Apr 17, 2015 at 08:05:57PM +0200, Jan Kiszka wrote:
>>>>>> On 2015-04-17 19:50, Gilles Chanteperdrix wrote:
>>>>>>> On Fri, Apr 17, 2015 at 07:34:30PM +0200, Jan Kiszka wrote:
>>>>>>>> Hi,
>>>>>>>>
>>>>>>>> analyzing page faults of an application that prefers to set its own
>>>>>>>> stacks, I noticed a problem in Xenomai (2 and 3), at least from the
>>>>>>>> usability POV: We document the minimum stack stack as PTHREAD_STACK_MIN
>>>>>>>> + 1 page, at least in Xenomai 3, and we enforce that on thread creation.
>>>>>>>> However, enforcement is doomed to fail if the stack is preallocated (and
>>>>>>>> that too small).
>>>>>>>>
>>>>>>>> As we cannot detect if the user set a stack address in pthread_attr_t, I
>>>>>>>> would suggest to fail thread creation instead of performing it with
>>>>>>>> improper parameters. Other suggestions? If not, I would prepare a patch
>>>>>>>> for Xenomai 3 (for 2 only if desired).
>>>>>>>
>>>>>>> It seems to me we can detect the parameters in the pthread_attr_t
>>>>>>> using pthread_attr_getstack. So, we can get __wrap_pthread_create to
>>>>>>> fail if the size is not sufficient.
>>>>>>
>>>>>> Nope, unfortunately not:
>>>>>>
>>>>>> "If the pthread_attr_getstack() function is called before the stackaddr
>>>>>> attribute has been set, the behavior is unspecified."
>>>>>
>>>>> It is unspecified by POSIX, but Xenomai supports only two
>>>>> implementations, glibc and uClibc, so, we can look at what these two
>>>>> libraries do. I would bet they return you a NULL stack pointer or
>>>>> something.
>>>>
>>>> I would have expected that, too, but the results for glibc seem random.
>>>> Plus there is the risk that something changes, thus we become
>>>> version-dependent.
>>>
>>> Ok then, what about the influence of pthread_attr_setstack() on
>>> pthread_attr_getstacksize(), maybe more luck there?
>>
>> setstack defines the size getstacksize returns. So does setstacksize.
>>
>> What happens during setstacksize is apparently that the address is set
>> to NULL - size. But, again, that is just the current glibc behaviour.
> 
> Yes, ok, but in order to test the stack size passed by the user, it
> simply means we can use getstacksize and bail out if not sufficient.

Sure, that would be the best way.

Along that, I would recommend raising the minimum to 2*PTHREAD_STACK_MIN.

Jan

-- 
Siemens AG, Corporate Technology, CT RTC ITP SES-DE
Corporate Competence Center Embedded Linux

Re: [Xenomai] Dealing with too small thread stacks

[Gilles Chanteperdrix]

On Fri, Apr 17, 2015 at 08:26:32PM +0200, Jan Kiszka wrote:
> On 2015-04-17 20:24, Gilles Chanteperdrix wrote:
> > On Fri, Apr 17, 2015 at 08:17:26PM +0200, Jan Kiszka wrote:
> >> On 2015-04-17 20:12, Gilles Chanteperdrix wrote:
> >>> On Fri, Apr 17, 2015 at 08:10:40PM +0200, Jan Kiszka wrote:
> >>>> On 2015-04-17 20:08, Gilles Chanteperdrix wrote:
> >>>>> On Fri, Apr 17, 2015 at 08:05:57PM +0200, Jan Kiszka wrote:
> >>>>>> On 2015-04-17 19:50, Gilles Chanteperdrix wrote:
> >>>>>>> On Fri, Apr 17, 2015 at 07:34:30PM +0200, Jan Kiszka wrote:
> >>>>>>>> Hi,
> >>>>>>>>
> >>>>>>>> analyzing page faults of an application that prefers to set its own
> >>>>>>>> stacks, I noticed a problem in Xenomai (2 and 3), at least from the
> >>>>>>>> usability POV: We document the minimum stack stack as PTHREAD_STACK_MIN
> >>>>>>>> + 1 page, at least in Xenomai 3, and we enforce that on thread creation.
> >>>>>>>> However, enforcement is doomed to fail if the stack is preallocated (and
> >>>>>>>> that too small).
> >>>>>>>>
> >>>>>>>> As we cannot detect if the user set a stack address in pthread_attr_t, I
> >>>>>>>> would suggest to fail thread creation instead of performing it with
> >>>>>>>> improper parameters. Other suggestions? If not, I would prepare a patch
> >>>>>>>> for Xenomai 3 (for 2 only if desired).
> >>>>>>>
> >>>>>>> It seems to me we can detect the parameters in the pthread_attr_t
> >>>>>>> using pthread_attr_getstack. So, we can get __wrap_pthread_create to
> >>>>>>> fail if the size is not sufficient.
> >>>>>>
> >>>>>> Nope, unfortunately not:
> >>>>>>
> >>>>>> "If the pthread_attr_getstack() function is called before the stackaddr
> >>>>>> attribute has been set, the behavior is unspecified."
> >>>>>
> >>>>> It is unspecified by POSIX, but Xenomai supports only two
> >>>>> implementations, glibc and uClibc, so, we can look at what these two
> >>>>> libraries do. I would bet they return you a NULL stack pointer or
> >>>>> something.
> >>>>
> >>>> I would have expected that, too, but the results for glibc seem random.
> >>>> Plus there is the risk that something changes, thus we become
> >>>> version-dependent.
> >>>
> >>> Ok then, what about the influence of pthread_attr_setstack() on
> >>> pthread_attr_getstacksize(), maybe more luck there?
> >>
> >> setstack defines the size getstacksize returns. So does setstacksize.
> >>
> >> What happens during setstacksize is apparently that the address is set
> >> to NULL - size. But, again, that is just the current glibc behaviour.
> > 
> > Yes, ok, but in order to test the stack size passed by the user, it
> > simply means we can use getstacksize and bail out if not sufficient.
> 
> Sure, that would be the best way.
> 
> Along that, I would recommend raising the minimum to 2*PTHREAD_STACK_MIN.

What for ? We know that the system crashes if the stack is too
small, but that is the consequence of the user choice, there is not
much we can do about it. What I would concentrate on, and I think we
had that on xenomai 2.x is to have a reasonable default stack size.
Actually, we could wrap pthread_attr_init to define our own default
and be done with it.

-- 
					    Gilles.

Re: [Xenomai] Dealing with too small thread stacks

[Jan Kiszka]

On 2015-04-17 20:30, Gilles Chanteperdrix wrote:
> On Fri, Apr 17, 2015 at 08:26:32PM +0200, Jan Kiszka wrote:
>> On 2015-04-17 20:24, Gilles Chanteperdrix wrote:
>>> On Fri, Apr 17, 2015 at 08:17:26PM +0200, Jan Kiszka wrote:
>>>> On 2015-04-17 20:12, Gilles Chanteperdrix wrote:
>>>>> On Fri, Apr 17, 2015 at 08:10:40PM +0200, Jan Kiszka wrote:
>>>>>> On 2015-04-17 20:08, Gilles Chanteperdrix wrote:
>>>>>>> On Fri, Apr 17, 2015 at 08:05:57PM +0200, Jan Kiszka wrote:
>>>>>>>> On 2015-04-17 19:50, Gilles Chanteperdrix wrote:
>>>>>>>>> On Fri, Apr 17, 2015 at 07:34:30PM +0200, Jan Kiszka wrote:
>>>>>>>>>> Hi,
>>>>>>>>>>
>>>>>>>>>> analyzing page faults of an application that prefers to set its own
>>>>>>>>>> stacks, I noticed a problem in Xenomai (2 and 3), at least from the
>>>>>>>>>> usability POV: We document the minimum stack stack as PTHREAD_STACK_MIN
>>>>>>>>>> + 1 page, at least in Xenomai 3, and we enforce that on thread creation.
>>>>>>>>>> However, enforcement is doomed to fail if the stack is preallocated (and
>>>>>>>>>> that too small).
>>>>>>>>>>
>>>>>>>>>> As we cannot detect if the user set a stack address in pthread_attr_t, I
>>>>>>>>>> would suggest to fail thread creation instead of performing it with
>>>>>>>>>> improper parameters. Other suggestions? If not, I would prepare a patch
>>>>>>>>>> for Xenomai 3 (for 2 only if desired).
>>>>>>>>>
>>>>>>>>> It seems to me we can detect the parameters in the pthread_attr_t
>>>>>>>>> using pthread_attr_getstack. So, we can get __wrap_pthread_create to
>>>>>>>>> fail if the size is not sufficient.
>>>>>>>>
>>>>>>>> Nope, unfortunately not:
>>>>>>>>
>>>>>>>> "If the pthread_attr_getstack() function is called before the stackaddr
>>>>>>>> attribute has been set, the behavior is unspecified."
>>>>>>>
>>>>>>> It is unspecified by POSIX, but Xenomai supports only two
>>>>>>> implementations, glibc and uClibc, so, we can look at what these two
>>>>>>> libraries do. I would bet they return you a NULL stack pointer or
>>>>>>> something.
>>>>>>
>>>>>> I would have expected that, too, but the results for glibc seem random.
>>>>>> Plus there is the risk that something changes, thus we become
>>>>>> version-dependent.
>>>>>
>>>>> Ok then, what about the influence of pthread_attr_setstack() on
>>>>> pthread_attr_getstacksize(), maybe more luck there?
>>>>
>>>> setstack defines the size getstacksize returns. So does setstacksize.
>>>>
>>>> What happens during setstacksize is apparently that the address is set
>>>> to NULL - size. But, again, that is just the current glibc behaviour.
>>>
>>> Yes, ok, but in order to test the stack size passed by the user, it
>>> simply means we can use getstacksize and bail out if not sufficient.
>>
>> Sure, that would be the best way.
>>
>> Along that, I would recommend raising the minimum to 2*PTHREAD_STACK_MIN.
> 
> What for ? We know that the system crashes if the stack is too
> small, but that is the consequence of the user choice, there is not
> much we can do about it. What I would concentrate on, and I think we
> had that on xenomai 2.x is to have a reasonable default stack size.
> Actually, we could wrap pthread_attr_init to define our own default
> and be done with it.
> 

The current minimum PTHREAD_STACK_MIN + PAGE doesn't work reliably (see
my other posting). Thus we can either give up on checking the minimum
completely (much simpler) or raise it to a value that has at least an
effect.

Jan

-- 
Siemens AG, Corporate Technology, CT RTC ITP SES-DE
Corporate Competence Center Embedded Linux

Re: [Xenomai] Dealing with too small thread stacks

[Gilles Chanteperdrix]

On Fri, Apr 17, 2015 at 08:34:00PM +0200, Jan Kiszka wrote:
> On 2015-04-17 20:30, Gilles Chanteperdrix wrote:
> > On Fri, Apr 17, 2015 at 08:26:32PM +0200, Jan Kiszka wrote:
> >> On 2015-04-17 20:24, Gilles Chanteperdrix wrote:
> >>> On Fri, Apr 17, 2015 at 08:17:26PM +0200, Jan Kiszka wrote:
> >>>> On 2015-04-17 20:12, Gilles Chanteperdrix wrote:
> >>>>> On Fri, Apr 17, 2015 at 08:10:40PM +0200, Jan Kiszka wrote:
> >>>>>> On 2015-04-17 20:08, Gilles Chanteperdrix wrote:
> >>>>>>> On Fri, Apr 17, 2015 at 08:05:57PM +0200, Jan Kiszka wrote:
> >>>>>>>> On 2015-04-17 19:50, Gilles Chanteperdrix wrote:
> >>>>>>>>> On Fri, Apr 17, 2015 at 07:34:30PM +0200, Jan Kiszka wrote:
> >>>>>>>>>> Hi,
> >>>>>>>>>>
> >>>>>>>>>> analyzing page faults of an application that prefers to set its own
> >>>>>>>>>> stacks, I noticed a problem in Xenomai (2 and 3), at least from the
> >>>>>>>>>> usability POV: We document the minimum stack stack as PTHREAD_STACK_MIN
> >>>>>>>>>> + 1 page, at least in Xenomai 3, and we enforce that on thread creation.
> >>>>>>>>>> However, enforcement is doomed to fail if the stack is preallocated (and
> >>>>>>>>>> that too small).
> >>>>>>>>>>
> >>>>>>>>>> As we cannot detect if the user set a stack address in pthread_attr_t, I
> >>>>>>>>>> would suggest to fail thread creation instead of performing it with
> >>>>>>>>>> improper parameters. Other suggestions? If not, I would prepare a patch
> >>>>>>>>>> for Xenomai 3 (for 2 only if desired).
> >>>>>>>>>
> >>>>>>>>> It seems to me we can detect the parameters in the pthread_attr_t
> >>>>>>>>> using pthread_attr_getstack. So, we can get __wrap_pthread_create to
> >>>>>>>>> fail if the size is not sufficient.
> >>>>>>>>
> >>>>>>>> Nope, unfortunately not:
> >>>>>>>>
> >>>>>>>> "If the pthread_attr_getstack() function is called before the stackaddr
> >>>>>>>> attribute has been set, the behavior is unspecified."
> >>>>>>>
> >>>>>>> It is unspecified by POSIX, but Xenomai supports only two
> >>>>>>> implementations, glibc and uClibc, so, we can look at what these two
> >>>>>>> libraries do. I would bet they return you a NULL stack pointer or
> >>>>>>> something.
> >>>>>>
> >>>>>> I would have expected that, too, but the results for glibc seem random.
> >>>>>> Plus there is the risk that something changes, thus we become
> >>>>>> version-dependent.
> >>>>>
> >>>>> Ok then, what about the influence of pthread_attr_setstack() on
> >>>>> pthread_attr_getstacksize(), maybe more luck there?
> >>>>
> >>>> setstack defines the size getstacksize returns. So does setstacksize.
> >>>>
> >>>> What happens during setstacksize is apparently that the address is set
> >>>> to NULL - size. But, again, that is just the current glibc behaviour.
> >>>
> >>> Yes, ok, but in order to test the stack size passed by the user, it
> >>> simply means we can use getstacksize and bail out if not sufficient.
> >>
> >> Sure, that would be the best way.
> >>
> >> Along that, I would recommend raising the minimum to 2*PTHREAD_STACK_MIN.
> > 
> > What for ? We know that the system crashes if the stack is too
> > small, but that is the consequence of the user choice, there is not
> > much we can do about it. What I would concentrate on, and I think we
> > had that on xenomai 2.x is to have a reasonable default stack size.
> > Actually, we could wrap pthread_attr_init to define our own default
> > and be done with it.
> > 
> 
> The current minimum PTHREAD_STACK_MIN + PAGE doesn't work reliably (see
> my other posting). Thus we can either give up on checking the minimum
> completely (much simpler) or raise it to a value that has at least an
> effect.

Again, trying to enforce a minimum looks like a bad idea to me. I
would rather have pthread_attr_init set a default value. And 
larger than PTHREAD_STACK_MIN + pagesize (but much smaller than the
default 2MiB). I believe POSIX does not define what value is set by
pthread_attr_init, so we can do what we want.

-- 
					    Gilles.

Re: [Xenomai] Dealing with too small thread stacks

[Gilles Chanteperdrix]

On Fri, Apr 17, 2015 at 08:34:00PM +0200, Jan Kiszka wrote:
> On 2015-04-17 20:30, Gilles Chanteperdrix wrote:
> > On Fri, Apr 17, 2015 at 08:26:32PM +0200, Jan Kiszka wrote:
> >> On 2015-04-17 20:24, Gilles Chanteperdrix wrote:
> >>> On Fri, Apr 17, 2015 at 08:17:26PM +0200, Jan Kiszka wrote:
> >>>> On 2015-04-17 20:12, Gilles Chanteperdrix wrote:
> >>>>> On Fri, Apr 17, 2015 at 08:10:40PM +0200, Jan Kiszka wrote:
> >>>>>> On 2015-04-17 20:08, Gilles Chanteperdrix wrote:
> >>>>>>> On Fri, Apr 17, 2015 at 08:05:57PM +0200, Jan Kiszka wrote:
> >>>>>>>> On 2015-04-17 19:50, Gilles Chanteperdrix wrote:
> >>>>>>>>> On Fri, Apr 17, 2015 at 07:34:30PM +0200, Jan Kiszka wrote:
> >>>>>>>>>> Hi,
> >>>>>>>>>>
> >>>>>>>>>> analyzing page faults of an application that prefers to set its own
> >>>>>>>>>> stacks, I noticed a problem in Xenomai (2 and 3), at least from the
> >>>>>>>>>> usability POV: We document the minimum stack stack as PTHREAD_STACK_MIN
> >>>>>>>>>> + 1 page, at least in Xenomai 3, and we enforce that on thread creation.
> >>>>>>>>>> However, enforcement is doomed to fail if the stack is preallocated (and
> >>>>>>>>>> that too small).
> >>>>>>>>>>
> >>>>>>>>>> As we cannot detect if the user set a stack address in pthread_attr_t, I
> >>>>>>>>>> would suggest to fail thread creation instead of performing it with
> >>>>>>>>>> improper parameters. Other suggestions? If not, I would prepare a patch
> >>>>>>>>>> for Xenomai 3 (for 2 only if desired).
> >>>>>>>>>
> >>>>>>>>> It seems to me we can detect the parameters in the pthread_attr_t
> >>>>>>>>> using pthread_attr_getstack. So, we can get __wrap_pthread_create to
> >>>>>>>>> fail if the size is not sufficient.
> >>>>>>>>
> >>>>>>>> Nope, unfortunately not:
> >>>>>>>>
> >>>>>>>> "If the pthread_attr_getstack() function is called before the stackaddr
> >>>>>>>> attribute has been set, the behavior is unspecified."
> >>>>>>>
> >>>>>>> It is unspecified by POSIX, but Xenomai supports only two
> >>>>>>> implementations, glibc and uClibc, so, we can look at what these two
> >>>>>>> libraries do. I would bet they return you a NULL stack pointer or
> >>>>>>> something.
> >>>>>>
> >>>>>> I would have expected that, too, but the results for glibc seem random.
> >>>>>> Plus there is the risk that something changes, thus we become
> >>>>>> version-dependent.
> >>>>>
> >>>>> Ok then, what about the influence of pthread_attr_setstack() on
> >>>>> pthread_attr_getstacksize(), maybe more luck there?
> >>>>
> >>>> setstack defines the size getstacksize returns. So does setstacksize.
> >>>>
> >>>> What happens during setstacksize is apparently that the address is set
> >>>> to NULL - size. But, again, that is just the current glibc behaviour.
> >>>
> >>> Yes, ok, but in order to test the stack size passed by the user, it
> >>> simply means we can use getstacksize and bail out if not sufficient.
> >>
> >> Sure, that would be the best way.
> >>
> >> Along that, I would recommend raising the minimum to 2*PTHREAD_STACK_MIN.
> > 
> > What for ? We know that the system crashes if the stack is too
> > small, but that is the consequence of the user choice, there is not
> > much we can do about it. What I would concentrate on, and I think we
> > had that on xenomai 2.x is to have a reasonable default stack size.
> > Actually, we could wrap pthread_attr_init to define our own default
> > and be done with it.
> > 
> 
> The current minimum PTHREAD_STACK_MIN + PAGE doesn't work reliably (see
> my other posting). Thus we can either give up on checking the minimum
> completely (much simpler) or raise it to a value that has at least an
> effect.

As in xenomai 2.x, I would suggest the maximum of __WORDSIZE * 1024
and PTHREAD_STACK_MIN as a default stack size.

I think the idea of adding pagesize to PTHREAD_STACK_MIN comes from
the implementation of signals which were pulling siginfo structs
from kernel after each syscall. Since this thing is long gone (even
in xenomai 2.6 I believe), we have no reason to try and enforce
another minimum than POSIX.

-- 
					    Gilles.

Re: [Xenomai] Dealing with too small thread stacks

[Jan Kiszka]

On 2015-04-17 19:34, Jan Kiszka wrote:
> Hi,
> 
> analyzing page faults of an application that prefers to set its own
> stacks, I noticed a problem in Xenomai (2 and 3), at least from the
> usability POV: We document the minimum stack stack as PTHREAD_STACK_MIN
> + 1 page, at least in Xenomai 3, and we enforce that on thread creation.
> However, enforcement is doomed to fail if the stack is preallocated (and
> that too small).

Another aspect in this: I just realized that the glibc tends to ignore
our "+ getpagesize()" in the stack calculation. That's at least true for
the x86-64 systems with glibc 2.19 on which I tested.

Jan

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