Re: [PATCH, newlib] Allow locking routine to be retargeted

[Thomas Preudhomme <thomas dot preudhomme at foss dot arm dot com>]

On 11/11/16 11:56, Sebastian Huber wrote:
> ----- Thomas Preudhomme <thomas.preudhomme@foss.arm.com> schrieb:
> > On 10/11/16 20:32, Freddie Chopin wrote:
> [...]
> > > This could work, if only malloc() was not protected with the statically
> > > created _LOCK_T lock itself, which would need such initialization
> > > too... I don't see any other way to write such function for an RTOS
> > > which has mutexes with size greater than "uintptr_t" (probably huge
> > > majority). You could do it with a static pool of storage for mutexes,
> > > but then you'd have to know upfront how many you want and your RTOS
> > > would have to allow creation of locks in provided storage (most of them
> > > allow that, but not all). Maybe I just don't see the simplest solution
> > > to this problem?
> >
> > Why do mutex needs to be more than a single integer? How big are mutex in RTOS
> > typically in your experience?
>
> This depends on what features the RTOS wants to support.  For example SMP, locking protocols, blocking on a wait queue, user-provided or external-provided storage, nesting, etc.
>
> For RTEMS a recursive mutex with support for priority inheritance (or OMIP) and SMP (optional) needs 20 bytes on a 32-bit machine for example:
>
> https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;a=blob;f=newlib/libc/sys/rtems/include/sys/lock.h;h=e0d77cb614b348a949e23ca4c543b1ce6a055b6d;hb=HEAD#l55

Right, I didn't think of extra features. The one thing I could think of would be 
to remove usage of the static initializers and only use the function initializer 
which could do a malloc if needed. The issue you described with malloc can be 
avoided by redefining malloc_lock so as not to call the generic lock function.

Now whether this is worth the trouble is another issue because that would 
require more work. Thanks a lot for the feedback, I'm glad you pointed out the 
pitfalls in that approach. It is time to go back to the drawing board.

Best regards,

Thomas