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Re: [RFC][BZ #16159] Detecting that recursive mutex recursed.


On Wed, Nov 13, 2013 at 04:14:20PM -0500, Rich Felker wrote:
> On Wed, Nov 13, 2013 at 10:02:56PM +0100, OndÅej BÃlka wrote:
> > On Wed, Nov 13, 2013 at 02:37:46PM -0500, Rich Felker wrote:
> > > On Wed, Nov 13, 2013 at 08:03:56PM +0100, OndÅej BÃlka wrote:
> > > > On Wed, Nov 13, 2013 at 01:12:57PM -0500, Rich Felker wrote:
> > > > > On Wed, Nov 13, 2013 at 02:46:08PM +0100, Ondrej Bilka wrote:
> > > 
> > > > Alternative solution that I wanted to avoid is to use additional
> > > > thread-local variable to detect signals. A count would be maintained by
> > > > atomic increment/decrement.
> > > > 
> > > > Do you know how to generate cmpxchgb instruction without lock prefix as
> > > > there is no need of interthread synchronization?
> > > 
> > > Why do you need any atomicity or asm? In principle, read, modify,
> > > write is perfectly valid for this usage as long as the write is atomic
> > > (e.g. not using 2 16-bit writes to implement a 32-bit write). If
> > > you're not happy assumin 32-bit writes are atomic, then yes, a little
> > > more work would be required.
> > >
> > You cannot do that as you are at mercy of compiler. With volatile
> > variable gcc will not use read-modify-write. Without that gcc in
> > int x = 0;
> > int main(){
> >    int z=1;
> >    x++;
> >    if (x<3)
> >     z=0;
> > 
> >    x--;
> >  return z;
> > }
> > happily optimizes x++ and x-- away.
> 
> You forgot the mutex lock or unlock (full barrier) in between.
> 
No, we are in part that uses atomics for synchronization.


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