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Re: [RFC][BZ #16159] Detecting that recursive mutex recursed.
- From: Rich Felker <dalias at aerifal dot cx>
- To: OndÅej BÃlka <neleai at seznam dot cz>
- Cc: libc-alpha at sourceware dot org
- Date: Wed, 13 Nov 2013 21:54:52 -0500
- Subject: Re: [RFC][BZ #16159] Detecting that recursive mutex recursed.
- Authentication-results: sourceware.org; auth=none
- References: <20131113134608 dot GA5437 at popelka dot ms dot mff dot cuni dot cz> <20131113181257 dot GT24286 at brightrain dot aerifal dot cx> <20131113190356 dot GA7913 at domone dot podge> <20131113193746 dot GV24286 at brightrain dot aerifal dot cx> <20131113210256 dot GA18664 at domone dot podge> <20131113211420 dot GW24286 at brightrain dot aerifal dot cx> <20131113212523 dot GB18664 at domone dot podge>
On Wed, Nov 13, 2013 at 10:25:23PM +0100, OndÅej BÃlka wrote:
> 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.
In that case, any compiler barrier should work. You could just make x
volatile or use a memory-clobber asm block.
Rich