Re: Re: [PATCH] fix threaded grep for machines with only one cpu

From: Nicolas Pitre <nico@fluxnic.net>
Date: Feb 16, 2010, 18:59 UTC
Message-ID: <alpine.LFD.2.00.1002161352080.1946@xanadu.home>
In-Reply-To: <20100216180209.GA1532@book.hvoigt.net>

On Tue, 16 Feb 2010, Heiko Voigt wrote:

Show 37 quoted lines

> On Mon, Feb 15, 2010 at 06:39:48PM -0800, Junio C Hamano wrote:
> > Junio C Hamano <gitster@pobox.com> writes:
> > 
> > > Is this the answer to my question?
> 
> Sorry I forgot to add a reference to the thread from which the patch
> originated: http://groups.google.com/group/msysgit/msg/5da53cf1ccf417cf
> 
> > IOW, please try this patch.  I am planning to queue it to 'maint' as part
> > of 1.7.0.1 if this is the right solution (which I obviously think it is).
> 
> Yes your patch does it correctly I just verified that the segfaults are
> gone as well. I think your solution is even nicer than mine. Thanks.
> 
> > -- >8 --
> > From: Junio C Hamano <gitster@pobox.com>
> > Date: Mon, 15 Feb 2010 18:34:28 -0800
> > Subject: [PATCH] Fix use of mutex in threaded grep
> [...]
> > diff --git a/builtin-grep.c b/builtin-grep.c
> > index 26d4deb..5c1545e 100644
> > --- a/builtin-grep.c
> > +++ b/builtin-grep.c
> > @@ -81,8 +81,8 @@ static pthread_mutex_t read_sha1_mutex;
> >  
> >  #define grep_lock() pthread_mutex_lock(&grep_mutex)
> >  #define grep_unlock() pthread_mutex_unlock(&grep_mutex)
> > -#define read_sha1_lock() pthread_mutex_lock(&read_sha1_mutex)
> > -#define read_sha1_unlock() pthread_mutex_unlock(&read_sha1_mutex)
> > +#define read_sha1_lock() do { if (use_threads) pthread_mutex_lock(&read_sha1_mutex); } while (0)
> > +#define read_sha1_unlock() do { if (use_threads) pthread_mutex_unlock(&read_sha1_mutex); } while (0)
> 
> One minor thing: Would it not be even nicer having the while loop inside
> the if clause? E.g like this
> 
> #define read_sha1_lock() if (use_threads) do { pthread_mutex_lock(&read_sha1_mutex); } while (0)
> #define read_sha1_unlock() if (use_threads) do { pthread_mutex_unlock(&read_sha1_mutex); } while (0)

No.  Think what happens if you have code like this:

	if (foo == 1)
		read_sha1_lock();
	else
		baz();

Nicolas