Date: Tue, 18 Mar 2014 19:38:03 +0000 From: "Meyer, Conrad" <conrad.meyer@isilon.com> To: John Baldwin <jhb@freebsd.org>, "freebsd-hackers@freebsd.org" <freebsd-hackers@freebsd.org> Cc: Bryan Drewery <bdrewery@freebsd.org> Subject: RE: [PATCH] amd64/pcpu.h: Use Clang builtins for clarity when referencing thread's pcpu Message-ID: <A3CAF0E84A34A540B4C74454358E003F1292F043@MX103CL02.corp.emc.com> In-Reply-To: <201403181452.13685.jhb@freebsd.org> References: <1394821826-19412-1-git-send-email-conrad.meyer@isilon.com>, <201403181452.13685.jhb@freebsd.org>
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From: John Baldwin [jhb@freebsd.org]
Sent: Tuesday, March 18, 2014 11:52 AM
To: freebsd-hackers@freebsd.org
Cc: Meyer, Conrad; Bryan Drewery
Subject: Re: [PATCH] amd64/pcpu.h: Use Clang builtins for clarity when referencing thread's pcpu
> > - How atomic does PCPU_INC() need to be? It looks like it updates cpu-local
> > counters; as long as it's a single asm instruction, should it be fine
> > w.r.t. interrupts? The existing implementation does NOT use the 'lock; ' prefix.
>
> I think a single instruction is fine.
Unfortunately, I'm seeing crashes under stress in internal testing that I can't attribute to anything else. I'm not sure why Clang would generate more than one instruction for any of this, but I haven't probed too deeply.
> > diff --git a/sys/amd64/include/pcpu.h b/sys/amd64/include/pcpu.h
> > index fe898e9..68892fc 100644
> > --- a/sys/amd64/include/pcpu.h
> > +++ b/sys/amd64/include/pcpu.h
> > +#define curthread __extension__ ({ \
> > + *((volatile __pcpu_type(pc_curthread) __GS_RELATIVE *) \
> > + __pcpu_offset(pc_curthread)); \
> > +})
>
> Would be nice to not lose the __pure2 attribute for curthread (you
> might need it to still be an inline function to keep that)
Yeah, I think you would need it to be a function for that. As is I don't think Clang has any reason to optimize away redundant loads from a volatile pointer.
Thanks,
Conrad
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