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Date: Mon, 2 Jun 2014 16:46:32 -0400 (EDT) From: Mikulas Patocka <mpatocka@...hat.com> To: Linus Torvalds <torvalds@...ux-foundation.org> cc: Peter Zijlstra <peterz@...radead.org>, "James E.J. Bottomley" <jejb@...isc-linux.org>, Helge Deller <deller@....de>, John David Anglin <dave.anglin@...l.net>, Parisc List <linux-parisc@...r.kernel.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Paul McKenney <paulmck@...ux.vnet.ibm.com>, "Vinod, Chegu" <chegu_vinod@...com>, Waiman Long <Waiman.Long@...com>, Thomas Gleixner <tglx@...utronix.de>, Rik van Riel <riel@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>, Davidlohr Bueso <davidlohr@...com>, Peter Anvin <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>, "Chandramouleeswaran, Aswin" <aswin@...com>, "Norton, Scott J" <scott.norton@...com>, Jason Low <jason.low2@...com> Subject: Re: [PATCH v2] introduce atomic_pointer to fix a race condition in cancelable mcs spinlocks On Mon, 2 Jun 2014, Linus Torvalds wrote: > On Mon, Jun 2, 2014 at 9:25 AM, Peter Zijlstra <peterz@...radead.org> wrote: > > > > And I can't say I'm a particular fan of these ops either, as alternative > > I'm almost inclined to just exclude parisc from using opt spinning. > > Please do. > > There is no way in hell that we should introduce a magic new > atomic_pointer thing for parisc. And the idea somebody had to change > ACCESS_ONCE() for PA-RISC (I'm not going to go back to find who to > blame) is just horribly wrong too, since it's not even necessary for > any normal use: the special "load-and-store-zero" instruction isn't > actually used for "real" data, it's used only for the special > spinlocks afaik, so doing it for all ACCESS_ONCE() users would be > wrong even on PA-RISC. For any normal data, the usual "just load the > value, making sure the compiler doesn't reload it" is perfectly fine - > even on PA-RISC. > > Now, if PA-RISC was a major architecture, we'd have to figure this > out. But as it is, PA-RISC is just about the shittiest RISC ever > invented (with original sparc being a strong contender), and let's > face it, nobody really uses it. It's a "fun project", but it is not > something that we should use to mess up either ACCESS_ONCE() or the > MCS locks. > > Just make PA-RISC use its own locks, not any of the new fancy ones. > > Linus And what else do you want to do? Peter Zijlstra said "I've been using xchg() and cmpxchg() without such consideration for quite a while." - so it basically implies that the kernel is full of such races, mcs_spinlock is just the most visible one that crashes the kernel first. It's not only parisc - tile32, arc, metag (maybe hexagon) are broken too, because they don't have cmpxchg in hardware. We have atomic_t, atomic64_t, atomic_long_t that can be sanely used even on architectures without hardware cmpxchg - so I ask - why can't we have atomic_pointer_t with the same semantics? (pointer type conversion issues can be solved, as it is done in the PATCH v2) Mikulas -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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