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From: Stafford Horne <shorne@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Guo Ren <guoren@kernel.org>,
	linux-riscv <linux-riscv@lists.infradead.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-csky@vger.kernel.org,
	linux-arch <linux-arch@vger.kernel.org>,
	Guo Ren <guoren@linux.alibaba.com>, Will Deacon <will@kernel.org>,
	Ingo Molnar <mingo@redhat.com>, Waiman Long <longman@redhat.com>,
	Arnd Bergmann <arnd@arndb.de>, Anup Patel <anup@brainfault.org>
Subject: Re: [PATCH v4 3/4] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS_XCHG32
Date: Wed, 31 Mar 2021 07:35:14 +0900
Message-ID: <20210330223514.GE1171117@lianli.shorne-pla.net> (raw)
In-Reply-To: <YGNNCEAMSWbBU+hd@hirez.programming.kicks-ass.net>

On Tue, Mar 30, 2021 at 06:08:40PM +0200, Peter Zijlstra wrote:
> On Tue, Mar 30, 2021 at 11:13:55AM +0800, Guo Ren wrote:
> > On Mon, Mar 29, 2021 at 8:50 PM Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > On Mon, Mar 29, 2021 at 08:01:41PM +0800, Guo Ren wrote:
> > > > u32 a = 0x55aa66bb;
> > > > u16 *ptr = &a;
> > > >
> > > > CPU0                       CPU1
> > > > =========             =========
> > > > xchg16(ptr, new)     while(1)
> > > >                                     WRITE_ONCE(*(ptr + 1), x);
> > > >
> > > > When we use lr.w/sc.w implement xchg16, it'll cause CPU0 deadlock.
> > >
> > > Then I think your LL/SC is broken.
> > >
> > > That also means you really don't want to build super complex locking
> > > primitives on top, because that live-lock will percolate through.
> 
> > Do you mean the below implementation has live-lock risk?
> > +static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
> > +{
> > +       u32 old, new, val = atomic_read(&lock->val);
> > +
> > +       for (;;) {
> > +               new = (val & _Q_LOCKED_PENDING_MASK) | tail;
> > +               old = atomic_cmpxchg(&lock->val, val, new);
> > +               if (old == val)
> > +                       break;
> > +
> > +               val = old;
> > +       }
> > +       return old;
> > +}
> 
> That entirely depends on the architecture (and cmpxchg() impementation).
> 
> There are a number of cases:
> 
>  * architecture has cmpxchg() instruction (x86, s390, sparc, etc.).
> 
>   - architecture provides fwd progress (x86)
>   - architecture requires backoff for progress (sparc)
> 
>  * architecture does not have cmpxchg, and implements it using LL/SC.
> 
>   and here things get *really* interesting, because while an
>   architecture can have LL/SC fwd progress, that does not translate into
>   cmpxchg() also having the same guarantees and all bets are off.
> 
> The real bummer is that C can do cmpxchg(), but there is no way it can
> do LL/SC. And even if we'd teach C how to do LL/SC, it couldn't be
> generic because architectures lacking it can't emulate it using
> cmpxchg() (there's a fun class of bugs there).
> 
> So while the above code might be the best we can do in generic code,
> it's really up to the architecture to make it work.

I just want to chime in here, there may be a better spot in the thread to
mention this but, for OpenRISC I did implement some generic 8/16-bit xchg code
which I have on my todo list somwhere to replace the other generic
implementations like that in mips.

  arch/openrisc/include/asm/cmpxchg.h

The idea would be that architectures just implement these methods:

  long cmpxchg_u32(*ptr,old,new)
  long xchg_u32(*ptr,val)

Then the rest of the generic header would implement cmpxchg.

-Stafford

  reply index

Thread overview: 63+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-27 18:06 [PATCH v4 0/4] riscv: Add qspinlock/qrwlock guoren
2021-03-27 18:06 ` [PATCH v4 1/4] riscv: cmpxchg.h: Cleanup unused code guoren
2021-03-27 18:06 ` [PATCH v4 2/4] riscv: cmpxchg.h: Merge macros guoren
2021-03-27 21:25   ` Arnd Bergmann
2021-03-28  1:50     ` Guo Ren
2021-03-27 18:06 ` [PATCH v4 3/4] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS_XCHG32 guoren
2021-03-27 18:43   ` Waiman Long
2021-03-28  1:48     ` Guo Ren
2021-03-29  7:50   ` Peter Zijlstra
2021-03-29  9:41     ` Arnd Bergmann
2021-03-29 11:16       ` Peter Zijlstra
2021-03-29 11:29         ` Peter Zijlstra
2021-03-29 12:52           ` Guo Ren
2021-03-29 13:56             ` Arnd Bergmann
2021-03-30  2:26               ` Guo Ren
2021-03-30  5:51                 ` Anup Patel
2021-03-30  6:26                   ` Guo Ren
2021-03-30  7:11                 ` Arnd Bergmann
2021-03-31  4:18                   ` Guo Ren
2021-03-31  5:33                     ` Paul Campbell
2021-04-05 16:12                       ` Guo Ren
2021-03-31  6:44                     ` Guo Ren
2021-03-31  7:12                       ` Arnd Bergmann
2021-03-29 11:19     ` Guo Ren
2021-03-29 11:26       ` Peter Zijlstra
2021-03-29 12:01         ` Guo Ren
2021-03-29 12:49           ` Peter Zijlstra
2021-03-30  3:13             ` Guo Ren
2021-03-30  4:54               ` Anup Patel
2021-03-30  6:27                 ` Guo Ren
2021-03-30  8:31               ` David Laight
2021-03-30 14:09               ` Waiman Long
2021-03-31 14:47                 ` Guo Ren
2021-04-05 16:45                 ` Guo Ren
2021-03-30 16:08               ` Peter Zijlstra
2021-03-30 22:35                 ` Stafford Horne [this message]
2021-03-31  7:23                   ` Arnd Bergmann
2021-03-31 12:31                     ` Stafford Horne
2021-03-31 15:10                       ` Guo Ren
2021-04-06  8:51                         ` Stafford Horne
2021-04-06  3:50                     ` Guo Ren
2021-04-06  8:56                       ` Stafford Horne
2021-04-07  8:42                         ` Arnd Bergmann
2021-04-07 11:36                           ` Peter Zijlstra
2021-04-07 11:57                             ` Arnd Bergmann
2021-04-07 12:02                             ` Peter Zijlstra
2021-04-05 16:40                 ` Guo Ren
2021-03-31 15:22             ` Guo Ren
2021-04-06  7:15               ` Peter Zijlstra
2021-04-07  9:42                 ` Christoph Hellwig
2021-04-07 14:29                   ` Christoph Müllner
2021-04-07 14:34                     ` Christoph Hellwig
2021-04-07 15:51                     ` Peter Zijlstra
2021-04-07 16:44                       ` Peter Zijlstra
2021-04-07 15:52                     ` Peter Zijlstra
2021-04-07 16:54                       ` Peter Zijlstra
2021-04-07 16:00                     ` Peter Zijlstra
2021-04-07 19:50                       ` Christoph Müllner
2021-04-06 17:24               ` Boqun Feng
2021-04-07  9:26                 ` Peter Zijlstra
2021-03-29 12:13         ` Anup Patel
2021-03-29 12:54           ` Peter Zijlstra
2021-03-27 18:06 ` [PATCH v4 4/4] riscv: Convert custom spinlock/rwlock to generic qspinlock/qrwlock guoren

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