From: "Christoph Müllner" <christophm30@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Palmer Dabbelt <palmer@dabbelt.com>,
Anup Patel <anup@brainfault.org>, Guo Ren <guoren@kernel.org>,
linux-riscv <linux-riscv@lists.infradead.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Guo Ren <guoren@linux.alibaba.com>,
catalin.marinas@arm.com, will.deacon@arm.com,
Arnd Bergmann <arnd@arndb.de>
Subject: Re: [PATCH] riscv: locks: introduce ticket-based spinlock implementation
Date: Tue, 13 Apr 2021 12:25:00 +0200 [thread overview]
Message-ID: <CAHB2gtTzEuD7j-+5ztui0eV6UNiEisBTgoK+2Sr=Z0b4PPXRyA@mail.gmail.com> (raw)
In-Reply-To: <YHVl8fFfdQDn+QYW@hirez.programming.kicks-ass.net>
On Tue, Apr 13, 2021 at 11:37 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Tue, Apr 13, 2021 at 11:22:40AM +0200, Christoph Müllner wrote:
>
> > > For ticket locks you really only needs atomic_fetch_add() and
> > > smp_store_release() and an architectural guarantees that the
> > > atomic_fetch_add() has fwd progress under contention and that a sub-word
> > > store (through smp_store_release()) will fail the SC.
> > >
> > > Then you can do something like:
> > >
> > > void lock(atomic_t *lock)
> > > {
> > > u32 val = atomic_fetch_add(1<<16, lock); /* SC, gives us RCsc */
> > > u16 ticket = val >> 16;
> > >
> > > for (;;) {
> > > if (ticket == (u16)val)
> > > break;
> > > cpu_relax();
> > > val = atomic_read_acquire(lock);
> > > }
> > > }
> > >
> > > void unlock(atomic_t *lock)
> > > {
> > > u16 *ptr = (u16 *)lock + (!!__BIG_ENDIAN__);
> > > u32 val = atomic_read(lock);
> > >
> > > smp_store_release(ptr, (u16)val + 1);
> > > }
> > >
> > > That's _almost_ as simple as a test-and-set :-) It isn't quite optimal
> > > on x86 for not being allowed to use a memop on unlock, since its being
> > > forced into a load-store because of all the volatile, but whatever.
> >
> > What about trylock()?
> > I.e. one could implement trylock() without a loop, by letting
> > trylock() fail if the SC fails.
> > That looks safe on first view, but nobody does this right now.
>
> Generic code has to use cmpxchg(), and then you get something like this:
>
> bool trylock(atomic_t *lock)
> {
> u32 old = atomic_read(lock);
>
> if ((old >> 16) != (old & 0xffff))
> return false;
>
> return atomic_try_cmpxchg(lock, &old, old + (1<<16)); /* SC, for RCsc */
> }
This approach requires two loads (atomic_read() and cmpxchg()), which
is not required.
Detecting this pattern and optimizing it in a compiler is quite unlikely.
A bit less generic solution would be to wrap the LL/SC (would be
mandatory in this case)
instructions and do something like this:
uint32_t __smp_load_acquire_reserved(void*);
int __smp_store_release_conditional(void*, uint32_t);
typedef union {
uint32_t v32;
struct {
uint16_t owner;
uint16_t next;
};
} arch_spinlock_t;
int trylock(arch_spinlock_t *lock)
{
arch_spinlock_t l;
int success;
do {
l.v32 = __smp_load_acquire_reserved(lock);
if (l.owner != l.next)
return 0;
l.next++;
success = __smp_store_release_conditional(lock, l.v32);
} while (!success);
return success;
}
But here we can't tell the compiler to optimize the code between LL and SC...
>
> That will try and do the full LL/SC loop, because it wants to complete
> the cmpxchg, but in generic code we have no other option.
>
> (Is this what C11's weak cmpxchg is for?)
next prev parent reply other threads:[~2021-04-13 10:25 UTC|newest]
Thread overview: 49+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-24 10:14 [PATCH] riscv: locks: introduce ticket-based spinlock implementation guoren
2021-03-24 11:09 ` Peter Zijlstra
2021-03-24 12:10 ` Guo Ren
[not found] ` <CAM4kBBK7_s9U2vJbq68yC8WdDEfPQTaCOvn1xds3Si5B-Wpw+A@mail.gmail.com>
2021-03-24 12:23 ` Peter Zijlstra
2021-03-24 12:24 ` Guo Ren
2021-03-24 12:31 ` Peter Zijlstra
2021-03-24 12:28 ` Anup Patel
2021-03-24 12:37 ` Peter Zijlstra
2021-03-24 12:53 ` Anup Patel
2021-04-11 21:11 ` Palmer Dabbelt
2021-04-12 13:32 ` Christoph Müllner
2021-04-12 14:51 ` Peter Zijlstra
2021-04-12 21:21 ` Christoph Müllner
2021-04-12 17:33 ` Palmer Dabbelt
2021-04-12 21:54 ` Christoph Müllner
2021-04-13 8:03 ` Peter Zijlstra
2021-04-13 8:17 ` Peter Zijlstra
2021-04-14 2:26 ` Guo Ren
2021-04-14 7:08 ` Peter Zijlstra
2021-04-14 9:05 ` Peter Zijlstra
2021-04-14 10:16 ` [RFC][PATCH] locking: Generic ticket-lock Peter Zijlstra
2021-04-14 12:39 ` Guo Ren
2021-04-14 12:55 ` Peter Zijlstra
2021-04-14 13:08 ` Peter Zijlstra
2021-04-14 15:59 ` David Laight
2021-04-14 12:45 ` Peter Zijlstra
2021-04-14 21:02 ` Stafford Horne
2021-04-14 20:47 ` Stafford Horne
2021-04-15 8:09 ` Peter Zijlstra
2021-04-15 9:02 ` Catalin Marinas
2021-04-15 9:22 ` Will Deacon
2021-04-15 9:24 ` Peter Zijlstra
2021-04-19 17:35 ` Will Deacon
2021-04-23 6:44 ` Palmer Dabbelt
2021-04-13 9:22 ` [PATCH] riscv: locks: introduce ticket-based spinlock implementation Christoph Müllner
2021-04-13 9:30 ` Catalin Marinas
2021-04-13 9:55 ` Christoph Müllner
2021-04-14 0:23 ` Guo Ren
2021-04-14 9:17 ` Catalin Marinas
2021-04-13 9:35 ` Peter Zijlstra
2021-04-13 10:25 ` Christoph Müllner [this message]
2021-04-13 10:45 ` Catalin Marinas
2021-04-13 10:54 ` David Laight
2021-04-14 5:54 ` Guo Ren
2021-04-13 11:04 ` Christoph Müllner
2021-04-13 13:19 ` Guo Ren
-- strict thread matches above, loose matches on Subject: below --
2021-09-19 16:53 guoren
2021-09-25 14:47 ` Guo Ren
2021-10-21 13:13 ` Peter Zijlstra
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