Re: [dpdk-dev] [EXT] [PATCH v3 5/6] spinlock: reimplement with atomic one-way barrier builtins

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

> >
> > On Thu, 2018-12-27 at 12:13 +0800, Gavin Hu wrote:
> > -------------------------------------------------------------------
> > > ---
> > > The __sync builtin based implementation generates full memory
> > > barriers ('dmb ish') on Arm platforms. Using C11 atomic builtins to
> > > generate one way barriers.
> > >
> > > Here is the assembly code of __sync_compare_and_swap builtin.
> > > __sync_bool_compare_and_swap(dst, exp, src);
> > >    0x000000000090f1b0 <+16>:    e0 07 40 f9 ldr x0, [sp, #8]
> > >    0x000000000090f1b4 <+20>:    e1 0f 40 79 ldrh    w1, [sp, #6]
> > >    0x000000000090f1b8 <+24>:    e2 0b 40 79 ldrh    w2, [sp, #4]
> > >    0x000000000090f1bc <+28>:    21 3c 00 12 and w1, w1, #0xffff
> > >    0x000000000090f1c0 <+32>:    03 7c 5f 48 ldxrh   w3, [x0]
> > >    0x000000000090f1c4 <+36>:    7f 00 01 6b cmp w3, w1
> > >    0x000000000090f1c8 <+40>:    61 00 00 54 b.ne    0x90f1d4
> > > <rte_atomic16_cmpset+52>  // b.any
> > >    0x000000000090f1cc <+44>:    02 fc 04 48 stlxrh  w4, w2, [x0]
> > >    0x000000000090f1d0 <+48>:    84 ff ff 35 cbnz    w4, 0x90f1c0
> > > <rte_atomic16_cmpset+32>
> > >    0x000000000090f1d4 <+52>:    bf 3b 03 d5 dmb ish
> > >    0x000000000090f1d8 <+56>:    e0 17 9f 1a cset    w0, eq  // eq =
> > > none
> > >
> > > The benchmarking results showed 3X performance gain on Cavium
> > > ThunderX2 and
> > > 13% on Qualcomm Falmon and 3.7% on 4-A72 Marvell macchiatobin.
                                            ^^^^^^
Typo, should be Falkor

> > > Here is the example test result on TX2:
> > >
> > > *** spinlock_autotest without this patch *** Core [123] Cost Time =
> > > 639822 us Core [124] Cost Time = 633253 us Core [125] Cost Time =
> > > 646030 us Core [126] Cost Time = 643189 us Core [127] Cost Time =
> > > 647039 us Total Cost Time = 95433298 us
> > >
> > > *** spinlock_autotest with this patch *** Core [123] Cost Time =
> > > 163615 us Core [124] Cost Time = 166471 us Core [125] Cost Time =
> > > 189044 us Core [126] Cost Time = 195745 us Core [127] Cost Time =
> > > 78423 us Total Cost Time = 27339656 us
> > >
> > > Signed-off-by: Gavin Hu <gavin.hu@arm.com>
> > > Reviewed-by: Phil Yang <phil.yang@arm.com>
> > > Reviewed-by: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>
> > > Reviewed-by: Ola Liljedahl <Ola.Liljedahl@arm.com>
> > > Reviewed-by: Steve Capper <Steve.Capper@arm.com>
> > > ---
> > >  lib/librte_eal/common/include/generic/rte_spinlock.h | 18
> > > +++++++++++++-----
> > >  1 file changed, 13 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/lib/librte_eal/common/include/generic/rte_spinlock.h
> > > b/lib/librte_eal/common/include/generic/rte_spinlock.h
> > > index c4c3fc31e..87ae7a4f1 100644
> > > --- a/lib/librte_eal/common/include/generic/rte_spinlock.h
> > > +++ b/lib/librte_eal/common/include/generic/rte_spinlock.h
> > > @@ -61,9 +61,14 @@ rte_spinlock_lock(rte_spinlock_t *sl);  static
> > > inline void  rte_spinlock_lock(rte_spinlock_t *sl)  {
> > > -	while (__sync_lock_test_and_set(&sl->locked, 1))
> > > -		while(sl->locked)
> > > +	int exp = 0;
> > > +
> > > +	while (!__atomic_compare_exchange_n(&sl->locked, &exp, 1, 0,
> > > +				__ATOMIC_ACQUIRE, __ATOMIC_RELAXED))
> > {
> >
> > How about remove explict exp = 0 and change to
> > __atomic_test_and_set(flag, __ATOMIC_ACQUIRE);
> >
> > i.e
> > while (_atomic_test_and_set(flag, __ATOMIC_ACQUIRE))
> >
> Will do it in v4.
The operand for '__atomic_test_and_set' needs to be a bool or char. This means, sl->locked need to be changed to bool or char. But the API 'rte_spinlock_lock_tm' also uses sl->locked. The requirements of 'rte_spinlock_lock_tm' need to be considered.

> >
> > > +		while (__atomic_load_n(&sl->locked, __ATOMIC_RELAXED))
> > >  			rte_pause();
> > > +		exp = 0;
> >
> > We can remove exp = 0 with above scheme.
> >
> > > +	}
> > >  }
> > >  #endif
> > >
> > > @@ -80,7 +85,7 @@ rte_spinlock_unlock (rte_spinlock_t *sl);  static
> > > inline void  rte_spinlock_unlock (rte_spinlock_t *sl)  {
> > > -	__sync_lock_release(&sl->locked);
> > > +	__atomic_store_n(&sl->locked, 0, __ATOMIC_RELEASE);
> >  }
> > >  #endif
> > >
> > > @@ -99,7 +104,10 @@ rte_spinlock_trylock (rte_spinlock_t *sl);
> > > static inline int  rte_spinlock_trylock (rte_spinlock_t *sl)  {
> > > -	return __sync_lock_test_and_set(&sl->locked,1) == 0;
> > > +	int exp = 0;
> > > +	return __atomic_compare_exchange_n(&sl->locked, &exp, 1,
> > > +				0, /* disallow spurious failure */
> > > +				__ATOMIC_ACQUIRE, __ATOMIC_RELAXED);
> >
> > Here to remove explicit exp.
> >
> > return (__atomic_test_and_set(flag, __ATOMIC_ACQUIRE) == 0)
> 
> Will do it in v4.
> 
> > >  }
> > >  #endif
> > >
> > > @@ -113,7 +121,7 @@ rte_spinlock_trylock (rte_spinlock_t *sl)
> > >   */
> > >  static inline int rte_spinlock_is_locked (rte_spinlock_t *sl)  {
> > > -	return sl->locked;
> > > +	return __atomic_load_n(&sl->locked, __ATOMIC_ACQUIRE);
> >
> > __ATOMIC_RELAXED would be enough here. Right ?
> Yes, it is enough for current DPDK uses, used for testing and assertions only.
> 
> For general applicability, we set acquire as concerned about it is used for
> reading protected data while the lock is not taken by anybody.
> In this use case, Acquire will properly see all updates from before the lock
> was released, but this is still dangerous, as during the course, someone else
> might have taken the lock and changed the data.
> 
> Anyway, I will set Relaxed in v4 as the above use scenario was not
> recommended and not present in DPDK.
IMO, once the API is provided the application can make use of it in anyway it wants. Once use case I can think of is follows:

if (rte_spinlock_is_locked(&sl) == true) {
	access_shared_data();
}

access_shared_data() can get hoisted/executed speculatively before the 'if' statement. This needs to be prevented, hence we need 'acquire' semantics.

> 
> >
> > >  }
> > >
> > >  /**