On Thu, Mar 07, 2019 at 05:35:46PM -0800, Vineet Gupta wrote:
> - ARCv2 LLSC based spinlocks smp_mb() both before and after the LLSC
> instructions, which is not required per lkmm ACQ/REL semantics.
> smp_mb() is only needed _after_ lock and _before_ unlock.
> So remove the extra barriers.
Right; I have memories of mentioning this earlier ;-)
> Signed-off-by: Vineet Gupta <vgupta@xxxxxxxxxxxx>
> ---
> arch/arc/include/asm/spinlock.h | 45 +++++++++++------------------------------
> 1 file changed, 12 insertions(+), 33 deletions(-)
>
> diff --git a/arch/arc/include/asm/spinlock.h b/arch/arc/include/asm/spinlock.h
> index 2ba04a7db621..be603859fb04 100644
> --- a/arch/arc/include/asm/spinlock.h
> +++ b/arch/arc/include/asm/spinlock.h
> @@ -21,8 +21,6 @@ static inline void arch_spin_lock(arch_spinlock_t *lock)
> {
> unsigned int val;
>
> - smp_mb();
> -
> __asm__ __volatile__(
> "1: llock %[val], [%[slock]] \n"
> " breq %[val], %[LOCKED], 1b \n" /* spin while LOCKED */
> @@ -34,6 +32,14 @@ static inline void arch_spin_lock(arch_spinlock_t *lock)
> [LOCKED] "r" (__ARCH_SPIN_LOCK_LOCKED__)
> : "memory", "cc");
>
> + /*
> + * ACQUIRE barrier to ensure load/store after taking the lock
> + * don't "bleed-up" out of the critical section (leak-in is allowed)
> + * http://www.spinics.net/lists/kernel/msg2010409.html
> + *
> + * ARCv2 only has load-load, store-store and all-all barrier
> + * thus need the full all-all barrier
> + */
> smp_mb();
> }
Two things:
- have you considered doing a ticket lock instead of the test-and-set
lock? Ticket locks are not particularly difficult to implement (see
arch/arm/include/asm/spinlock.h for an example) and have much better
worst case performance.
(also; you can then easily convert to qrwlock, removing your custom
rwlock implementation)
- IFF (and please do verify this with your hardware people) the bnz
after your scond can be considered a proper control dependency and
thereby guarantees later stores will not bubble up, then you can get
away with adding an smp_rmb(), see smp_acquire__after_ctrl_dep() and
its comment.
Your unlock will still need the smp_mb() before, such that the whole
things will be RCsc.
> @@ -309,8 +290,7 @@ static inline void arch_spin_unlock(arch_spinlock_t *lock)
> : "memory");
>
> /*
> - * superfluous, but keeping for now - see pairing version in
> - * arch_spin_lock above
> + * see pairing version/comment in arch_spin_lock above
> */
> smp_mb();
> }
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