Messages in this thread Patch in this message | | | Date | Mon, 5 Nov 2018 22:49:21 +0000 | From | Will Deacon <> | Subject | Re: [PATCH v2] bit_spinlock: introduce smp_cond_load_relaxed |
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[+PeterZ -- please include him on stuff like this]
Hi Gao,
On Tue, Oct 30, 2018 at 02:04:41PM +0800, Gao Xiang wrote: > It is better to use wrapped smp_cond_load_relaxed > instead of open-coded busy waiting for bit_spinlock. > > Signed-off-by: Gao Xiang <gaoxiang25@huawei.com> > --- > > change log v2: > - fix the incorrect expression !(VAL >> (bitnum & (BITS_PER_LONG-1))) > - the test result is described in the following reply.
Please include the results in the commit message, so that this change is justified.
> diff --git a/include/linux/bit_spinlock.h b/include/linux/bit_spinlock.h > index bbc4730a6505..d5f922b5ffd9 100644 > --- a/include/linux/bit_spinlock.h > +++ b/include/linux/bit_spinlock.h > @@ -15,22 +15,19 @@ > */ > static inline void bit_spin_lock(int bitnum, unsigned long *addr) > { > - /* > - * Assuming the lock is uncontended, this never enters > - * the body of the outer loop. If it is contended, then > - * within the inner loop a non-atomic test is used to > - * busywait with less bus contention for a good time to > - * attempt to acquire the lock bit. > - */ > - preempt_disable(); > #if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK) > - while (unlikely(test_and_set_bit_lock(bitnum, addr))) { > - preempt_enable(); > - do { > - cpu_relax(); > - } while (test_bit(bitnum, addr)); > + const unsigned int bitshift = bitnum & (BITS_PER_LONG - 1); > + > + while (1) { > + smp_cond_load_relaxed(&addr[BIT_WORD(bitnum)], > + !((VAL >> bitshift) & 1)); > preempt_disable(); > + if (!test_and_set_bit_lock(bitnum, addr)) > + break; > + preempt_enable(); > } > +#else > + preempt_disable();
This appears to introduce a bunch of overhead for the uncontended fastpath. How about the much-simpler-but-completely-untested (tm) patch below?
Will
--->8
diff --git a/include/asm-generic/bitops/lock.h b/include/asm-generic/bitops/lock.h index 3ae021368f48..9de8d3544630 100644 --- a/include/asm-generic/bitops/lock.h +++ b/include/asm-generic/bitops/lock.h @@ -6,6 +6,15 @@ #include <linux/compiler.h> #include <asm/barrier.h> +static inline void spin_until_bit_unlock(unsigned int nr, + volatile unsigned long *p) +{ + unsigned long mask = BIT_MASK(bitnum); + + p += BIT_WORD(nr); + smp_cond_load_relaxed(p, VAL & mask); +} + /** * test_and_set_bit_lock - Set a bit and return its old value, for lock * @nr: Bit to set diff --git a/include/linux/bit_spinlock.h b/include/linux/bit_spinlock.h index bbc4730a6505..d711c62e718c 100644 --- a/include/linux/bit_spinlock.h +++ b/include/linux/bit_spinlock.h @@ -26,9 +26,7 @@ static inline void bit_spin_lock(int bitnum, unsigned long *addr) #if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK) while (unlikely(test_and_set_bit_lock(bitnum, addr))) { preempt_enable(); - do { - cpu_relax(); - } while (test_bit(bitnum, addr)); + spin_until_bit_unlock(bitnum, addr); preempt_disable(); } #endif
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