[lkml]   [2015]   [May]   [29]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [RFC][PATCH 5/5] percpu-rwsem: Optimize readers and reduce global impact
Sorry for delay, finally I found the time to read this series...
The code matches our previous discussion and I believe it is correct.

Reviewed-by: Oleg Nesterov <>

Just one nit below,

On 05/26, Peter Zijlstra wrote:
> struct percpu_rw_semaphore {
> - unsigned int __percpu *fast_read_ctr;
> - atomic_t write_ctr;
> + unsigned int __percpu *refcount;
> + int state;


> +enum { readers_slow, readers_block };

Now that we rely on rss_sync() and thus we do not have "readers_fast",
I think that "bool reader_should_block" will look better.

> +void percpu_down_write(struct percpu_rw_semaphore *sem)
> {

so it does


state = BLOCK;


wait_event(sem->writer, readers_active_check(sem));

and this looks correct.

The only nontrivial thing we need to ensure is that
per_cpu_sum(*sem->refcount) == 0 can't be false positive. False
negative is fine.

And this means that if we see the result of this_cpu_dec() we must
not miss the result of the previous this_cpu_inc() on another CPU.
same or _another_ CPU.

And this is true because if the reader does dec() on another CPU
it does up_read() and this is only possible if down_read() didn't
see state == BLOCK.

But if it didn't see state == BLOCK then the writer must see the
result of the previous down_read()->inc().

IOW, we just rely on STORE-MB-LOAD, just the writer does LOAD
multiple times in per_cpu_sum():


state = BLOCK; refcount[X]++;

mb(); mb();

sum = 0; if (state != BLOCK)
sum += refcount[0]; return; /* success* /
sum += refcount[1];
... refcount[X]--;
sum += refcount[NR_CPUS];

If the reader wins and takes the lock, then its addition to
refcount[X] must be accounted by the writer.

The writer can obviously miss dec() from the reader, but we rely
on wake_up/wait_event in this case.


 \ /
  Last update: 2015-05-29 22:01    [W:0.199 / U:35.228 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site