Re: [PATCH 02/11] mm/page_alloc: Convert per-cpu list protection to local_lock

[Peter Zijlstra]

On Fri, Apr 09, 2021 at 02:32:56PM +0100, Mel Gorman wrote:
> That said, there are some curious users already.
> fs/squashfs/decompressor_multi_percpu.c looks like it always uses the
> local_lock in CPU 0's per-cpu structure instead of stabilising a per-cpu
> pointer. 

I'm not sure how you read that.

You're talking about this:

  local_lock(&msblk->stream->lock);

right? Note that msblk->stream is a per-cpu pointer, so
&msblk->stream->lock is that same per-cpu pointer with an offset on.

The whole think relies on:

	&per_cpu_ptr(msblk->stream, cpu)->lock == per_cpu_ptr(&msblk->stream->lock, cpu)

Which is true because the lhs:

	(local_lock_t *)((msblk->stream + per_cpu_offset(cpu)) + offsetof(struct squashfs_stream, lock))

and the rhs:

	(local_lock_t *)((msblk->stream + offsetof(struct squashfs_stream, lock)) + per_cpu_offset(cpu))

are identical, because addition is associative.

> drivers/block/zram/zcomp.c appears to do the same although for
> at least one of the zcomp_stream_get() callers, the CPU is pinned for
> other reasons (bit spin lock held). I think it happens to work anyway
> but it's weird and I'm not a fan.

Same thing.