Messages in this thread | | | Date | Fri, 9 Apr 2021 20:55:39 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 02/11] mm/page_alloc: Convert per-cpu list protection to local_lock |
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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.
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