Messages in this thread | | | From | David Howells <> | Subject | Do we need to correct barriering in circular-buffers.rst? | Date | Wed, 18 Sep 2019 16:43:00 +0100 |
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Will Deacon <will@kernel.org> wrote:
> If I'm understanding your code correctly (big 'if'), then you have things > like this in pipe_read(): > > > unsigned int head = READ_ONCE(pipe->head); > unsigned int tail = pipe->tail; > unsigned int mask = pipe->buffers - 1; > > if (tail != head) { > struct pipe_buffer *buf = &pipe->bufs[tail & mask]; > > [...] > > written = copy_page_to_iter(buf->page, buf->offset, chars, to); > > > where you want to make sure you don't read from 'buf->page' until after > you've read the updated head index. Is that right? If so, then READ_ONCE() > will not give you that guarantee on architectures such as Power and Arm, > because the 'if (tail != head)' branch can be speculated and the buffer > can be read before we've got around to looking at the head index. > > So I reckon you need smp_load_acquire() in this case. pipe_write() might be > ok with the control dependency because CPUs don't tend to make speculative > writes visible, but I didn't check it carefully and the compiler can do > crazy stuff in this area, so I'd be inclined to use smp_load_acquire() here > too unless you really need the last ounce of performance.
Yeah, I probably do.
Documentation/core-api/circular-buffers.rst might be wrong, then, I think.
It mandates using smp_store_release() to update buffer->head in the producer and buffer->tail in the consumer - but these need pairing with memory barriers used when reading buffer->head and buffer->tail on the other side. Currently, for the producer we have:
spin_lock(&producer_lock);
unsigned long head = buffer->head; /* The spin_unlock() and next spin_lock() provide needed ordering. */ unsigned long tail = READ_ONCE(buffer->tail);
if (CIRC_SPACE(head, tail, buffer->size) >= 1) { /* insert one item into the buffer */ struct item *item = buffer[head];
produce_item(item);
smp_store_release(buffer->head, (head + 1) & (buffer->size - 1));
/* wake_up() will make sure that the head is committed before * waking anyone up */ wake_up(consumer); }
spin_unlock(&producer_lock);
I think the ordering comment about spin_unlock and spin_lock is wrong. There's no requirement to have a spinlock on either side - and in any case, both sides could be inside their respective locked sections when accessing the buffer. The READ_ONCE() would theoretically provide the smp_read_barrier_depends() to pair with the smp_store_release() in the consumer. Maybe I should change this to:
spin_lock(&producer_lock);
/* Barrier paired with consumer-side store-release on tail */ unsigned long tail = smp_load_acquire(buffer->tail); unsigned long head = buffer->head;
if (CIRC_SPACE(head, tail, buffer->size) >= 1) { /* insert one item into the buffer */ struct item *item = buffer[head];
produce_item(item);
smp_store_release(buffer->head, (head + 1) & (buffer->size - 1));
/* wake_up() will make sure that the head is committed before * waking anyone up */ wake_up(consumer); }
spin_unlock(&producer_lock);
The consumer is currently:
spin_lock(&consumer_lock);
/* Read index before reading contents at that index. */ unsigned long head = smp_load_acquire(buffer->head); unsigned long tail = buffer->tail;
if (CIRC_CNT(head, tail, buffer->size) >= 1) {
/* extract one item from the buffer */ struct item *item = buffer[tail];
consume_item(item);
/* Finish reading descriptor before incrementing tail. */ smp_store_release(buffer->tail, (tail + 1) & (buffer->size - 1)); }
spin_unlock(&consumer_lock);
which I think is okay.
And yes, I note that this does use smp_load_acquire(buffer->head) in the consumer - which I should also be doing.
David
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