Hi David, On Tue, Sep 17, 2019 at 02:51:35PM +0100, David Howells wrote: > Will Deacon <will@xxxxxxxxxx> wrote: > > > > + /* Barrier: head belongs to the write side, so order reading > > > + * the data after reading the head pointer. > > > + */ > > > + unsigned int head = READ_ONCE(pipe->head); > > > > Hmm, I don't understand this. Since READ_ONCE() doesn't imply a barrier, > > how are you enforcing the read-read ordering in the CPU? > > It does imply a barrier: smp_read_barrier_depends(). I believe that's I fed your incomplete sentence to https://talktotransformer.com/ : It does imply a barrier: smp_read_barrier_depends(). I believe that's correct. (I'm not a coder so I assume it just means it's a dependency. Maybe this works for other languages too.) but I have a feeling that's not what you meant. I guess AI isn't quite ready to rule the world. > > What is the purpose of saying "This may need to insert a barrier"? Can this > > function be overridden or something? > > I mean it's arch-dependent whether READ_ONCE() inserts a barrier or not. Ok, but why would the caller care? > > Saying that "This inserts a barrier" feels misleading, because READ_ONCE() > > doesn't do that. > > Yes it does - on the Alpha: > > [arch/alpha/include/asm/barrier.h] > #define read_barrier_depends() __asm__ __volatile__("mb": : :"memory") > > [include/asm-generic/barrier.h] > #ifndef __smp_read_barrier_depends > #define __smp_read_barrier_depends() read_barrier_depends() > #endif > ... > #ifndef smp_read_barrier_depends > #define smp_read_barrier_depends() __smp_read_barrier_depends() > #endif > > [include/linux/compiler.h] > #define __READ_ONCE(x, check) \ > ({ \ > union { typeof(x) __val; char __c[1]; } __u; \ > if (check) \ > __read_once_size(&(x), __u.__c, sizeof(x)); \ > else \ > __read_once_size_nocheck(&(x), __u.__c, sizeof(x)); \ > smp_read_barrier_depends(); /* Enforce dependency ordering from x */ \ > __u.__val; \ > }) > #define READ_ONCE(x) __READ_ONCE(x, 1) > > See: > > commit 76ebbe78f7390aee075a7f3768af197ded1bdfbb > Author: Will Deacon <will.deacon@xxxxxxx> > Date: Tue Oct 24 11:22:47 2017 +0100 > locking/barriers: Add implicit smp_read_barrier_depends() to READ_ONCE() Ah, that guy. I tried emailing him but he didn't reply. Seriously though, READ_ONCE() implies a barrier on Alpha, but its portable barrier semantics are only that it can be used to head an address dependency, a bit like rcu_dereference(). You shouldn't be relying on the stronger ordering provided by Alpha, and I doubt that you really are. 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. Will
