Re: [PATCH 8/8] blk-mq: drain I/O when all CPUs in a hctx are offline

[Ming Lei]

On Thu, May 28, 2020 at 06:37:47AM -0700, Bart Van Assche wrote:
> On 2020-05-27 22:19, Ming Lei wrote:
> > On Wed, May 27, 2020 at 08:33:48PM -0700, Bart Van Assche wrote:
> >> My understanding is that operations that have acquire semantics pair
> >> with operations that have release semantics. I haven't been able to find
> >> any documentation that shows that smp_mb__after_atomic() has release
> >> semantics. So I looked up its definition. This is what I found:
> >>
> >> $ git grep -nH 'define __smp_mb__after_atomic'
> >> arch/ia64/include/asm/barrier.h:49:#define __smp_mb__after_atomic()
> >> barrier()
> >> arch/mips/include/asm/barrier.h:133:#define __smp_mb__after_atomic()
> >> smp_llsc_mb()
> >> arch/s390/include/asm/barrier.h:50:#define __smp_mb__after_atomic()
> >> barrier()
> >> arch/sparc/include/asm/barrier_64.h:57:#define __smp_mb__after_atomic()
> >> barrier()
> >> arch/x86/include/asm/barrier.h:83:#define __smp_mb__after_atomic()	do {
> >> } while (0)
> >> arch/xtensa/include/asm/barrier.h:20:#define __smp_mb__after_atomic()	
> >> barrier()
> >> include/asm-generic/barrier.h:116:#define __smp_mb__after_atomic()
> >> __smp_mb()
> >>
> >> My interpretation of the above is that not all smp_mb__after_atomic()
> >> implementations have release semantics. Do you agree with this conclusion?
> > 
> > I understand smp_mb__after_atomic() orders set_bit(BLK_MQ_S_INACTIVE)
> > and reading the tag bit which is done in blk_mq_all_tag_iter().
> > 
> > So the two pair of OPs are ordered:
> > 
> > 1) if one request(tag bit) is allocated before setting BLK_MQ_S_INACTIVE,
> > the tag bit will be observed in blk_mq_all_tag_iter() from blk_mq_hctx_has_requests(),
> > so the request will be drained.
> > 
> > OR
> > 
> > 2) if one request(tag bit) is allocated after setting BLK_MQ_S_INACTIVE,
> > the request(tag bit) will be released and retried on another CPU
> > finally, see __blk_mq_alloc_request().
> > 
> > Cc Paul and linux-kernel list.
> 
> I do not agree with the above conclusion. My understanding of
> acquire/release labels is that if the following holds:
> (1) A store operation that stores the value V into memory location M has
> a release label.
> (2) A load operation that reads memory location M has an acquire label.
> (3) The load operation (2) retrieves the value V that was stored by (1).
> 
> that the following ordering property holds: all load and store
> instructions that happened before the store instruction (1) in program
> order are guaranteed to happen before the load and store instructions
> that follow (2) in program order.
> 
> In the ARM manual these semantics have been described as follows: "A
> Store-Release instruction is multicopy atomic when observed with a
> Load-Acquire instruction".
> 
> In this case the load-acquire operation is the
> "test_and_set_bit_lock(nr, word)" statement from the sbitmap code. That
> code is executed indirectly by blk_mq_get_tag(). Since there is no
> matching store-release instruction in __blk_mq_alloc_request() for
> 'word', ordering of the &data->hctx->state and 'tag' memory locations is
> not guaranteed by the acquire property of the "test_and_set_bit_lock(nr,
> word)" statement from the sbitmap code.

If the order isn't guaranteed, either of the following two documents has to be wrong:

Documentation/memory-barriers.txt:
	...
	In all cases there are variants on "ACQUIRE" operations and "RELEASE" operations
	for each construct.  These operations all imply certain barriers:
	
	 (1) ACQUIRE operation implication:
	
	     Memory operations issued after the ACQUIRE will be completed after the
	     ACQUIRE operation has completed.

Documentation/atomic_bitops.txt:
	...
	Except for a successful test_and_set_bit_lock() which has ACQUIRE semantics and
	clear_bit_unlock() which has RELEASE semantics.

Setting the tag bit is part of successful test_and_set_bit_lock(), which has ACQUIRE
semantics, and any Memory operations(test_bit(INACTIVE)) after the ACQUIRE will be
completed after the ACQUIRE has completed according to the above two documents.

Thanks,
Ming