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

[Ming Lei]

Hi Paul,

Thanks for your response!

On Thu, May 28, 2020 at 10:21:21AM -0700, Paul E. McKenney wrote:
> 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.
> 
> I feel like I just parachuted into the middle of the conversation,
> so let me start by giving a (silly) example illustrating the limits of
> smp_mb__{before,after}_atomic() that might be tangling things up.
> 
> But please please please avoid doing this in real code unless you have
> an extremely good reason included in a comment.
> 
> void t1(void)
> {
> 	WRITE_ONCE(a, 1);
> 	smp_mb__before_atomic();
> 	WRITE_ONCE(b, 1);  // Just Say No to code here!!!
> 	atomic_inc(&c);
> 	WRITE_ONCE(d, 1);  // Just Say No to code here!!!
> 	smp_mb__after_atomic();
> 	WRITE_ONCE(e, 1);
> }
> 
> void t2(void)
> {
> 	r1 = READ_ONCE(e);
> 	smp_mb();
> 	r2 = READ_ONCE(d);
> 	smp_mb();
> 	r3 = READ_ONCE(c);
> 	smp_mb();
> 	r4 = READ_ONCE(b);
> 	smp_mb();
> 	r5 = READ_ONCE(a);
> }
> 
> Each platform must provide strong ordering for either atomic_inc()
> on the one hand (as ia64 does) or for smp_mb__{before,after}_atomic()
> on the other (as powerpc does).  Note that both ia64 and powerpc are
> weakly ordered.
> 
> So ia64 could see (r1 == 1 && r2 == 0) on the one hand as well as (r4 ==
> 1 && r5 == 0).  So clearly smp_mb_{before,after}_atomic() need not have
> any ordering properties whatsoever.
> 
> Similarly, powerpc could see (r3 == 1 && r4 == 0) on the one hand as well
> as (r2 == 1 && r3 == 0) on the other.  Or even both at the same time.
> So clearly atomic_inc() need not have any ordering properties whatsoever.
> 
> But the combination of smp_mb__before_atomic() and the later atomic_inc()
> does provide full ordering, so that no architecture can see (r3 == 1 &&
> r5 == 0), and either of r1 or r2 can be substituted for r3.
> 
> Similarly, atomic_inc() and the late4r smp_mb__after_atomic() also
> provide full ordering, so that no architecture can see (r1 == 1 && r3 ==
> 0), and either r4 or r5 can be substituted for r3.
> 
> 
> So a call to set_bit() followed by a call to smp_mb__after_atomic() will
> provide a full memory barrier (implying release semantics) for any write
> access after the smp_mb__after_atomic() with respect to the set_bit() or
> any access preceding it.  But the set_bit() by itself won't have release
> semantics, nor will the smp_mb__after_atomic(), only their combination
> further combined with some write following the smp_mb__after_atomic().
> 
> More generally, there will be the equivalent of smp_mb() somewhere between
> the set_bit() and every access following the smp_mb__after_atomic().
> 
> Does that help, or am I missing the point?

Yeah, it does help.

BTW, can we replace the smp_mb__after_atomic() with smp_mb() for
ordering set_bit() and the memory OP following the smp_mb()?


Thanks,
Ming