Doc. No.: | WG21/P1478R3 |
---|---|
Date: | 2019-11-17 |
Reply-to: | Hans-J. Boehm |
Email: | hboehm@google.com |
Authors: | Hans-J. Boehm |
Audience: | LEWG |
Target: | Concurrency TS 2 |
Several prior papers have suggested mechanisms that allow for nonatomic accesses that behave like atomics in some way. There are several possible use cases. Here we focus on seqlocks which, in our experience, seem to generate the strongest demand for such a feature.
This proposal is intended to be as simple as possible. It is in theory, but only in theory,
a pure library facility that can be implemented without compiler support.
We expect that practical implementations will implement the new facilities as
aliases for existing memcpy
implementations. This cannot be done
by the user in portable code, since it requires additional assumptions about
the memcpy implementation. Hence there is a strong argument for including it
in the standard library.
There have been a number of prior proposals in this space. Most recently P0690 suggested "tearable atomics". Other solutions were proposed in N3710, which suggested more complex handling for speculative nonatomics loads. This proposal is closest in title to P0603. In a sense this returns to the original intent of that proposal, and is arguably the simplest and narrowest proposal.
A fairly common technique to implement low cost read-mostly synchronization is to protect a block of data with an atomic version or sequence number. The writer increments the sequence number to an odd value, updates the data, and then updates the sequence number again, restoring it to an even value. The reader checks the sequence number before and after reading the data; if the two sequence number values read either differ, or are odd, the data is discarded and the operation retried.
This has the advantage that data can be updated without allocation, and that readers do not modify memory, and thus don't risk cache contention. It seems to also be a popular technique for protecting data in memory shared between processes.
Seqlock readers typically execute code along the following lines:
do { seq1 = seq_no.load(memory_order_acquire); data = shared_data; atomic_thread_fence(memory_order_acquire); int seq2 = seq_no.load(memory_order_relaxed); } while (seq1 != seq2 || seq1 & 1); use data;
For details, see Boehm, Can seqlocks get along with progrmming language memory models.
It is important that the sequence number reads not be reordered with the data reads.
That is ensured by the initial memory_order_acquire
load, and by the
explicit fence. But fences only order atomic accesses, and the read of
shared_data
still races with updates. Thus for the fence to be
effective, and to avoid the data race, the accesses access to shared_data
must be atomic in spite of the fact that any data read while a write is
occurring will be discarded.
In the general case, there are good semantic reasons to require that all data
accesses inside such a seqlock "critical section" must be atomic.
If we read a pointer p
as part of reading the data, and then
read *p
as well, the code inside the critical section may
read from a bad address if the read of p
happened to see
a half-updated pointer value.
In such cases, there is probably no way to avoid reading the pointer with a
conventional atomic load, and that's exactly what's desired.
However, in many cases, particularly in the multiple process case,
seqlock data consists of a single trivially copyable object, and the
seqlock "critical section" consists of a simple copy operation.
Under normal circumstances, this could have been written using memcpy
.
But that's unacceptable here, since memcpy
does not generate atomic accesses,
and is (according to our specification anyway) susceptable to data races.
Currently to write such code correctly, we need to basically decompose such data into many small lock-free atomic subobjects, and copy them a piece at a time. Treating the data as a single large atomic object would defeat the purpose of the seqlock, since the atomic copy operation would acquire a conventional lock. Our proposal essentially adds a convenient library facility to automate this decomposition into small objects.
We propose that both the copy from shared_data
, and the following
fence be replaced by a new atomic_load_per_byte_memcpy
call.
We propose to introduce two additional versions of memcpy
to
resolve the above issues. They guarantee that either source or destination
accesses are byte-wise atomic:
atomic_load_per_byte_memcpy(void* dest, void* source, size_t count, memory_order order)
directly addresses the seqlock reader problem. Like memcpy
, it
requires that the source and destination ranges do not overlap. It also requires that
order
is memory_order_acquire
or memory_order_relaxed
. (It is unclear that memory_order_seq_cst
makes sense here since much of the point here is to allow reordering of the
byte reads. Though we originally proposed to allow it, there was no support for it
in SG1, and we are unwilling to defend it.)
This behaves roughly as if:
for (size_t i = 0; i < count; ++i) { reinterpret_cast<char*>(dest)[i] = atomic_ref<char>(reinterpret_cast<char*>(source)[i]).load(memory_order_relaxed); } atomic_thread_fence(order);
Note that on standard hardware, it should be OK to actually perform the copy at larger than byte granularity. Copying multiple bytes as part of one operation is indistinguishable from running them so quickly that the intermediate state is not observed. In fact, we expect that existing assembly memcpy implementations will suffice when suffixed with the required fence.
With atomic_load_per_byte_memcpy
, the canonical seqlock reader code
becomes:
Foo data; // Trivially copyable. do { seq1 = seq_no.load(memory_order_acquire); atomic_load_per_byte_memcpy(&data, &shared_data, sizeof(Foo), memory_order_acquire); int seq2 = seq_no.load(memory_order_relaxed); } while (seq1 != seq2 || seq1 & 1); use data;
Note that for purposes of reasoning about memory ordering, treating the
memcpy as a single memory_order_acquire
operation conveys the correct
intuition; the memcpy operation is effectively ordered before the second sequence
number read.
The atomic_load_per_byte_memcpy
operation would introduce a data race and hence
undefined behavior if the source where simultaneously updated by an ordinary
memcpy
. Similarly, we would expect undefined behavior if the
writer updates the source using atomic operations of a different granularity.
To facilitate correct use, we need to also provide a corresponding version of
memcpy
that updates memory using atomic byte stores.
We thus also propose
atomic_store_per_byte_memcpy(void* dest, void* source, size_t count, memory_order order)
,
where order
is memory_order_release
or memory_order_relaxed
. (Memory_order_seq_cst
again barely makes
sense.)
It behaves roughly as if:
atomic_thread_fence(order); for (size_t i = 0; i < count; ++i) { atomic_ref<char>(reinterpret_cast<char*>(dest)[i]).store( reinterpret_cast<char*>(source)[i], memory_order_relaxed); }
There is a question as to whether the order
argument should be part of the
interface, and if so, whether this is the right way to handle it.
Excluding the order
argument, and requiring the programmer to explicitly
write the fence simplifies this proposal further. But I believe there are
convincing reasons to include it:
atomic_store_per_byte_memcpy(..., memory_order_release)
and an
atomic_load_per_byte_memcpy(..., memory_order_acquire)
that reads the resulting values
establishes a synchronizes_with relationship, as expected.
atomic_load_per_byte_memcpy(..., memory_order_acquire)
cannot contribute to an out-of-thin-air result, and hence there is no need to add
overhead to prevent that.
Resolution: Include the memory order argument.
Unfortunately, defining this construct in terms of an explicit fence overconstrains the hardware a bit; if the block being copied is short enough to be copied e.g. by a single ARMv8 load-acquire instruction, this would disallow that implementation, since the fence can also establish ordering in conjunction with other earlier atomic loads, while the load-acquire instruction cannot.
An alternative is to include the order
argument, but not
to define it in terms of a fence. This is slightly more complex, but allows
the above load-acquire implementation. Resolution:
Do not define it in terms of a fence. The wording below uses a different
formulation that provides weaker guarantees.
There were discussions in Cologne and Belfast as to whether
memory_order_seq_cst
is a reasonable memory order argument.
We concluded, at least for now, that it isn't. Reasonable interpretations
may be possible, but the fact that individual byte operations can still be
reordered makes it confusing.
The facility here is fundamentally a C level facility, making it potentially possible to include it in C as well. This would raise the same namespace issues that P0943 is trying to address, but compatibility should be possible.
It is clearly possible to put a higher-level type-safe layer on top of this that copies trivially copyable objects rather than bytes. It is not completely clear which level we should standardize. Preliminary resolution: Standardize the low-level primitive first; it's relatively easy for the user to implement the type-safe version.
Add the following to the atomics section in Concurrency TS 2. The eventual goal is to move
this into C++23 if we do not include a more general facility in the meantime.
It doesn't look to me like the definition of memcpy
in
the C standard works in the present context. Thus this is phrased somewhat differently.
The
atomic_load_per_byte_memcpy()
andatomic_store_per_byte_memcpy()
functions support concurrent programming idioms in which values may be read while being written, but the value is trusted only when it can be determined after the fact that a race did not occur. [ Note: So-called "seqlocks" are the canonical example of such an idiom. --end note ]
atomic_load_per_byte_memcpy(void* dest, void* source, size_t count, memory_order order)
- Requires:
memory_order
shall bememory_order_acquire
ormemory_order_relaxed
.- Effects:
- Copies
count
consecutive bytes pointed to bysource
into consecutive bytes pointed to bydest
. If this operation reads and modifies the same byte, the behavior is undefined. Each individual load operation from a source byte is an atomic operation.
atomic_store_per_byte_memcpy(void* dest, void* source, size_t count, memory_order order)
- Requires:
memory_order
shall bememory_order_release
ormemory_order_relaxed
.- Effects:
- Copies
count
consecutive bytes pointed to bysource
into consecutive bytes pointed to bydest
. If this operation reads and modifies the same byte, the behavior is undefined. Each individual store operation to a destination byte is an atomic operation.- Synchronization
- If any of the atomic byte loads performed by an
atomic_load_per_byte_memcpy()
call A withmemory_order_acquire
argument take their value from an atomic byte store performed byatomic_store_per_byte_memcpy()
call B withmemory_order_release
argument, then the start of B strongly happens before the completion of A.
Note that the naming is intentionally C/WG14-compatible, in that it starts with
atomic_
. This is enough of an esoteric and experts-only facility
that long names are probably OK.
This provides the minimal ordering guarantee require by seqlocks and the like. It does not promise synchronization with other atomic operations. We could later strengthen this. It is unclear to me that we want to promise more without a use case.
R0 was the initial proposal. Recommended specifications in terms of leading and trailing fences. This was intended to be in the pre-Kona 2019 mailing, but didn't make it due to a technical glitch and the author's failure to notice the technical glitch. SG1 had a preliminary discussion in Kona anyway, which informed R1.
R1 is the first attempt at wording. This is no longer based on leading or trailing
fences, thus potentially allowing memory_order_acquire
/
memory_order_release
operations with a small constant size argument to
be compiled to a single instruction on ARMv8 and similar architectures.
Various parts of the paper were also updated to reflect the preliminary discussion in Kona.
R2 tweaked the wording, largely in response to SG1 discussion in Cologne. We now talk about bytes rather than characters. Both functions were renamed. The synchronization clause was slightly tweaked. Added introductory paragraph to wording. Explicitly target Concurency TS 2 for now, since there was concern about preempting a more general facility.
R3 modified the wording to disallow overlapping source and destination ranges, as instructed by SG1. Updated some explanatory text in preparation for LEWG review.