Re: Linux 3.19-rc3

Peter Hurley
2015-02-02T19:03:40Z

On 02/02/2015 11:11 AM, Paul E. McKenney wrote:
> On Tue, Jan 20, 2015 at 09:03:12AM -0500, Peter Hurley wrote:
>> On 01/19/2015 07:30 PM, Paul E. McKenney wrote:
>>> On Tue, Jan 06, 2015 at 12:47:53PM -0800, Paul E. McKenney wrote:
>>>> On Tue, Jan 06, 2015 at 02:57:37PM -0500, Peter Hurley wrote:
>>>
>>> [ . . . ]
>>>
>>>> David Miller's call, actually.
>>>>
>>>> But the rule is that if it is an atomic read-modify-write operation and it
>>>> returns a value, then the operation itself needs to include full memory
>>>> barriers before and after (as in the caller doesn't need to add them).
>>>> Otherwise, the operation does not need to include memory ordering.
>>>> Since xchg(), atomic_xchg(), and atomic_long_xchg() all return a value,
>>>> their implementations must include full memory barriers before and after.
>>>>
>>>> Pretty straightforward. ;-)
>>>
>>> Hello again, Peter,
>>>
>>> Were you going to push a patch clarifying this?
>>
>> Hi Paul,
>>
>> As you pointed out, atomic_ops.txt is for arch implementors, so I wasn't
>> planning on patching that file.
>>
>> I've been meaning to write up something specifically for everyone else but
>> my own bugs have kept me from that. [That, and I'm not sure what I write
>> will be suitable for Documentation.]
>
> Well, upon revisiting this after coming back from travel, I am more inclined
> to agree that a change would be good. I doubt if you would be the only
> person who might be confused. So how about the following patch?

I think this is much clearer to both audiences. Thanks!

Regards,
Peter Hurley

[minor corrections below]

> ------------------------------------------------------------------------
>
> documentation: Clarify memory-barrier semantics of atomic operations
>
> All value-returning atomic read-modify-write operations must provide full
> memory-barrier semantics on both sides of the operation. This commit
> clarifies the documentation to make it clear that these memory-barrier
> semantics are provided by the operations themselves, not by their callers.
>
> Reported-by: Peter Hurley <peter@hurleysoftware.com>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>
> diff --git a/Documentation/atomic_ops.txt b/Documentation/atomic_ops.txt
> index 183e41bdcb69..672201e79c49 100644
> --- a/Documentation/atomic_ops.txt
> +++ b/Documentation/atomic_ops.txt
> @@ -201,11 +201,11 @@ These routines add 1 and subtract 1, respectively, from the given
> atomic_t and return the new counter value after the operation is
> performed.
>
> -Unlike the above routines, it is required that explicit memory
> -barriers are performed before and after the operation. It must be
> -done such that all memory operations before and after the atomic
> -operation calls are strongly ordered with respect to the atomic
> -operation itself.
> +Unlike the above routines, it is required that these primitives
> +inlude explicit memory barriers that are performed before and after
^^^^^^
include|provide
> +the operation. It must be done such that all memory operations before
> +and after the atomic operation calls are strongly ordered with respect
> +to the atomic operation itself.
>
> For example, it should behave as if a smp_mb() call existed both
> before and after the atomic operation.
> @@ -233,21 +233,21 @@ These two routines increment and decrement by 1, respectively, the
> given atomic counter. They return a boolean indicating whether the
> resulting counter value was zero or not.
>
> -It requires explicit memory barrier semantics around the operation as
> -above.
> +Again, these primitive provide explicit memory barrier semantics around
^
s
> +the atomic operation.
>
> int atomic_sub_and_test(int i, atomic_t *v);
>
> This is identical to atomic_dec_and_test() except that an explicit
> -decrement is given instead of the implicit "1". It requires explicit
> -memory barrier semantics around the operation.
> +decrement is given instead of the implicit "1". This primitive must
> +provide explicit memory barrier semantics around the operation.
>
> int atomic_add_negative(int i, atomic_t *v);
>
> -The given increment is added to the given atomic counter value. A
> -boolean is return which indicates whether the resulting counter value
> -is negative. It requires explicit memory barrier semantics around the
> -operation.
> +The given increment is added to the given atomic counter value. A boolean
> +is return which indicates whether the resulting counter value is negative.
> +This primitive must provide explicit memory barrier semantics around
> +the operation.
>
> Then:
>
> @@ -257,7 +257,7 @@ This performs an atomic exchange operation on the atomic variable v, setting
> the given new value. It returns the old value that the atomic variable v had
> just before the operation.
>
> -atomic_xchg requires explicit memory barriers around the operation.
> +atomic_xchg must provide explicit memory barriers around the operation.
>
> int atomic_cmpxchg(atomic_t *v, int old, int new);
>
> @@ -266,7 +266,7 @@ with the given old and new values. Like all atomic_xxx operations,
> atomic_cmpxchg will only satisfy its atomicity semantics as long as all
> other accesses of *v are performed through atomic_xxx operations.
>
> -atomic_cmpxchg requires explicit memory barriers around the operation.
> +atomic_cmpxchg must provide explicit memory barriers around the operation.
>
> The semantics for atomic_cmpxchg are the same as those defined for 'cas'
> below.
> @@ -279,8 +279,8 @@ If the atomic value v is not equal to u, this function adds a to v, and
> returns non zero. If v is equal to u then it returns zero. This is done as
> an atomic operation.
>
> -atomic_add_unless requires explicit memory barriers around the operation
> -unless it fails (returns 0).
> +atomic_add_unless must provide explicit memory barriers around the
> +operation unless it fails (returns 0).
>
> atomic_inc_not_zero, equivalent to atomic_add_unless(v, 1, 0)
>
> @@ -460,9 +460,9 @@ the return value into an int. There are other places where things
> like this occur as well.
>
> These routines, like the atomic_t counter operations returning values,
> -require explicit memory barrier semantics around their execution. All
> -memory operations before the atomic bit operation call must be made
> -visible globally before the atomic bit operation is made visible.
> +must provide explicit memory barrier semantics around their execution.
> +All memory operations before the atomic bit operation call must be
> +made visible globally before the atomic bit operation is made visible.
> Likewise, the atomic bit operation must be visible globally before any
> subsequent memory operation is made visible. For example:
>
> @@ -536,8 +536,9 @@ except that two underscores are prefixed to the interface name.
> These non-atomic variants also do not require any special memory
> barrier semantics.
>
> -The routines xchg() and cmpxchg() need the same exact memory barriers
> -as the atomic and bit operations returning values.
> +The routines xchg() and cmpxchg() must provide the same exact
> +memory-barrier semantics as the atomic and bit operations returning
> +values.
>
> Spinlocks and rwlocks have memory barrier expectations as well.
> The rule to follow is simple:
>