Re: Linux 3.19-rc3

Paul E. McKenney
2015-02-02T16:11:23Z

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?

Thanx, Paul

------------------------------------------------------------------------

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
+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
+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: