docs: document atomic_load_acquire and atomic_store_release

We will use them in the next patch, document what they do.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

docs/devel/atomics.txt

@@ -122,20 +122,30 @@ In general, if the algorithm you are writing includes both writes
 and reads on the same side, it is generally simpler to use sequentially
 consistent primitives.
 
-When using this model, variables are accessed with atomic_read() and
+When using this model, variables are accessed with:
-atomic_set(), and restrictions to the ordering of accesses is enforced
+
+- atomic_read() and atomic_set(); these prevent the compiler from
+  optimizing accesses out of existence and creating unsolicited
+  accesses, but do not otherwise impose any ordering on loads and
+  stores: both the compiler and the processor are free to reorder
+  them.
+
+- atomic_load_acquire(), which guarantees the LOAD to appear to
+  happen, with respect to the other components of the system,
+  before all the LOAD or STORE operations specified afterwards.
+  Operations coming before atomic_load_acquire() can still be
+  reordered after it.
+
+- atomic_store_release(), which guarantees the STORE to appear to
+  happen, with respect to the other components of the system,
+  after all the LOAD or STORE operations specified afterwards.
+  Operations coming after atomic_store_release() can still be
+  reordered after it.
+
+Restrictions to the ordering of accesses can also be specified
 using the memory barrier macros: smp_rmb(), smp_wmb(), smp_mb(),
 smp_mb_acquire(), smp_mb_release(), smp_read_barrier_depends().
 
-atomic_read() and atomic_set() prevents the compiler from using
-optimizations that might otherwise optimize accesses out of existence
-on the one hand, or that might create unsolicited accesses on the other.
-In general this should not have any effect, because the same compiler
-barriers are already implied by memory barriers.  However, it is useful
-to do so, because it tells readers which variables are shared with
-other threads, and which are local to the current thread or protected
-by other, more mundane means.
-
 Memory barriers control the order of references to shared memory.
 They come in six kinds:
 
@@ -232,7 +242,7 @@ make atomic_mb_set() the more expensive operation.
 
 There are two common cases in which atomic_mb_read and atomic_mb_set
 generate too many memory barriers, and thus it can be useful to manually
-place barriers instead:
+place barriers, or use atomic_load_acquire/atomic_store_release instead:
 
 - when a data structure has one thread that is always a writer
   and one thread that is always a reader, manual placement of
@@ -243,18 +253,15 @@ place barriers instead:
      thread 1                                thread 1
      -------------------------               ------------------------
      (other writes)
-                                             smp_mb_release()
+     atomic_mb_set(&a, x)                    atomic_store_release(&a, x)
-     atomic_mb_set(&a, x)                    atomic_set(&a, x)
+     atomic_mb_set(&b, y)                    atomic_store_release(&b, y)
-                                             smp_wmb()
-     atomic_mb_set(&b, y)                    atomic_set(&b, y)
 
                                        =>
      thread 2                                thread 2
      -------------------------               ------------------------
-     y = atomic_mb_read(&b)                  y = atomic_read(&b)
+     y = atomic_mb_read(&b)                  y = atomic_load_acquire(&b)
-                                             smp_rmb()
+     x = atomic_mb_read(&a)                  x = atomic_load_acquire(&a)
-     x = atomic_mb_read(&a)                  x = atomic_read(&a)
+     (other reads)
-                                             smp_mb_acquire()
 
   Note that the barrier between the stores in thread 1, and between
   the loads in thread 2, has been optimized here to a write or a
@@ -276,7 +283,6 @@ place barriers instead:
                                              smp_mb_acquire();
 
   Similarly, atomic_mb_set() can be transformed as follows:
-  smp_mb():
 
                                              smp_mb_release();
      for (i = 0; i < 10; i++)          =>    for (i = 0; i < 10; i++)
@@ -284,6 +290,8 @@ place barriers instead:
                                              smp_mb();
 
 
+  The other thread can still use atomic_mb_read()/atomic_mb_set().
+
 The two tricks can be combined.  In this case, splitting a loop in
 two lets you hoist the barriers out of the loops _and_ eliminate the
 expensive smp_mb():
@@ -296,8 +304,6 @@ expensive smp_mb():
                                                atomic_set(&a[i], false);
                                              smp_mb();
 
-  The other thread can still use atomic_mb_read()/atomic_mb_set()
-
 
 Memory barrier pairing
 ----------------------
@@ -386,10 +392,7 @@ and memory barriers, and the equivalents in QEMU:
   note that smp_store_mb() is a little weaker than atomic_mb_set().
   atomic_mb_read() compiles to the same instructions as Linux's
   smp_load_acquire(), but this should be treated as an implementation
-  detail.  QEMU does have atomic_load_acquire() and atomic_store_release()
+  detail.
-  macros, but for now they are only used within atomic.h.  This may
-  change in the future.
-
 
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