Cc: Linus Torvalds <torvalds@...>, Alan Cox <alan@...>, Paul E. McKenney <paulmck@...>, Andrew Morton <akpm@...>, Zdenek Kabelac <zdenek.kabelac@...>, <davem@...>, Rafael J. Wysocki <rjw@...>, Pierre Ossman <drzeus-mmc@...>, Kernel development list <linux-kernel@...>, pm list <linux-pm@...>
Here's an updated version of the patch, including the issue Alan Cox
brought up.
Alan Stern
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Atomicity of reads of write for pointers and integral types (other than
long long) should be documented, along with the limitations imposed by
the compiler.
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
---
Index: usb-2.6/Documentation/atomic_ops.txt
===================================================================
--- usb-2.6.orig/Documentation/atomic_ops.txt
+++ usb-2.6/Documentation/atomic_ops.txt
@@ -21,6 +21,24 @@ local_t is very similar to atomic_t. If
updated by one CPU, local_t is probably more appropriate. Please see
Documentation/local_ops.txt for the semantics of local_t.
+For all properly-aligned pointer and integral types other than long
+long, the kernel requires simple reads and writes to be atomic with
+respect to each other. That is, if one CPU reads a pointer value at
+the same time as another CPU overwrites the pointer, it is guaranteed
+that the reader will obtain either the old or the new value of the
+pointer, never some mish-mash combination of the two. Likewise, if
+one CPU writes a long value at the same time as another CPU does, it
+is guaranteed that one or the other of the values will end up stored
+in memory, not some mish-mash combination of bits.
+
+Thus, if all you need is atomicity of reading and writing then you can
+use plain old ints, longs, or pointers; you don't need to use
+atomic_t. But note: This guarantee emphatically does not apply to
+long long values or unaligned values! Note also that gcc does not
+guarantee to compile all C assignment expressions into simple writes.
+For example, a statement like "x = a + b" might cause gcc to emit code
+equivalent to "x = a; x += b", which is decidedly non-atomic.
+
The first operations to implement for atomic_t's are the initializers and
plain reads.
--
