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| -rw-r--r-- | Documentation/memory-barriers.txt | 19 |
1 files changed, 9 insertions, 10 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index cc53f47..cf0d541 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -287,7 +287,7 @@ Memory barriers come in four basic varieties: A write barrier is a partial ordering on stores only; it is not required to have any effect on loads. - A CPU can be viewed as as commiting a sequence of store operations to the + A CPU can be viewed as committing a sequence of store operations to the memory system as time progresses. All stores before a write barrier will occur in the sequence _before_ all the stores after the write barrier. @@ -418,7 +418,7 @@ There are certain things that the Linux kernel memory barriers do not guarantee: indirect effect will be the order in which the second CPU sees the effects of the first CPU's accesses occur, but see the next point: - (*) There is no guarantee that the a CPU will see the correct order of effects + (*) There is no guarantee that a CPU will see the correct order of effects from a second CPU's accesses, even _if_ the second CPU uses a memory barrier, unless the first CPU _also_ uses a matching memory barrier (see the subsection on "SMP Barrier Pairing"). @@ -489,7 +489,7 @@ lines. The pointer P might be stored in an odd-numbered cache line, and the variable B might be stored in an even-numbered cache line. Then, if the even-numbered bank of the reading CPU's cache is extremely busy while the odd-numbered bank is idle, one can see the new value of the pointer P (&B), -but the old value of the variable B (1). +but the old value of the variable B (2). Another example of where data dependency barriers might by required is where a @@ -749,7 +749,7 @@ some effectively random order, despite the write barrier issued by CPU 1: : : -If, however, a read barrier were to be placed between the load of E and the +If, however, a read barrier were to be placed between the load of B and the load of A on CPU 2: CPU 1 CPU 2 @@ -1466,9 +1466,8 @@ instruction itself is complete. On a UP system - where this wouldn't be a problem - the smp_mb() is just a compiler barrier, thus making sure the compiler emits the instructions in the -right order without actually intervening in the CPU. Since there there's only -one CPU, that CPU's dependency ordering logic will take care of everything -else. +right order without actually intervening in the CPU. Since there's only one +CPU, that CPU's dependency ordering logic will take care of everything else. ATOMIC OPERATIONS @@ -1645,9 +1644,9 @@ functions: The PCI bus, amongst others, defines an I/O space concept - which on such CPUs as i386 and x86_64 cpus readily maps to the CPU's concept of I/O - space. However, it may also mapped as a virtual I/O space in the CPU's - memory map, particularly on those CPUs that don't support alternate - I/O spaces. + space. However, it may also be mapped as a virtual I/O space in the CPU's + memory map, particularly on those CPUs that don't support alternate I/O + spaces. Accesses to this space may be fully synchronous (as on i386), but intermediary bridges (such as the PCI host bridge) may not fully honour |
