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Diffstat (limited to 'Documentation/memory-barriers.txt')
| -rw-r--r-- | Documentation/memory-barriers.txt | 17 |
1 files changed, 13 insertions, 4 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index 06e14efd8662..61b7317bcf2e 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -88,7 +88,6 @@ CONTENTS (*) The effects of the cpu cache. - - Cache coherency. - Cache coherency vs DMA. - Cache coherency vs MMIO. @@ -396,6 +395,11 @@ Memory barriers come in four basic varieties: (2) Address-dependency barriers (historical). + [!] This section is marked as HISTORICAL: it covers the long-obsolete + smp_read_barrier_depends() macro, the semantics of which are now + implicit in all marked accesses. For more up-to-date information, + including how compiler transformations can sometimes break address + dependencies, see Documentation/RCU/rcu_dereference.rst. An address-dependency barrier is a weaker form of read barrier. In the case where two loads are performed such that the second depends on the @@ -556,6 +560,11 @@ There are certain things that the Linux kernel memory barriers do not guarantee: ADDRESS-DEPENDENCY BARRIERS (HISTORICAL) ---------------------------------------- +[!] This section is marked as HISTORICAL: it covers the long-obsolete +smp_read_barrier_depends() macro, the semantics of which are now implicit +in all marked accesses. For more up-to-date information, including +how compiler transformations can sometimes break address dependencies, +see Documentation/RCU/rcu_dereference.rst. As of v4.15 of the Linux kernel, an smp_mb() was added to READ_ONCE() for DEC Alpha, which means that about the only people who need to pay attention @@ -667,8 +676,6 @@ include/linux/rcupdate.h. This permits the current target of an RCU'd pointer to be replaced with a new modified target, without the replacement target appearing to be incompletely initialised. -See also the subsection on "Cache Coherency" for a more thorough example. - CONTROL DEPENDENCIES -------------------- @@ -2175,9 +2182,11 @@ set_current_state() may be wrapped by: which therefore also imply a general memory barrier after setting the state. The whole sequence above is available in various canned forms, all of which -interpolate the memory barrier in the right place: +interpolate the memory barrier in the right place, for example: wait_event(); + wait_event_cmd(); + wait_event_exclusive_cmd(); wait_event_interruptible(); wait_event_interruptible_exclusive(); wait_event_interruptible_timeout(); |