Commit-ID: 66ce3a4dcb9f06fc6f15656b9434b4259fb4e319 Gitweb: http://git.kernel.org/tip/66ce3a4dcb9f06fc6f15656b9434b4259fb4e319 Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> AuthorDate: Fri, 30 Jun 2017 16:18:28 -0700 Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> CommitDate: Thu, 17 Aug 2017 07:29:57 -0700 doc: Update memory-barriers.txt for read-to-write dependencies The memory-barriers.txt document contains an obsolete passage stating that smp_read_barrier_depends() is required to force ordering for read-to-write dependencies. We now know that this is not required, even for DEC Alpha. This commit therefore updates this passage to state that read-to-write dependencies are respected even without smp_read_barrier_depends(). Reported-by: Lance Roy <ldr709@xxxxxxxxx> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> Cc: David Howells <dhowells@xxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Jonathan Corbet <corbet@xxxxxxx> Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> Cc: Andrea Parri <parri.andrea@xxxxxxxxx> Cc: Jade Alglave <j.alglave@xxxxxxxxx> Cc: Luc Maranget <luc.maranget@xxxxxxxx> [ paulmck: Reference control-dependencies sections and use WRITE_ONCE() per Will Deacon. Correctly place split-cache paragraph while there. ] Acked-by: Will Deacon <will.deacon@xxxxxxx> --- Documentation/memory-barriers.txt | 41 +++++++++++++++++++++++---------------- 1 file changed, 24 insertions(+), 17 deletions(-) diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index c4ddfcd..e2ee0a1 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -594,7 +594,24 @@ between the address load and the data load: This enforces the occurrence of one of the two implications, and prevents the third possibility from arising. -A data-dependency barrier must also order against dependent writes: + +[!] Note that this extremely counterintuitive situation arises most easily on +machines with split caches, so that, for example, one cache bank processes +even-numbered cache lines and the other bank processes odd-numbered cache +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 (2). + + +A data-dependency barrier is not required to order dependent writes +because the CPUs that the Linux kernel supports don't do writes +until they are certain (1) that the write will actually happen, (2) +of the location of the write, and (3) of the value to be written. +But please carefully read the "CONTROL DEPENDENCIES" section and the +Documentation/RCU/rcu_dereference.txt file: The compiler can and does +break dependencies in a great many highly creative ways. CPU 1 CPU 2 =============== =============== @@ -603,29 +620,19 @@ A data-dependency barrier must also order against dependent writes: <write barrier> WRITE_ONCE(P, &B); Q = READ_ONCE(P); - <data dependency barrier> - *Q = 5; + WRITE_ONCE(*Q, 5); -The data-dependency barrier must order the read into Q with the store -into *Q. This prohibits this outcome: +Therefore, no data-dependency barrier is required to order the read into +Q with the store into *Q. In other words, this outcome is prohibited, +even without a data-dependency barrier: (Q == &B) && (B == 4) Please note that this pattern should be rare. After all, the whole point of dependency ordering is to -prevent- writes to the data structure, along with the expensive cache misses associated with those writes. This pattern -can be used to record rare error conditions and the like, and the ordering -prevents such records from being lost. - - -[!] Note that this extremely counterintuitive situation arises most easily on -machines with split caches, so that, for example, one cache bank processes -even-numbered cache lines and the other bank processes odd-numbered cache -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 (2). +can be used to record rare error conditions and the like, and the CPUs' +naturally occurring ordering prevents such records from being lost. The data dependency barrier is very important to the RCU system, -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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