The patch titled Documentation/volatile-considered-harmful.txt: correct cpu_relax() documentation has been removed from the -mm tree. Its filename was documentation-volatile-considered-harmfultxt-correct-cpu_relax-documentation.patch This patch was dropped because it was merged into mainline or a subsystem tree The current -mm tree may be found at http://userweb.kernel.org/~akpm/mmotm/ ------------------------------------------------------ Subject: Documentation/volatile-considered-harmful.txt: correct cpu_relax() documentation From: Russell King <rmk@xxxxxxxxxxxxxxxx> cpu_relax() is documented in volatile-considered-harmful.txt to be a memory barrier. However, everyone with the exception of Blackfin and possibly ia64 defines cpu_relax() to be a compiler barrier. Make the documentation reflect the general concensus. Linus sayeth: : I don't think it was ever the intention that it would be seen as anything : but a compiler barrier, although it is obviously implied that it might : well perform some per-architecture actions that have "memory barrier-like" : semantics. : : After all, the whole and only point of the "cpu_relax()" thing is to tell : the CPU that we're busy-looping on some event. : : And that "event" might be (and often is) about reading the same memory : location over and over until it changes to what we want it to be. So it's : quite possible that on various architectures the "cpu_relax()" could be : about making sure that such a tight loop on loads doesn't starve cache : transactions, for example - and as such look a bit like a memory barrier : from a CPU standpoint. : : But it's not meant to have any kind of architectural memory ordering : semantics as far as the kernel is concerned - those must come from other : sources. Signed-off-by: Russell King <rmk+kernel@xxxxxxxxxxxxxxxx> Cc: <linux-arch@xxxxxxxxxxxxxxx> Acked-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- Documentation/volatile-considered-harmful.txt | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff -puN Documentation/volatile-considered-harmful.txt~documentation-volatile-considered-harmfultxt-correct-cpu_relax-documentation Documentation/volatile-considered-harmful.txt --- a/Documentation/volatile-considered-harmful.txt~documentation-volatile-considered-harmfultxt-correct-cpu_relax-documentation +++ a/Documentation/volatile-considered-harmful.txt @@ -63,9 +63,9 @@ way to perform a busy wait is: cpu_relax(); The cpu_relax() call can lower CPU power consumption or yield to a -hyperthreaded twin processor; it also happens to serve as a memory barrier, -so, once again, volatile is unnecessary. Of course, busy-waiting is -generally an anti-social act to begin with. +hyperthreaded twin processor; it also happens to serve as a compiler +barrier, so, once again, volatile is unnecessary. Of course, busy- +waiting is generally an anti-social act to begin with. There are still a few rare situations where volatile makes sense in the kernel: _ Patches currently in -mm which might be from rmk@xxxxxxxxxxxxxxxx are origin.patch arm-convert-proc-cpu-aligment-to-seq_file.patch arch-arm-include-asm-elfh-forward-declare-the-task-struct.patch bitops-rename-for_each_bit-to-for_each_set_bit-mtd.patch -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html