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author | Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> | 2007-11-05 14:50:54 -0800 |
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committer | Linus Torvalds <torvalds@woody.linux-foundation.org> | 2007-11-05 15:12:32 -0800 |
commit | e1265205c0ee3919c3f2c750662630154c8faab2 (patch) | |
tree | 904809bdacafb24d82d69c564251c3967ded9986 /Documentation/local_ops.txt | |
parent | bf2cdef30667d0d3d09c6934c95d1fb87c43345a (diff) | |
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local_t Documentation update
Grant Grundler was asking for more detail about correct usage of local
atomic operations and suggested adding the resulting summary to
local_ops.txt.
"Please add a bit more detail. If DaveM is correct (he normally is), then
there must be limits on how the local_t can be used in the kernel process
and interrupt contexts. I'd like those rules spelled out very clearly
since it's easy to get wrong and tracking down such a bug is quite painful."
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Signed-off-by: Grant Grundler <grundler@parisc-linux.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'Documentation/local_ops.txt')
-rw-r--r-- | Documentation/local_ops.txt | 23 |
1 files changed, 23 insertions, 0 deletions
diff --git a/Documentation/local_ops.txt b/Documentation/local_ops.txt index 4269a11..1a45f11 100644 --- a/Documentation/local_ops.txt +++ b/Documentation/local_ops.txt @@ -68,6 +68,29 @@ typedef struct { atomic_long_t a; } local_t; variable can be read when reading some _other_ cpu's variables. +* Rules to follow when using local atomic operations + +- Variables touched by local ops must be per cpu variables. +- _Only_ the CPU owner of these variables must write to them. +- This CPU can use local ops from any context (process, irq, softirq, nmi, ...) + to update its local_t variables. +- Preemption (or interrupts) must be disabled when using local ops in + process context to make sure the process won't be migrated to a + different CPU between getting the per-cpu variable and doing the + actual local op. +- When using local ops in interrupt context, no special care must be + taken on a mainline kernel, since they will run on the local CPU with + preemption already disabled. I suggest, however, to explicitly + disable preemption anyway to make sure it will still work correctly on + -rt kernels. +- Reading the local cpu variable will provide the current copy of the + variable. +- Reads of these variables can be done from any CPU, because updates to + "long", aligned, variables are always atomic. Since no memory + synchronization is done by the writer CPU, an outdated copy of the + variable can be read when reading some _other_ cpu's variables. + + * How to use local atomic operations #include <linux/percpu.h> |