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local_t Documentation update

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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>
This commit is contained in:
Mathieu Desnoyers 2007-11-05 14:50:54 -08:00 committed by Linus Torvalds
parent bf2cdef306
commit e1265205c0
1 changed files with 23 additions and 0 deletions
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23
Documentation/local_ops.txt
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@ -45,6 +45,29 @@ long fails. The definition looks like :
typedef struct { atomic_long_t a; } local_t;
* 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synchronization is done by the writer CPU, an outdated copy of the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.
* Rules to follow when using local atomic operations
- Variables touched by local ops must be per cpu variables.