diff options
Diffstat (limited to 'Documentation/local_ops.txt')
| -rw-r--r-- | Documentation/local_ops.txt | 48 |
1 files changed, 47 insertions, 1 deletions
diff --git a/Documentation/local_ops.txt b/Documentation/local_ops.txt index b0aca0705d1..1a45f11e645 100644 --- a/Documentation/local_ops.txt +++ b/Documentation/local_ops.txt @@ -27,7 +27,7 @@ CPU which owns the data. Therefore, care must taken to make sure that only one CPU writes to the local_t data. This is done by using per cpu data and making sure that we modify it from within a preemption safe context. It is however permitted to read local_t data from any CPU : it will then appear to be written -out of order wrt other memory writes on the owner CPU. +out of order wrt other memory writes by the owner CPU. * Implementation for a given architecture @@ -45,6 +45,52 @@ 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 Since no memory + synchronization is done by the writer CPU, an outdated copy of the + variable can be read when reading some _other_ cpu's variables.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. +- _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> |