On Wed, Aug 29, 2007 at 08:19:53AM -0400, Mathieu Desnoyers wrote: > local_t Documentation update 2 > > 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." > > It applies on top of 2.6.23-rc3-mm1. > > Signed-off-by: Mathieu Desnoyers <[EMAIL PROTECTED]> > CC: Grant Grundler <[EMAIL PROTECTED]>
Signed-off-by: Grant Grundler <[EMAIL PROTECTED]> thanks! grant > --- > Documentation/local_ops.txt | 23 +++++++++++++++++++++++ > 1 file changed, 23 insertions(+) > > Index: linux-2.6-lttng/Documentation/local_ops.txt > =================================================================== > --- linux-2.6-lttng.orig/Documentation/local_ops.txt 2007-08-29 > 08:09:34.000000000 -0400 > +++ linux-2.6-lttng/Documentation/local_ops.txt 2007-08-29 > 08:15:37.000000000 -0400 > @@ -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 + synchronization is done by the writer CPU, an outdated copy of the 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. > + > + > * How to use local atomic operations > > #include <linux/percpu.h> > > -- > Mathieu Desnoyers > Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal > OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/