The handling of preempt_count() is inconsistent accross kernel configurations. On kernels which have PREEMPT_COUNT=n preempt_disable/enable() and the lock/unlock functions are not affecting the preempt count, only local_bh_disable/enable() and _bh variants of locking, soft interrupt delivery, hard interrupt and NMI context affect it.
It's therefore impossible to have a consistent set of checks which provide information about the context in which a function is called. In many cases it makes sense to have seperate functions for seperate contexts, but there are valid reasons to avoid that and handle different calling contexts conditionally.
The lack of such indicators which work on all kernel configuratios is a constant source of trouble because developers either do not understand the implications or try to work around this inconsistency in weird ways. Neither seem these issues be catched by reviewers and testing.
Recently merged code does:
gfp = preemptible() ? GFP_KERNEL : GFP_ATOMIC;
Looks obviously correct, except for the fact that preemptible() is unconditionally false for CONFIF_PREEMPT_COUNT=n, i.e. all allocations in that code use GFP_ATOMIC on such kernels.
Attempts to make preempt count unconditional and consistent have been rejected in the past with handwaving performance arguments.
Freshly conducted benchmarks did not reveal any measurable impact from enabling preempt count unconditionally. On kernels with CONFIG_PREEMPT_NONE or CONFIG_PREEMPT_VOLUNTARY the preempt count is only incremented and decremented but the result of the decrement is not tested. Contrary to that enabling CONFIG_PREEMPT which tests the result has a small but measurable impact due to the conditional branch/call.
It's about time to make essential functionality of the kernel consistent accross the various preemption models.
Enable CONFIG_PREEMPT_COUNT unconditionally. Follow up changes will remove the #ifdeffery and remove the config option at the end.
Signed-off-by: Thomas Gleixner tglx@linutronix.de --- kernel/Kconfig.preempt | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-)
--- a/kernel/Kconfig.preempt +++ b/kernel/Kconfig.preempt @@ -75,8 +75,7 @@ config PREEMPT_RT endchoice
config PREEMPT_COUNT - bool + def_bool y
config PREEMPTION bool - select PREEMPT_COUNT