On Wed, 3 Mar 2021, Matthew Wilcox wrote: > > Can this be allocated in an interrupt context? > > > > And I am not sure how local_t relates to that? Percpu counters can be used > > in an interrupt context without the overhead of the address calculations > > that are required by a local_t. > > As I understand the patch, this counts the number of partially free slabs. > So if we start to free an object from a completely full slab in process > context, as "load x, add one to x, store x" and take an interrupt > between loading x and adding one to x, that interrupt handler might > free a different object from another completely full slab. that would > also load the same x, add one to it and store x, but then the process > context would add one to the old x, overwriting the updated value from > interrupt context. this_cpu operations are "atomic" vs. preemption but on some platforms not vs interrupts. That could be an issue in kmem_cache_free(). This would need a modification to the relevant this_cpu ops so that interrupts are disabled on those platforms. Like this_cpu_inc_irq() or so? > it's not the likeliest of races, and i don't know how important it is > that these counters remain accurate. but using a local_t instead of > a percpu long would fix the problem. i don't know why you think that > a local_t needs "address calculations". perhaps you've misremembered > what a local_t is? local_t does not include the address arithmetic that the this_cpu operation can implicitly perform on x86 f.e. with an segment register or maybe another register on another platform thereby avoiding the need to disable preemption or interrupts. Therefore a manual calculation of the target address for a local_t operation needs to be done beforehand which usually means disabling interrupts and/or preemption for the code segment. Otherwise we may end up on a different processor due to scheduler or other interruptions and use the percpu counter value of a different processor which could be racy.
