ebiederm@xxxxxxxxxxxx (Eric W. Biederman) writes: > Alexey Gladkov <gladkov.alexey@xxxxxxxxx> writes: > >> On Mon, Jan 18, 2021 at 12:34:29PM -0800, Linus Torvalds wrote: >>> On Mon, Jan 18, 2021 at 11:46 AM Alexey Gladkov >>> <gladkov.alexey@xxxxxxxxx> wrote: >>> > >>> > Sorry about that. I thought that this code is not needed when switching >>> > from int to refcount_t. I was wrong. >>> >>> Well, you _may_ be right. I personally didn't check how the return >>> value is used. >>> >>> I only reacted to "it certainly _may_ be used, and there is absolutely >>> no comment anywhere about why it wouldn't matter". >> >> I have not found examples where checked the overflow after calling >> refcount_inc/refcount_add. >> >> For example in kernel/fork.c:2298 : >> >> current->signal->nr_threads++; >> atomic_inc(¤t->signal->live); >> refcount_inc(¤t->signal->sigcnt); >> >> $ semind search signal_struct.sigcnt >> def include/linux/sched/signal.h:83 refcount_t sigcnt; >> m-- kernel/fork.c:723 put_signal_struct if (refcount_dec_and_test(&sig->sigcnt)) >> m-- kernel/fork.c:1571 copy_signal refcount_set(&sig->sigcnt, 1); >> m-- kernel/fork.c:2298 copy_process refcount_inc(¤t->signal->sigcnt); >> >> It seems to me that the only way is to use __refcount_inc and then compare >> the old value with REFCOUNT_MAX >> >> Since I have not seen examples of such checks, I thought that this is >> acceptable. Sorry once again. I have not tried to hide these changes. > > The current ucount code does check for overflow and fails the increment > in every case. > > So arguably it will be a regression and inferior error handling behavior > if the code switches to the ``better'' refcount_t data structure. > > I originally didn't use refcount_t because silently saturating and not > bothering to handle the error makes me uncomfortable. > > Not having to acquire the ucounts_lock every time seems nice. Perhaps > the path forward would be to start with stupid/correct code that always > takes the ucounts_lock for every increment of ucounts->count, that is > later replaced with something more optimal. > > Not impacting performance in the non-namespace cases and having good > performance in the other cases is a fundamental requirement of merging > code like this. So starting with something easy to comprehend and simple, may make it easier to figure out how to optimize the code. Eric
