On Tue, Jan 19, 2021 at 07:57:36PM -0600, Eric W. Biederman wrote: > 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. Did I understand your suggestion correctly that you suggest to use spin_lock for atomic_read and atomic_inc ? If so, then we are already incrementing the counter under ucounts_lock. ... if (atomic_read(&ucounts->count) == INT_MAX) ucounts = NULL; else atomic_inc(&ucounts->count); spin_unlock_irq(&ucounts_lock); return ucounts; something like this ? -- Rgrds, legion
