+++ Dmitry Torokhov [24/05/17 19:27 -0700]:
On Thu, May 25, 2017 at 03:00:17AM +0200, Luis R. Rodriguez wrote:On Wed, May 24, 2017 at 05:45:37PM -0700, Dmitry Torokhov wrote: > On Thu, May 25, 2017 at 02:14:52AM +0200, Luis R. Rodriguez wrote: > > On Fri, May 19, 2017 at 03:27:12PM -0700, Dmitry Torokhov wrote: > > > On Thu, May 18, 2017 at 08:24:43PM -0700, Luis R. Rodriguez wrote: > > > > In theory it is possible multiple concurrent threads will try to > > > > kmod_umh_threads_get() and as such atomic_inc(&kmod_concurrent) at > > > > the same time, therefore enabling a small time during which we've > > > > bumped kmod_concurrent but have not really enabled work. By using > > > > preemption we mitigate this a bit. > > > > > > > > Preemption is not needed when we kmod_umh_threads_put(). > > > > > > > > Signed-off-by: Luis R. Rodriguez <mcgrof@xxxxxxxxxx> > > > > --- > > > > kernel/kmod.c | 24 ++++++++++++++++++++++-- > > > > 1 file changed, 22 insertions(+), 2 deletions(-) > > > > > > > > diff --git a/kernel/kmod.c b/kernel/kmod.c > > > > index 563600fc9bb1..7ea11dbc7564 100644 > > > > --- a/kernel/kmod.c > > > > +++ b/kernel/kmod.c > > > > @@ -113,15 +113,35 @@ static int call_modprobe(char *module_name, int wait) > > > > > > > > static int kmod_umh_threads_get(void) > > > > { > > > > + int ret = 0; > > > > + > > > > + /* > > > > + * Disabling preemption makes sure that we are not rescheduled here > > > > + * > > > > + * Also preemption helps kmod_concurrent is not increased by mistake > > > > + * for too long given in theory two concurrent threads could race on > > > > + * atomic_inc() before we atomic_read() -- we know that's possible > > > > + * and but we don't care, this is not used for object accounting and > > > > + * is just a subjective threshold. The alternative is a lock. > > > > + */ > > > > + preempt_disable(); > > > > atomic_inc(&kmod_concurrent); > > > > if (atomic_read(&kmod_concurrent) <= max_modprobes) > > > > > > That is very "fancy" way to basically say: > > > > > > if (atomic_inc_return(&kmod_concurrent) <= max_modprobes) > > > > Do you mean to combine the atomic_inc() and atomic_read() in one as you noted > > (as that is not a change in this patch), *or* that using a memory barrier here > > with atomic_inc_return() should suffice to address the same and avoid an > > explicit preemption enable / disable ? > > I am saying that atomic_inc_return() will avoid situation where you have > more than one threads incrementing the counter and believing that they > are [not] allowed to start modprobe. > > I have no idea why you think preempt_disable() would help here. It only > ensures that current thread will not be preempted between the point > where you update the counter and where you check the result. It does not > stop interrupts nor does it affect other threads that might be updating > the same counter. The preemption was inspired by __module_get() and try_module_get(), was that rather silly ?As far as I can see prrempt_disable() was needed in __module_get() when modules user per-cpu refcounts: you did not want to move away from CPU while manipulating refcount. Now that modules use simple atomics for refcounting I think these preempt_disable() and preempt_enable() can be removed.
Yup, preempt_disable/enable was originally used for percpu module refcounting. AFAIK they are artifacts that remained from commit e1783a240f4 "use this_cpu_xx to dynamically allocate counters" and subsequently commit 2f35c41f58a "Replace module_ref with atomic_t refcnt" removed the need for it. Jessica -- To unsubscribe from this list: send the line "unsubscribe linux-kselftest" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
