Messages in this thread Patch in this message | | | Date | Thu, 25 May 2017 08:18:09 -0700 | From | Jessica Yu <> | Subject | Re: [PATCH 5/6] kmod: preempt on kmod_umh_threads_get() |
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+++ 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@kernel.org> >> > > > > --- >> > > > > 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
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