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Date: Fri, 19 May 2017 15:27:12 -0700 From: Dmitry Torokhov <dmitry.torokhov@...il.com> To: "Luis R. Rodriguez" <mcgrof@...nel.org> Cc: shuah@...nel.org, jeyu@...hat.com, rusty@...tcorp.com.au, ebiederm@...ssion.com, acme@...hat.com, corbet@....net, martin.wilck@...e.com, mmarek@...e.com, pmladek@...e.com, hare@...e.com, rwright@....com, jeffm@...e.com, DSterba@...e.com, fdmanana@...e.com, neilb@...e.com, linux@...ck-us.net, rgoldwyn@...e.com, subashab@...eaurora.org, xypron.glpk@....de, keescook@...omium.org, atomlin@...hat.com, mbenes@...e.cz, paulmck@...ux.vnet.ibm.com, dan.j.williams@...el.com, jpoimboe@...hat.com, davem@...emloft.net, mingo@...hat.com, akpm@...ux-foundation.org, torvalds@...ux-foundation.org, gregkh@...uxfoundation.org, linux-kselftest@...r.kernel.org, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH 5/6] kmod: preempt on kmod_umh_threads_get() 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@...nel.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) ... > - return 0; > + goto out; > + > atomic_dec(&kmod_concurrent); > - return -EBUSY; > + ret = -EBUSY; > +out: > + preempt_enable(); > + return ret; > } > > static void kmod_umh_threads_put(void) > { > + /* > + * Preemption is not needed given once work is done we can > + * pace ourselves on our way out. > + */ > atomic_dec(&kmod_concurrent); > } > > -- > 2.11.0 > Thanks. -- Dmitry
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