Messages in this thread | | | Date | Sat, 13 Apr 2013 21:42:06 -0700 | Subject | Re: [PATCH] module: Fix race condition between load and unload module | From | Anatol Pomozov <> |
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Hi
On Sat, Apr 13, 2013 at 8:35 PM, Al Viro <viro@zeniv.linux.org.uk> wrote: > On Fri, Apr 12, 2013 at 04:47:50PM -0700, Linus Torvalds wrote: >> This is a much more generic bug in kobjects, and I would hate to add >> some random workaround for just one case of this bug like you do. The >> more fundamental bug needs to be fixed too. >> >> I think the more fundamental bugfix is to just fix kobject_get() to >> return NULL if the refcount was zero, because in that case the kobject >> no longer really exists. >> >> So instead of having >> >> kref_get(&kobj->kref); >> >> it should do >> >> if (!atomic_inc_not_zero(&kobj->kref.refcount)) >> kobj = NULL; >> >> and I think that should fix your race automatically, no? Proper patch >> attached (but TOTALLY UNTESTED - it seems to compile, though). >> >> The problem is that we lose the warning for when the refcount is zero >> and somebody does a kobject_get(), but that is ok *assuming* that >> people actually check the return value of kobject_get() rather than >> just "know" that if they passed in a non-NULL kobj, they'll get it >> right back. >> >> Greg - please take a look... I'm adding Al to the discussion too, >> because Al just *loooves* these kinds of races ;) > > Unless I'm misreading what's going on, we have the following to thank for that: > /* remove from sysfs if the caller did not do it */ > if (kobj->state_in_sysfs) { > pr_debug("kobject: '%s' (%p): auto cleanup kobject_del\n", > kobject_name(kobj), kobj); > kobject_del(kobj); > } > in kobject_cleanup(). Why don't we require kobject_del() before the final > kobject_put(), if the sucker had been added? FWIW, I thought it *was* > required all along...
But kobject_release/kobject_cleanup function is called as a result of atomic decrement_compare. Until we perform the atomic operation we don't know whether it is final kobject_put() or not.
kobject_put() { if (atomic_sub_and_test(kobj->kref->refcount)) { // refcounter is decremented to 0 so cleanup sysfs kobject_release(kobj) } }
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