Messages in this thread | | | Subject | Re: [PATH -mm -v2] Fix a race condtion of oops_in_progress | From | Huang Ying <> | Date | Tue, 04 Nov 2008 09:41:17 +0800 |
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On Tue, 2008-11-04 at 02:44 +0800, Chris Snook wrote: > Huang Ying wrote: > > On Sat, 2008-11-01 at 00:42 +0800, Chris Snook wrote: > >> Huang Ying wrote: > >>> Hi, Chris, > >>> > >>> On Wed, 2008-10-29 at 08:51 -0600, Chris Snook wrote: > >>>> Huang Ying wrote: > >>>>> Fix a race condition accessing oops_in_progress. Which may be changed on > >>>>> multiple CPU simultaneously, but it is changed via non-atomic operation > >>>>> ++/--. This patch changes the definition of oops_in_process from int to > >>>>> atomic_t, and accessing method to atomic operations. > >>>> You also need barriers. I believe rmb() before atomic_read() and wmb() after > >>>> atomic_set() should suffice. > >>> I don't think that is necessary. I haven't found there is particular > >>> consistent requirement about oops_in_progress. > >> atomic_read() and atomic_set() don't inherently cause changes to be visible on > >> other CPUs any faster than ++ and -- operators. Sometimes it happens to work > >> out that way as a result of how the compiler and the CPU order operations, but > >> there's no semantic guarantee, and it could even take arbitrarily long under > >> some circumstances. If you want to use atomic ops to close the race, you need > >> to use barriers. > > > > As far as I know, barriers don't cause changes to be visible on other > > CPUs faster too. It just guarantees corresponding operations after will > > not get executed until that before have finished. And, I don't think we > > need make changes to be visible on other CPUs faster. > > You're correct that barrier() has no impact on other CPUs. wmb() and rmb() do. > If we don't need to make changes visible any faster, what's the point in using > atomic_set()? It's not any less racy. atomic_inc() and atomic_dec() would be > less racy, but you're not using those.
In default bust_spinlocks() implementation in lib/bust_spinlocks.c, atomic_inc() and atomic_dec_and_test() is used. Which is used by x86 too. In some other architecture, atomic_set() is used to replace "oops_in_progress = <xxx>". So this patch fixes architectures which use default bust_spinlocks(), other architectures can be fixed by corresponding architecture developers.
Best Regards, Huang Ying
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