Messages in this thread | | | Date | Mon, 16 Oct 2006 11:15:11 -0700 | From | Andrew Morton <> | Subject | Re: [patch] slab: Fix a cpu hotplug race condition while tuning slab cpu caches |
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On Mon, 16 Oct 2006 01:54:39 -0700 Ravikiran G Thirumalai <kiran@scalex86.org> wrote:
> Fix a cpu hotplug race condition while tuning slab cpu caches. > > CPU online (cpu_up) and tuning slab caches (do_tune_cpucache) > can race and can lead to a situation where we do not have an > arraycache allocated for a newly onlined cpu.
lock_cpu_hotplug() is a noxious thing which should be removed.
> The race can be explained as follows: > > cpu_online_map 00000111 > cpu_up(3) > cpuup_callback CPU_UP_PREPARE: > mutex_lock(&cache_chain_mutex); > ... > allocate_array_cache for cpu 3 > ... > mutex_unlock(&cache_chain_mutex); > ... slabinfo_write > ... mutex_lock(&cache_chain_mutex); > ... do_tune_cpucache > ... allocate new arraycache for cpu0,1,2, NULL rest > ... ... > mutex_lock(&cpu_bitmask_lock); ... > cpu_online_map 00001111 > mutex_unlock(&cpu_bitmask_lock); ... > on_each_cpu swap the new array_cache with old > ^^^^ > CPU 3 gets assigned with a NULL array cache
The problem is obvious: we have some data (the array caches) and we have a data structure which is used to look up that data (cpu_online_map). But we're releasing the lock while these two things are in an inconsistent state.
So you could have fixed this by taking cache_chain_mutex in CPU_UP_PREPARE and releasing it in CPU_ONLINE and CPU_UP_CANCELED.
> Hence, when do_ccupdate_local is run on CPU 3, CPU 3 gets a NULL array_cache, > and caused badness thereon. > > So don't allow cpus to come and go while in do_tune_cpucache. > The other code path of do_tune_cpucache through kmem_cache_create > is already protected through lock_cpu_hotplug. > > Signed-off-by: Alok N Kataria <alokk@calsoftinc.com> > Signed-off-by: Ravikiran Thirumalai <kiran@scalex86.org> > Signed-off-by: Shai Fultheim <shai@scalex86.org> > > Index: linux-2.6.19-rc1slab/mm/slab.c > =================================================================== > --- linux-2.6.19-rc1slab.orig/mm/slab.c 2006-10-13 12:35:02.578841000 -0700 > +++ linux-2.6.19-rc1slab/mm/slab.c 2006-10-13 12:35:46.848841000 -0700 > @@ -4072,6 +4072,7 @@ ssize_t slabinfo_write(struct file *file > return -EINVAL; > > /* Find the cache in the chain of caches. */ > + lock_cpu_hotplug(); > mutex_lock(&cache_chain_mutex); > res = -EINVAL; > list_for_each_entry(cachep, &cache_chain, next) { > @@ -4087,6 +4088,7 @@ ssize_t slabinfo_write(struct file *file > } > } > mutex_unlock(&cache_chain_mutex); > + unlock_cpu_hotplug(); > if (res >= 0) > res = count; > return res;
Given that this lock_cpu_hotplug() happens at a high level I guess it'll avoid the usual lock_cpu_hotplug() horrors and we can live with it. I assume lockdep was enabled when you were testing this?
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