Re: [patch] slab: Fix a cpu hotplug race condition while tuning slab cpu caches

On Mon, 16 Oct 2006 01:54:39 -0700
Ravikiran G Thirumalai <kiran@xxxxxxxxxxxx> 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@xxxxxxxxxxxxxx>
Signed-off-by: Ravikiran Thirumalai <kiran@xxxxxxxxxxxx>
Signed-off-by: Shai Fultheim <shai@xxxxxxxxxxxx>

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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