put_pid checks whether the current thread has the only reference to the pid with atomic_read() which does not have any memory barriers, and if so proceeds directly to kmem_cache_free(). As the result memory accesses to the object in kmem_cache_free() or user accesses to the object after reallocation (again without any memory barriers on fast path) can hoist above the atomic_read() check and conflict with memory accesses to the pid object in other threads before they released their references.
There is a control dependency between the atomic_read() check and kmem_cache_free(), but control dependencies are disregarded by some architectures. Documentation/memory-barriers.txt explicitly states: "A load-load control dependency requires a full read memory barrier. ... please note that READ_ONCE_CTRL() is not optional! [even for stores]" in the CONTROL DEPENDENCIES section. For example, if store to the first word of the object to build a freelist in kmem_cache_free() hoists above the check, stores to the first word in other threads can corrupt the memory allocator freelist. Use atomic_read_acquire() for the fast path check to hand off properly acquired object to memory allocator. The data race was found with KernelThreadSanitizer (KTSAN). Signed-off-by: Dmitry Vyukov <[email protected]> --- kernel/pid.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/pid.c b/kernel/pid.c index ca36879..3b0b13d 100644 --- a/kernel/pid.c +++ b/kernel/pid.c @@ -242,7 +242,7 @@ void put_pid(struct pid *pid) return; ns = pid->numbers[pid->level].ns; - if ((atomic_read(&pid->count) == 1) || + if ((atomic_read_acquire(&pid->count) == 1) || atomic_dec_and_test(&pid->count)) { kmem_cache_free(ns->pid_cachep, pid); put_pid_ns(ns); -- 2.6.0.rc0.131.gf624c3d -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
