kernel/pid.c: convert struct pid count to refcount_t

struct pid's count is an atomic_t field used as a refcount.  Use
refcount_t for it which is basically atomic_t but does additional
checking to prevent use-after-free bugs.

For memory ordering, the only change is with the following:

 -	if ((atomic_read(&pid->count) == 1) ||
 -	     atomic_dec_and_test(&pid->count)) {
 +	if (refcount_dec_and_test(&pid->count)) {
 		kmem_cache_free(ns->pid_cachep, pid);

Here the change is from: Fully ordered --> RELEASE + ACQUIRE (as per
refcount-vs-atomic.rst) This ACQUIRE should take care of making sure the
free happens after the refcount_dec_and_test().

The above hunk also removes atomic_read() since it is not needed for the
code to work and it is unclear how beneficial it is.  The removal lets
refcount_dec_and_test() check for cases where get_pid() happened before
the object was freed.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Joel Fernandes (Google) <[email protected]>
Reviewed-by: Andrea Parri <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Elena Reshetova <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Eric W. Biederman <[email protected]>
Cc: KJ Tsanaktsidis <[email protected]>
Cc: Michal Hocko <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

0 files changed, 0 insertions, 0 deletions