Messages in this thread | | | Date | Fri, 18 Sep 2015 10:51:56 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] kernel: fix data race in put_pid |
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On Thu, Sep 17, 2015 at 08:09:19PM +0200, Oleg Nesterov wrote: > On 09/17, Dmitry Vyukov wrote: > > > > I can update the patch description, but let me explain it here first. > > Yes thanks. > > > Here is the essence of what happens: > > Aha, so you really meant that 2 put_pid's can race with each other, > > > // thread 1 > > 1: pid->foo = 1; // foo is the first word of pid object > > // then it does put_pid > > 2: atomic_dec_and_test(&pid->count) // decrements count to 1 and > > returns false so the function returns > > > > // thread 2 > > // executes put_pid > > 3: atomic_load(&pid->count); // returns 1, so proceed to kmem_cache_free > > // then kmem_cache_free does: > > 4: *(void**)pid = head->freelist; > > 5: head->freelist = (void*)pid; > > > > This can be executed as: > > > > 4: *(void**)pid = head->freelist; > > 1: pid->foo = 1; // foo is the first word of pid object > > 2: atomic_dec_and_test(&pid->count) // decrements count to 1 and > > returns false so the function returns > > 3: atomic_load(&pid->count); // returns 1, so proceed to kmem_cache_free > > 5: head->freelist = (void*)pid; > > Unless I am totally confused, everything is simpler. We can forget > about the hoisting, freelist, etc. > > Thread 2 can see the result of atomic_dec_and_test(), but not the > result of "pid->foo = 1". In this case in can free the object which > can be re-allocated _before_ STORE(pid->foo) completes. Of course, > this would be really bad. > > I need to recheck, but afaics this is not possible. This optimization > is fine, but probably needs a comment.
For sure, this code doesn't make any sense to me.
> We rely on delayed_put_pid() > called by RCU. And note that nobody can write to this pid after it > is removed from the rcu-protected list. > > So I think this is false alarm, but I'll try to recheck tomorrow, it > is too late for me today.
As an alternative patch, could we not do:
void put_pid(struct pid *pid) { struct pid_namespace *ns;
if (!pid) return;
ns = pid->numbers[pid->level].ns; if ((atomic_read(&pid->count) == 1) || atomic_dec_and_test(&pid->count)) {
+ smp_read_barrier_depends(); /* ctrl-dep */
kmem_cache_free(ns->pid_cachep, pid); put_pid_ns(ns); } }
That would upgrade the atomic_read() path to a full READ_ONCE_CTRL(), and thereby avoid any of the kmem_cache_free() stores from leaking out. And its free, except on Alpha. Whereas the atomic_read_acquire() will generate a full memory barrier on whole bunch of archs.
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