Re: [PATCH 2/6] treewide: remove using list iterator after loop body as a ptr

Am 28.02.22 um 12:08 schrieb Jakob Koschel:

If the list does not contain the expected element, the value of list_for_each_entry() iterator will not point to a valid structure. To avoid type confusion in such case, the list iterator scope will be limited to list_for_each_entry() loop.

We explicitly have the list_entry_is_head() macro to test after a loop if the element pointer points to the head of the list instead of a valid list entry.

So at least from my side I absolutely don't think that this is a good idea.

In preparation to limiting scope of a list iterator to the list traversal loop, use a dedicated pointer to point to the found element. Determining if an element was found is then simply checking if the pointer is != NULL.

Since when do we actually want to do this?

Take this code here as an example:

diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c index 48afe96ae0f0..6c916416decc 100644 --- a/arch/x86/kernel/cpu/sgx/encl.c +++ b/arch/x86/kernel/cpu/sgx/encl.c @@ -450,7 +450,8 @@ static void sgx_mmu_notifier_release(struct mmu_notifier *mn, struct mm_struct *mm) { struct sgx_encl_mm *encl_mm = container_of(mn, struct sgx_encl_mm, mmu_notifier);

• struct sgx_encl_mm *tmp = NULL;

• struct sgx_encl_mm *found_encl_mm = NULL;

• struct sgx_encl_mm *tmp;

  /*

  • The enclave itself can remove encl_mm. Note, objects can't be moved

@@ -460,12 +461,13 @@ static void sgx_mmu_notifier_release(struct mmu_notifier *mn, list_for_each_entry(tmp, &encl_mm->encl->mm_list, list) { if (tmp == encl_mm) { list_del_rcu(&encl_mm->list);

• 	found_encl_mm = tmp;
  break;
  

  } } spin_unlock(&encl_mm->encl->mm_lock);

• if (tmp == encl_mm) {

• if (found_encl_mm) { synchronize_srcu(&encl_mm->encl->srcu); mmu_notifier_put(mn); }

I don't think that using the extra variable makes the code in any way more reliable or easier to read.

Regards, Christian.