Re: [PATCH 2/6] treewide: remove using list iterator after loop body as a ptr
From: Christian König <christian.koenig@amd.com>
Date: 2022-02-28 12:19:37
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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:
quoted hunk ↗ jump to hunk
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.