linux-kernel - [PATCH resend] Revert "async: simplify lowest_in

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Message-Id: <20171128104938.3921-1-linux@rasmusvillemoes.dk>
Date:   Tue, 28 Nov 2017 11:49:38 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Tejun Heo <tj@...nel.org>, Arjan van de Ven <arjan@...ux.intel.com>
Cc:     stable@...r.kernel.org, Adam Wallis <awallis@...eaurora.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-kernel@...r.kernel.org
Subject: [PATCH resend] Revert "async: simplify lowest_in_progress()"

This reverts commit 92266d6ef60c2381c980c6cdcb2a5c1667b36b49, which
was simply wrong: In the case where domain is NULL, we now use the
wrong offsetof() in the list_first_entry macro, so we don't actually
fetch the ->cookie value, but rather the eight bytes located
sizeof(struct list_head) further into the struct async_entry.

On 64 bit, that's the data member, while on 32 bit, that's a u64 built
from func and data in some order.

I think the bug happens to be harmless in practice: It obviously only
affects callers which pass a NULL domain, and AFAICT the only such
caller is

  async_synchronize_full() ->
  async_synchronize_full_domain(NULL) ->
  async_synchronize_cookie_domain(ASYNC_COOKIE_MAX, NULL)

and the ASYNC_COOKIE_MAX means that in practice we end up waiting for
the async_global_pending list to be empty - but it would break if
somebody happened to pass (void*)-1 as the data element to
async_schedule, and of course also if somebody ever does a
async_synchronize_cookie_domain(, NULL) with a "finite" cookie value.

Cc: stable@...r.kernel.org # 3.10+
Fixes: 92266d6ef60c "async: simplify lowest_in_progress()"
Acked-by: Tejun Heo <tj@...nel.org>
Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
---
Resending with Cc akpm, added ack from Tejun.

Maybe the "harmless in practice" means this isn't -stable
material. But I'm not completely confident my quick git grep'ing is
enough, and there might be affected code in one of the earlier kernels
that has since been removed, so I'll leave the decision to the stable
guys.

kernel/async.c | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/kernel/async.c b/kernel/async.c
index 2cbd3dd5940d..a893d6170944 100644
--- a/kernel/async.c
+++ b/kernel/async.c
@@ -84,20 +84,24 @@ static atomic_t entry_count;

 static async_cookie_t lowest_in_progress(struct async_domain *domain)
 {
-	struct list_head *pending;
+	struct async_entry *first = NULL;
 	async_cookie_t ret = ASYNC_COOKIE_MAX;
 	unsigned long flags;

 	spin_lock_irqsave(&async_lock, flags);

-	if (domain)
-		pending = &domain->pending;
-	else
-		pending = &async_global_pending;
+	if (domain) {
+		if (!list_empty(&domain->pending))
+			first = list_first_entry(&domain->pending,
+					struct async_entry, domain_list);
+	} else {
+		if (!list_empty(&async_global_pending))
+			first = list_first_entry(&async_global_pending,
+					struct async_entry, global_list);
+	}

-	if (!list_empty(pending))
-		ret = list_first_entry(pending, struct async_entry,
-				       domain_list)->cookie;
+	if (first)
+		ret = first->cookie;

 	spin_unlock_irqrestore(&async_lock, flags);
 	return ret;
-- 
2.11.0