Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
From:   Pasha Tatashin <pasha.tatashin@soleen.com>
To:     pasha.tatashin@soleen.com, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-m68k@lists.linux-m68k.org,
        anshuman.khandual@arm.com, willy@infradead.org,
        akpm@linux-foundation.org, william.kucharski@oracle.com,
        mike.kravetz@oracle.com, vbabka@suse.cz, geert@linux-m68k.org,
        schmitzmic@gmail.com, rostedt@goodmis.org, mingo@redhat.com,
        hannes@cmpxchg.org, guro@fb.com, songmuchun@bytedance.com,
        weixugc@google.com, gthelen@google.com, rientjes@google.com,
        pjt@google.com, hughd@google.com
Subject: [PATCH v3 9/9] mm: use atomic_cmpxchg_acquire in page_ref_freeze().
Date:   Wed, 26 Jan 2022 18:34:29 +0000
Message-Id: <20220126183429.1840447-10-pasha.tatashin@soleen.com>
In-Reply-To: <20220126183429.1840447-1-pasha.tatashin@soleen.com>
References: <20220126183429.1840447-1-pasha.tatashin@soleen.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

page_ref_freeze and page_ref_unfreeze are designed to be used as a pair.
They protect critical sections where struct page can be modified.

page_ref_unfreeze() is protected by _release() atomic operation, but
page_ref_freeze() is not as it is assumed that cmpxch provides the full
barrier.

Instead, use the appropriate atomic_cmpxchg_acquire() to ensure that
memory model is excplicitly followed.

Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
---
 include/linux/page_ref.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
index 9efabeff4e06..45be731d8919 100644
--- a/include/linux/page_ref.h
+++ b/include/linux/page_ref.h
@@ -308,7 +308,8 @@ static inline bool folio_try_get_rcu(struct folio *folio)
 
 static inline int page_ref_freeze(struct page *page, int count)
 {
-	int ret = likely(atomic_cmpxchg(&page->_refcount, count, 0) == count);
+	int old_val = atomic_cmpxchg_acquire(&page->_refcount, count, 0);
+	int ret = likely(old_val == count);
 
 	if (page_ref_tracepoint_active(page_ref_freeze))
 		__page_ref_freeze(page, count, ret);
-- 
2.35.0.rc0.227.g00780c9af4-goog