linux-kernel - [PATCH v3] mmap_lock: Change trace and locking order

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Message-ID: <20210907201456.4036910-1-Liam.Howlett@oracle.com>
Date:   Tue, 7 Sep 2021 20:15:19 +0000
From:   Liam Howlett <liam.howlett@...cle.com>
To:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
CC:     Steven Rostedt <rostedt@...dmis.org>,
        Michel Lespinasse <walken.cr@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>
Subject: [PATCH v3] mmap_lock: Change trace and locking order

The ordering of the printed messages from the mmap_lock trace can occur
out of order.  This results in confusing trace logs such as:

task          cpu     atomic counter: message
---------------------------------------------
task-749     [006] ....     14437980: mmap_lock_acquire_returned: mm=00000000c94d28b8 memcg_path= write=true success=true
task-750     [007] ....     14437981: mmap_lock_acquire_returned: mm=00000000c94d28b8 memcg_path= write=true success=true
task-749     [006] ....     14437983: mmap_lock_released: mm=00000000c94d28b8 memcg_path= write=true

When the actual series of evens are as follows:

task-749     [006]  mmap_lock_acquire_returned: mm=00000000c94d28b8 memcg_path= write=true success=true
task-749     [006]  mmap_lock_released: mm=00000000c94d28b8 memcg_path= write=true

task-750     [007]  mmap_lock_acquire_returned: mm=00000000c94d28b8 memcg_path= write=true success=true

The incorrect ordering of the trace log happens because the release log
is outside of the lock itself.  The ordering can be guaranteed by
protecting the acquire success and release trace logs by the lock.

Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
Suggested-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
---
 include/linux/mmap_lock.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
index 0540f0156f58..b179f1e3541a 100644
--- a/include/linux/mmap_lock.h
+++ b/include/linux/mmap_lock.h
@@ -101,14 +101,14 @@ static inline bool mmap_write_trylock(struct mm_struct *mm)
 
 static inline void mmap_write_unlock(struct mm_struct *mm)
 {
-	up_write(&mm->mmap_lock);
 	__mmap_lock_trace_released(mm, true);
+	up_write(&mm->mmap_lock);
 }
 
 static inline void mmap_write_downgrade(struct mm_struct *mm)
 {
-	downgrade_write(&mm->mmap_lock);
 	__mmap_lock_trace_acquire_returned(mm, false, true);
+	downgrade_write(&mm->mmap_lock);
 }
 
 static inline void mmap_read_lock(struct mm_struct *mm)
@@ -140,8 +140,8 @@ static inline bool mmap_read_trylock(struct mm_struct *mm)
 
 static inline void mmap_read_unlock(struct mm_struct *mm)
 {
-	up_read(&mm->mmap_lock);
 	__mmap_lock_trace_released(mm, false);
+	up_read(&mm->mmap_lock);
 }
 
 static inline bool mmap_read_trylock_non_owner(struct mm_struct *mm)
@@ -155,8 +155,8 @@ static inline bool mmap_read_trylock_non_owner(struct mm_struct *mm)
 
 static inline void mmap_read_unlock_non_owner(struct mm_struct *mm)
 {
-	up_read_non_owner(&mm->mmap_lock);
 	__mmap_lock_trace_released(mm, false);
+	up_read_non_owner(&mm->mmap_lock);
 }
 
 static inline void mmap_assert_locked(struct mm_struct *mm)
-- 
2.30.2