linux-kernel - [PATCH] mm: do not protect VMA lock object in vma_end

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Message-Id: <20231207094844.706407-1-agordeev@linux.ibm.com>
Date:   Thu,  7 Dec 2023 10:48:44 +0100
From:   Alexander Gordeev <agordeev@...ux.ibm.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Jann Horn <jannh@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: [PATCH] mm: do not protect VMA lock object in vma_end_read()

In early discussion to the implementation of vma_end_read()
Jann Horn pointed out that up_read() could access the VMA
lock object after it has already been acquired by someone
else. As result, up_read() is protected with RCU read lock:

	rcu_read_lock(); /* keeps vma alive */
	up_read(&vma->lock);
	rcu_read_unlock();

Since commit 3f5245538a19 ("locking/rwsem: Disable preemption
in all down_read*() and up_read() code paths") __up_read()
disables preemption internally and thus the need to protect
the VMA lock object does not exist anymore.

Link: https://lore.kernel.org/all/CAG48ez3sCwasFzKD5CsqMFA2W57-2fazd75g7r0NaA_BVNTLow@mail.gmail.com/
Cc: Jann Horn <jannh@...gle.com>
Cc: Suren Baghdasaryan <surenb@...gle.com>
Signed-off-by: Alexander Gordeev <agordeev@...ux.ibm.com>
---
 include/linux/mm.h | 2 --
 1 file changed, 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 418d26608ece..7b32bc75a4ab 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -683,9 +683,7 @@ static inline bool vma_start_read(struct vm_area_struct *vma)
 
 static inline void vma_end_read(struct vm_area_struct *vma)
 {
-	rcu_read_lock(); /* keeps vma alive till the end of up_read */
 	up_read(&vma->vm_lock->lock);
-	rcu_read_unlock();
 }
 
 /* WARNING! Can only be used if mmap_lock is expected to be write-locked */
-- 
2.40.1