linux-kernel - [PATCH] mm: get_cmdline use arg_lock instead of mmap

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Date:   Wed, 17 Apr 2019 14:03:47 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Mateusz Guzik <mguzik@...hat.com>,
        Michal Hocko <mhocko@...e.com>
Cc:     Mike Rapoport <rppt@...ux.ibm.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Arun KS <arunks@...eaurora.org>,
        Bartosz Golaszewski <brgl@...ev.pl>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] mm: get_cmdline use arg_lock instead of mmap_sem

The commit a3b609ef9f8b ("proc read mm's {arg,env}_{start,end} with mmap
semaphore taken.") added synchronization of reading argument/environment
boundaries under mmap_sem. Later commit 88aa7cc688d4 ("mm: introduce
arg_lock to protect arg_start|end and env_start|end in mm_struct")
avoided the coarse use of mmap_sem in similar situations.

get_cmdline can also use arg_lock instead of mmap_sem when it reads the
boundaries.

Signed-off-by: Michal Koutný <mkoutny@...e.com>
---
 mm/util.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/util.c b/mm/util.c
index d559bde497a9..568575cceefc 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -758,12 +758,12 @@ int get_cmdline(struct task_struct *task, char *buffer, int buflen)
 	if (!mm->arg_end)
 		goto out_mm;	/* Shh! No looking before we're done */
 
-	down_read(&mm->mmap_sem);
+	spin_lock(&mm->arg_lock);
 	arg_start = mm->arg_start;
 	arg_end = mm->arg_end;
 	env_start = mm->env_start;
 	env_end = mm->env_end;
-	up_read(&mm->mmap_sem);
+	spin_unlock(&mm->arg_lock);
 
 	len = arg_end - arg_start;
 
-- 
2.16.4