a suspected data race in /driver/mtd/ubi/build.c
Artem Bityutskiy
dedekind at infradead.org
Sat May 23 04:58:45 EDT 2009
On Tue, 2009-05-19 at 20:34 +0900, 홍신 shin hong wrote:
> Hi. I am reporting a susptected data race bug at
> ubi_attach_mtd_dev() in /driver/mtd/ubi/build.c .
>
> This function creates a kernel thread by calling
> kthread_create(ubi_thread, ubi .. ). and then
> it assigns ubi->thread_enabled = 1.
>
> However, ubi_thread() also reads ubi->thread_enabled.
> This may cause data race since the execution results
> would be differ depending on the scheduling.
>
> I think, ubi_attach_mtd_dev() should be modified to protect
> the writing aceess to ubi->thread_enabled
> by spin_lock(&ubi->wl_lock).
>
> But I do not have much background on the code.
> So please check this code and let me know your opinions.
Agreed, thanks for the report. I'm going to push the
following patch (sorry I wrote your name without the
hieroglyph characters because my vim refused them).
From: Artem Bityutskiy <Artem.Bityutskiy at nokia.com>
Subject: [PATCH] UBI: fix race condition
This patch fixes a minor problem where we may fail to wake
upe the UBI background thread. This is not fatal at all,
it may just result at sligtly worse performace for a short
period of time, just because the thread will be woken up
when real I/O on the UBI starts.
Anywey, the issue is the race condition between
'ubi_attach_mtd_dev()' and 'ubi_thread()'. If we do not
serialize them, the 'wake_up_process()' call may be done
before 'ubi_thread()' went seep, but after it checked
'ubi->thread_enabled'.
This issue was spotted by Shin Hong <hongshin at gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy at nokia.com>
---
drivers/mtd/ubi/build.c | 8 +++++++-
1 files changed, 7 insertions(+), 1 deletions(-)
diff --git a/drivers/mtd/ubi/build.c b/drivers/mtd/ubi/build.c
index 4048db8..c2c7827 100644
--- a/drivers/mtd/ubi/build.c
+++ b/drivers/mtd/ubi/build.c
@@ -380,7 +380,7 @@ static void free_user_volumes(struct ubi_device *ubi)
* @ubi: UBI device description object
*
* This function returns zero in case of success and a negative error code in
- * case of failure. Note, this function destroys all volumes if it failes.
+ * case of failure. Note, this function destroys all volumes if it fails.
*/
static int uif_init(struct ubi_device *ubi)
{
@@ -872,9 +872,15 @@ int ubi_attach_mtd_dev(struct mtd_info *mtd, int ubi_num, int vid_hdr_offset)
ubi->beb_rsvd_pebs);
ubi_msg("max/mean erase counter: %d/%d", ubi->max_ec, ubi->mean_ec);
+ /*
+ * The below lock makes sure we do not race with 'ubi_thread()' which
+ * checks @ubi->thread_enabled. Otherwise we may fail to wake it up.
+ */
+ spin_lock(&ubi->wl_lock);
if (!DBG_DISABLE_BGT)
ubi->thread_enabled = 1;
wake_up_process(ubi->bgt_thread);
+ spin_unlock(&ubi->wl_lock);
ubi_devices[ubi_num] = ubi;
return ubi_num;
--
1.6.0.6
--
Best regards,
Artem Bityutskiy (Битюцкий Артём)
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