[PATCH 5.15 698/731] pstore: Switch pmsg_lock to an rt_mutex to avoid priority inversion

Greg Kroah-Hartman
2022-12-28T16:00:37Z

From: John Stultz <jstultz@google.com>

[ Upstream commit 76d62f24db07f22ccf9bc18ca793c27d4ebef721 ]

Wei Wang reported seeing priority inversion caused latencies
caused by contention on pmsg_lock, and suggested it be switched
to a rt_mutex.

I was initially hesitant this would help, as the tasks in that
trace all seemed to be SCHED_NORMAL, so the benefit would be
limited to only nice boosting.

However, another similar issue was raised where the priority
inversion was seen did involve a blocked RT task so it is clear
this would be helpful in that case.

Cc: Wei Wang <wvw@google.com>
Cc: Midas Chien<midaschieh@google.com>
Cc: Connor O'Brien <connoro@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Anton Vorontsov <anton@enomsg.org>
Cc: Colin Cross <ccross@android.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: kernel-team@android.com
Fixes: 9d5438f462ab ("pstore: Add pmsg - user-space accessible pstore object")
Reported-by: Wei Wang <wvw@google.com>
Signed-off-by: John Stultz <jstultz@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20221214231834.3711880-1-jstultz@google.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/pstore/pmsg.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/pstore/pmsg.c b/fs/pstore/pmsg.c
index d8542ec2f38c..18cf94b597e0 100644
--- a/fs/pstore/pmsg.c
+++ b/fs/pstore/pmsg.c
@@ -7,9 +7,10 @@
#include <linux/device.h>
#include <linux/fs.h>
#include <linux/uaccess.h>
+#include <linux/rtmutex.h>
#include "internal.h"

-static DEFINE_MUTEX(pmsg_lock);
+static DEFINE_RT_MUTEX(pmsg_lock);

static ssize_t write_pmsg(struct file *file, const char __user *buf,
size_t count, loff_t *ppos)
@@ -28,9 +29,9 @@ static ssize_t write_pmsg(struct file *file, const char __user *buf,
if (!access_ok(buf, count))
return -EFAULT;

- mutex_lock(&pmsg_lock);
+ rt_mutex_lock(&pmsg_lock);
ret = psinfo->write_user(&record, buf);
- mutex_unlock(&pmsg_lock);
+ rt_mutex_unlock(&pmsg_lock);
return ret ? ret : count;
}

--
2.35.1