Patch "pstore: Revert pmsg_lock back to a normal mutex" has been added to the 6.2-stable tree

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This is a note to let you know that I've just added the patch titled

    pstore: Revert pmsg_lock back to a normal mutex

to the 6.2-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     pstore-revert-pmsg_lock-back-to-a-normal-mutex.patch
and it can be found in the queue-6.2 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 5dd13550f3ec289e60639931449afc8292afadb6
Author: John Stultz <jstultz@xxxxxxxxxx>
Date:   Wed Mar 8 20:40:43 2023 +0000

    pstore: Revert pmsg_lock back to a normal mutex
    
    [ Upstream commit 5239a89b06d6b199f133bf0ffea421683187f257 ]
    
    This reverts commit 76d62f24db07f22ccf9bc18ca793c27d4ebef721.
    
    So while priority inversion on the pmsg_lock is an occasional
    problem that an rt_mutex would help with, in uses where logging
    is writing to pmsg heavily from multiple threads, the pmsg_lock
    can be heavily contended.
    
    After this change landed, it was reported that cases where the
    mutex locking overhead was commonly adding on the order of 10s
    of usecs delay had suddenly jumped to ~msec delay with rtmutex.
    
    It seems the slight differences in the locks under this level
    of contention causes the normal mutexes to utilize the spinning
    optimizations, while the rtmutexes end up in the sleeping
    slowpath (which allows additional threads to pile on trying
    to take the lock).
    
    In this case, it devolves to a worse case senerio where the lock
    acquisition and scheduling overhead dominates, and each thread
    is waiting on the order of ~ms to do ~us of work.
    
    Obviously, having tons of threads all contending on a single
    lock for logging is non-optimal, so the proper fix is probably
    reworking pstore pmsg to have per-cpu buffers so we don't have
    contention.
    
    Additionally, Steven Rostedt has provided some furhter
    optimizations for rtmutexes that improves the rtmutex spinning
    path, but at least in my testing, I still see the test tripping
    into the sleeping path on rtmutexes while utilizing the spinning
    path with mutexes.
    
    But in the short term, lets revert the change to the rt_mutex
    and go back to normal mutexes to avoid a potentially major
    performance regression. And we can work on optimizations to both
    rtmutexes and finer-grained locking for pstore pmsg in the
    future.
    
    Cc: Wei Wang <wvw@xxxxxxxxxx>
    Cc: Midas Chien<midaschieh@xxxxxxxxxx>
    Cc: "Chunhui Li (李春辉)" <chunhui.li@xxxxxxxxxxxx>
    Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
    Cc: Kees Cook <keescook@xxxxxxxxxxxx>
    Cc: Anton Vorontsov <anton@xxxxxxxxxx>
    Cc: "Guilherme G. Piccoli" <gpiccoli@xxxxxxxxxx>
    Cc: Tony Luck <tony.luck@xxxxxxxxx>
    Cc: kernel-team@xxxxxxxxxxx
    Fixes: 76d62f24db07 ("pstore: Switch pmsg_lock to an rt_mutex to avoid priority inversion")
    Reported-by: "Chunhui Li (李春辉)" <chunhui.li@xxxxxxxxxxxx>
    Signed-off-by: John Stultz <jstultz@xxxxxxxxxx>
    Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
    Link: https://lore.kernel.org/r/20230308204043.2061631-1-jstultz@xxxxxxxxxx
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/fs/pstore/pmsg.c b/fs/pstore/pmsg.c
index ab82e5f053464..b31c9c72d90b4 100644
--- a/fs/pstore/pmsg.c
+++ b/fs/pstore/pmsg.c
@@ -7,10 +7,9 @@
 #include <linux/device.h>
 #include <linux/fs.h>
 #include <linux/uaccess.h>
-#include <linux/rtmutex.h>
 #include "internal.h"
 
-static DEFINE_RT_MUTEX(pmsg_lock);
+static DEFINE_MUTEX(pmsg_lock);
 
 static ssize_t write_pmsg(struct file *file, const char __user *buf,
 			  size_t count, loff_t *ppos)
@@ -29,9 +28,9 @@ static ssize_t write_pmsg(struct file *file, const char __user *buf,
 	if (!access_ok(buf, count))
 		return -EFAULT;
 
-	rt_mutex_lock(&pmsg_lock);
+	mutex_lock(&pmsg_lock);
 	ret = psinfo->write_user(&record, buf);
-	rt_mutex_unlock(&pmsg_lock);
+	mutex_unlock(&pmsg_lock);
 	return ret ? ret : count;
 }
 



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