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Message-ID: <20150821183050.GA19587@redhat.com>
Date: Fri, 21 Aug 2015 20:30:50 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Dave Chinner <david@...morbit.com>
Cc: Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: __sb_start_write() && force_trylock hack
On 08/20, Dave Chinner wrote:
>
> On Wed, Aug 19, 2015 at 05:00:26PM +0200, Oleg Nesterov wrote:
> >
> > Yes, we hold SB_FREEZE_WRITE lock, so recursive SB_FREEZE_FS is safe.
> >
> > But, this means that the comment in __sb_start_write() is still correct,
> > "XFS for example gets freeze protection on internal level twice" is true,
> > and we can not remove this force_trylock hack.
>
> You've hit a very rare corner case of a rare corner case.
Yes, I see, thanks.
Just fyi, I ran the tests again with the stupid debugging patch below
and I do not see anything new in dmesg.
So perhaps xfs_create() which does the "recursive" xfs_trans_alloc()
is the only source of double-lock in fs/xfs/.
Oleg.
---
diff --git a/fs/super.c b/fs/super.c
index a38ad91..32b1846 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -1153,13 +1153,57 @@ out:
return ERR_PTR(error);
}
+void __sb_writers_acquired(struct super_block *sb, int level)
+{
+ struct percpu_rw_semaphore *sem = sb->s_writers.rw_sem + level-1;
+ struct ltrace *lt = current->ltrace + level-1;
+
+ WARN_ON(percpu_rwsem_is_held(sem));
+
+ percpu_rwsem_acquire(sem, 1, _RET_IP_);
+
+ if (!lt->lock) {
+ lt->lock = sem;
+ save_stack_trace(<->trace);
+ }
+}
+EXPORT_SYMBOL(__sb_writers_acquired);
+
+void __sb_writers_release(struct super_block *sb, int level)
+{
+ struct percpu_rw_semaphore *sem = sb->s_writers.rw_sem + level-1;
+ struct ltrace *lt = current->ltrace + level-1;
+
+ WARN_ON(!lt->lock);
+
+ percpu_rwsem_release(sem, 1, _RET_IP_);
+
+ WARN_ON(percpu_rwsem_is_held(sem));
+ if (lt->lock == sem) {
+ lt->lock = NULL;
+ lt->trace.nr_entries = 0;
+ }
+}
+EXPORT_SYMBOL(__sb_writers_release);
+
+
/*
* This is an internal function, please use sb_end_{write,pagefault,intwrite}
* instead.
*/
void __sb_end_write(struct super_block *sb, int level)
{
+ struct percpu_rw_semaphore *sem = sb->s_writers.rw_sem + level-1;
+ struct ltrace *lt = current->ltrace + level-1;
+
+ WARN_ON(!lt->lock);
+
percpu_up_read(sb->s_writers.rw_sem + level-1);
+
+ if (lt->lock == sem && !percpu_rwsem_is_held(sem)) {
+ lt->lock = NULL;
+ lt->trace.nr_entries = 0;
+ }
}
EXPORT_SYMBOL(__sb_end_write);
@@ -1169,10 +1213,22 @@ EXPORT_SYMBOL(__sb_end_write);
*/
int __sb_start_write(struct super_block *sb, int level, bool wait)
{
+ struct percpu_rw_semaphore *sem = sb->s_writers.rw_sem + level-1;
+ struct ltrace *lt = current->ltrace + level-1;
bool force_trylock = false;
int ret = 1;
+ WARN_ON(lt->lock == sem && !percpu_rwsem_is_held(sem));
+
#ifdef CONFIG_LOCKDEP
+ if (wait && lt->lock == sem) {
+ pr_crit("XXXXX %s:%d lev=%d\n", current->comm, current->pid, level);
+ dump_stack();
+ debug_show_held_locks(current);
+ pr_crit("Prev Trace:\n");
+ print_stack_trace(<->trace, 0);
+ }
+
/*
* We want lockdep to tell us about possible deadlocks with freezing
* but it's it bit tricky to properly instrument it. Getting a freeze
@@ -1198,6 +1254,10 @@ int __sb_start_write(struct super_block *sb, int level, bool wait)
ret = percpu_down_read_trylock(sb->s_writers.rw_sem + level-1);
WARN_ON(force_trylock & !ret);
+ if (ret && !lt->lock) {
+ lt->lock = sem;
+ save_stack_trace(<->trace);
+ }
return ret;
}
EXPORT_SYMBOL(__sb_start_write);
diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
index 08d4fe4..33bf46a 100644
--- a/fs/xfs/xfs_log.c
+++ b/fs/xfs/xfs_log.c
@@ -614,7 +614,7 @@ xfs_log_mount(
int min_logfsbs;
if (!(mp->m_flags & XFS_MOUNT_NORECOVERY)) {
- xfs_notice(mp, "Mounting V%d Filesystem",
+ if (0) xfs_notice(mp, "Mounting V%d Filesystem",
XFS_SB_VERSION_NUM(&mp->m_sb));
} else {
xfs_notice(mp,
diff --git a/fs/xfs/xfs_log_recover.c b/fs/xfs/xfs_log_recover.c
index 480ebba..ed241dd 100644
--- a/fs/xfs/xfs_log_recover.c
+++ b/fs/xfs/xfs_log_recover.c
@@ -4607,7 +4607,7 @@ xlog_recover_finish(
: "internal");
log->l_flags &= ~XLOG_RECOVERY_NEEDED;
} else {
- xfs_info(log->l_mp, "Ending clean mount");
+ if (0) xfs_info(log->l_mp, "Ending clean mount");
}
return 0;
}
diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index 1fb16562..f680f3c 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -1573,7 +1573,7 @@ xfs_fs_put_super(
{
struct xfs_mount *mp = XFS_M(sb);
- xfs_notice(mp, "Unmounting Filesystem");
+ if (0) xfs_notice(mp, "Unmounting Filesystem");
xfs_filestream_unmount(mp);
xfs_unmountfs(mp);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index ce356f6..8514e65 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1385,10 +1385,8 @@ extern struct timespec current_fs_time(struct super_block *sb);
void
--
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