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Date: Wed, 17 Jun 2020 14:09:34 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Peter Zijlstra <peterz@...radead.org>
Cc: John Ogness <john.ogness@...utronix.de>,
Alexander Viro <viro@...iv.linux.org.uk>,
Thomas Gleixner <tglx@...utronix.de>,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/1] fs/namespace.c: use spinlock instead of busy loop
On 2020-06-17 13:27:19 [+0200], Peter Zijlstra wrote:
> Why can't you use a regular percpu-rwsem for this?
This is the percpu-rwsem version. The lock is per struct super_block not
per mount point. This is due to the restriction in
sb_prepare_remount_readonly().
diff --git a/fs/mount.h b/fs/mount.h
index 711a4093e475..344fae9931b5 100644
--- a/fs/mount.h
+++ b/fs/mount.h
@@ -4,6 +4,7 @@
#include <linux/poll.h>
#include <linux/ns_common.h>
#include <linux/fs_pin.h>
+#include <linux/percpu-rwsem.h>
struct mnt_namespace {
atomic_t count;
@@ -40,6 +41,7 @@ struct mount {
struct rcu_head mnt_rcu;
struct llist_node mnt_llist;
};
+ struct percpu_rw_semaphore mnt_writers_rws;
#ifdef CONFIG_SMP
struct mnt_pcp __percpu *mnt_pcp;
#else
diff --git a/fs/namespace.c b/fs/namespace.c
index a28e4db075ed..cc05c922d7d6 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -311,30 +311,17 @@ static int mnt_is_readonly(struct vfsmount *mnt)
int __mnt_want_write(struct vfsmount *m)
{
struct mount *mnt = real_mount(m);
+ struct super_block *sb = m->mnt_sb;
int ret = 0;
- preempt_disable();
- mnt_inc_writers(mnt);
- /*
- * The store to mnt_inc_writers must be visible before we pass
- * MNT_WRITE_HOLD loop below, so that the slowpath can see our
- * incremented count after it has set MNT_WRITE_HOLD.
- */
- smp_mb();
- while (READ_ONCE(mnt->mnt.mnt_flags) & MNT_WRITE_HOLD)
- cpu_relax();
- /*
- * After the slowpath clears MNT_WRITE_HOLD, mnt_is_readonly will
- * be set to match its requirements. So we must not load that until
- * MNT_WRITE_HOLD is cleared.
- */
- smp_rmb();
- if (mnt_is_readonly(m)) {
- mnt_dec_writers(mnt);
+ percpu_down_read(&sb->mnt_writers_rws);
+
+ if (mnt_is_readonly(m))
ret = -EROFS;
- }
- preempt_enable();
+ else
+ mnt_inc_writers(mnt);
+ percpu_up_read(&sb->mnt_writers_rws);
return ret;
}
@@ -459,53 +446,30 @@ void mnt_drop_write_file(struct file *file)
}
EXPORT_SYMBOL(mnt_drop_write_file);
-static int mnt_make_readonly(struct mount *mnt)
+static int mnt_make_readonly(struct super_block *sb, struct mount *mnt)
{
int ret = 0;
+ percpu_down_write(&sb->mnt_writers_rws);
lock_mount_hash();
- mnt->mnt.mnt_flags |= MNT_WRITE_HOLD;
- /*
- * After storing MNT_WRITE_HOLD, we'll read the counters. This store
- * should be visible before we do.
- */
- smp_mb();
- /*
- * With writers on hold, if this value is zero, then there are
- * definitely no active writers (although held writers may subsequently
- * increment the count, they'll have to wait, and decrement it after
- * seeing MNT_READONLY).
- *
- * It is OK to have counter incremented on one CPU and decremented on
- * another: the sum will add up correctly. The danger would be when we
- * sum up each counter, if we read a counter before it is incremented,
- * but then read another CPU's count which it has been subsequently
- * decremented from -- we would see more decrements than we should.
- * MNT_WRITE_HOLD protects against this scenario, because
- * mnt_want_write first increments count, then smp_mb, then spins on
- * MNT_WRITE_HOLD, so it can't be decremented by another CPU while
- * we're counting up here.
- */
if (mnt_get_writers(mnt) > 0)
ret = -EBUSY;
else
mnt->mnt.mnt_flags |= MNT_READONLY;
- /*
- * MNT_READONLY must become visible before ~MNT_WRITE_HOLD, so writers
- * that become unheld will see MNT_READONLY.
- */
- smp_wmb();
- mnt->mnt.mnt_flags &= ~MNT_WRITE_HOLD;
+
unlock_mount_hash();
+ percpu_up_write(&sb->mnt_writers_rws);
return ret;
}
-static int __mnt_unmake_readonly(struct mount *mnt)
+static int __mnt_unmake_readonly(struct super_block *sb, struct mount *mnt)
{
+ percpu_down_write(&sb->mnt_writers_rws);
lock_mount_hash();
mnt->mnt.mnt_flags &= ~MNT_READONLY;
unlock_mount_hash();
+ percpu_up_write(&sb->mnt_writers_rws);
return 0;
}
@@ -514,21 +478,22 @@ int sb_prepare_remount_readonly(struct super_block *sb)
struct mount *mnt;
int err = 0;
- /* Racy optimization. Recheck the counter under MNT_WRITE_HOLD */
+ /* Racy optimization. Recheck the counter under mnt_writers_rws. */
if (atomic_long_read(&sb->s_remove_count))
return -EBUSY;
+ percpu_down_write(&sb->mnt_writers_rws);
lock_mount_hash();
+
list_for_each_entry(mnt, &sb->s_mounts, mnt_instance) {
if (!(mnt->mnt.mnt_flags & MNT_READONLY)) {
- mnt->mnt.mnt_flags |= MNT_WRITE_HOLD;
- smp_mb();
if (mnt_get_writers(mnt) > 0) {
err = -EBUSY;
break;
}
}
}
+
if (!err && atomic_long_read(&sb->s_remove_count))
err = -EBUSY;
@@ -536,11 +501,9 @@ int sb_prepare_remount_readonly(struct super_block *sb)
sb->s_readonly_remount = 1;
smp_wmb();
}
- list_for_each_entry(mnt, &sb->s_mounts, mnt_instance) {
- if (mnt->mnt.mnt_flags & MNT_WRITE_HOLD)
- mnt->mnt.mnt_flags &= ~MNT_WRITE_HOLD;
- }
+
unlock_mount_hash();
+ percpu_up_write(&sb->mnt_writers_rws);
return err;
}
@@ -1036,7 +999,7 @@ static struct mount *clone_mnt(struct mount *old, struct dentry *root,
}
mnt->mnt.mnt_flags = old->mnt.mnt_flags;
- mnt->mnt.mnt_flags &= ~(MNT_WRITE_HOLD|MNT_MARKED|MNT_INTERNAL);
+ mnt->mnt.mnt_flags &= ~(MNT_MARKED|MNT_INTERNAL);
atomic_inc(&sb->s_active);
mnt->mnt.mnt_sb = sb;
@@ -2440,7 +2403,8 @@ static bool can_change_locked_flags(struct mount *mnt, unsigned int mnt_flags)
return true;
}
-static int change_mount_ro_state(struct mount *mnt, unsigned int mnt_flags)
+static int change_mount_ro_state(struct super_block *sb,
+ struct mount *mnt, unsigned int mnt_flags)
{
bool readonly_request = (mnt_flags & MNT_READONLY);
@@ -2448,9 +2412,9 @@ static int change_mount_ro_state(struct mount *mnt, unsigned int mnt_flags)
return 0;
if (readonly_request)
- return mnt_make_readonly(mnt);
+ return mnt_make_readonly(sb, mnt);
- return __mnt_unmake_readonly(mnt);
+ return __mnt_unmake_readonly(sb, mnt);
}
/*
@@ -2509,7 +2473,7 @@ static int do_reconfigure_mnt(struct path *path, unsigned int mnt_flags)
return -EPERM;
down_write(&sb->s_umount);
- ret = change_mount_ro_state(mnt, mnt_flags);
+ ret = change_mount_ro_state(sb, mnt, mnt_flags);
if (ret == 0)
set_mount_attributes(mnt, mnt_flags);
up_write(&sb->s_umount);
diff --git a/fs/super.c b/fs/super.c
index a288cd60d2ae..b6036e5fbe8d 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -210,6 +210,8 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags,
INIT_LIST_HEAD(&s->s_mounts);
s->s_user_ns = get_user_ns(user_ns);
init_rwsem(&s->s_umount);
+ percpu_init_rwsem(&s->mnt_writers_rws);
+
lockdep_set_class(&s->s_umount, &type->s_umount_key);
/*
* sget() can have s_umount recursion.
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 45cc10cdf6dd..cc259b7c4ee8 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1458,6 +1458,7 @@ struct super_block {
#endif
struct hlist_bl_head s_roots; /* alternate root dentries for NFS */
struct list_head s_mounts; /* list of mounts; _not_ for fs use */
+ struct percpu_rw_semaphore mnt_writers_rws;
struct block_device *s_bdev;
struct backing_dev_info *s_bdi;
struct mtd_info *s_mtd;
diff --git a/include/linux/mount.h b/include/linux/mount.h
index bf8cc4108b8f..8daf217108e8 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -32,7 +32,6 @@ struct fs_context;
#define MNT_READONLY 0x40 /* does the user want this to be r/o? */
#define MNT_SHRINKABLE 0x100
-#define MNT_WRITE_HOLD 0x200
#define MNT_SHARED 0x1000 /* if the vfsmount is a shared mount */
#define MNT_UNBINDABLE 0x2000 /* if the vfsmount is a unbindable mount */
@@ -49,7 +48,7 @@ struct fs_context;
| MNT_READONLY)
#define MNT_ATIME_MASK (MNT_NOATIME | MNT_NODIRATIME | MNT_RELATIME )
-#define MNT_INTERNAL_FLAGS (MNT_SHARED | MNT_WRITE_HOLD | MNT_INTERNAL | \
+#define MNT_INTERNAL_FLAGS (MNT_SHARED | MNT_INTERNAL | \
MNT_DOOMED | MNT_SYNC_UMOUNT | MNT_MARKED)
#define MNT_INTERNAL 0x4000
--
2.27.0
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