[RFC PATCH 1/1] fs/namespace.c: use spinlock instead of busy loop
From: John Ogness
Date: Wed Jun 17 2020 - 06:41:30 EST
The MNT_WRITE_HOLD flag is used to manually implement a per-cpu
optimized rwsem using busy waiting. This implementation is a problem
on PREEMPT_RT because write_seqlock() on @mount_lock (i.e. taking a
spinlock) does not disable preemption. This allows a writer to
preempt a task that has set MNT_WRITE_HOLD and thus that writer will
live lock in __mnt_want_write() due to busy looping with preemption
disabled.
Reimplement the same semantics using per-cpu spinlocks. This
provides lockdep coverage and makes the code RT ready.
Since this reverts some of the optimization work of
commit d3ef3d7351cc ("fs: mnt_want_write speedup")
lmbench lat_mmap tests were performed to verify that there is no
obvious performance degradation.
Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
---
Here is the detailed test information...
TEST COMMAND
lat_mmap -P $pval -W 32 -N 50 64m file
OUTPUT FORMAT
pval: avg std
RESULTS (32 CPUs)
No Forced Preemption
BEFORE AFTER
1: 275.60 1.82 274.40 0.55
2: 296.20 3.83 286.80 1.92
4: 310.20 4.44 304.40 2.51
8: 359.20 2.28 357.80 2.95
16: 417.40 2.51 412.20 3.90
32: 625.60 2.07 622.00 3.08
64: 1202.60 15.87 1202.20 6.14
No Forced Preemption, no PTI
BEFORE AFTER
1: 278.00 2.12 274.40 1.52
2: 301.00 3.67 289.80 6.06
4: 333.40 7.73 303.80 2.39
8: 389.80 3.56 351.80 3.42
16: 425.00 3.46 408.20 4.87
32: 606.00 1.22 605.60 1.82
64: 1193.60 7.09 1184.80 4.27
Voluntary Kernel Preemption
BEFORE AFTER
1: 277.80 1.30 278.20 1.10
2: 291.20 0.84 286.60 2.30
4: 310.00 1.87 304.80 1.30
8: 360.00 2.55 354.60 1.14
16: 414.00 1.58 414.00 2.35
32: 619.60 5.50 607.00 3.74
64: 1224.00 8.40 1219.60 6.19
Voluntary Kernel Preemption, no PTI
BEFORE AFTER
1: 277.80 4.66 276.40 0.89
2: 291.40 6.54 286.40 3.58
4: 310.00 1.22 315.40 1.14
8: 357.20 0.84 361.40 2.61
16: 405.60 2.88 407.60 2.51
32: 615.40 2.30 611.60 5.55
64: 1219.80 9.91 1207.40 10.88
Preemptible Kernel
BEFORE AFTER
1: 283.80 0.45 286.80 0.84
2: 293.40 2.51 294.40 3.51
4: 318.20 1.30 315.60 1.95
8: 367.00 0.71 363.00 1.22
16: 416.20 1.64 413.20 4.87
32: 628.80 2.28 617.40 2.97
64: 1277.20 9.88 1254.20 4.87
Preemptible Kernel, no PTI
BEFORE AFTER
1: 283.00 1.73 288.40 1.67
2: 305.80 2.05 297.00 3.24
4: 321.40 4.34 318.60 2.79
8: 370.20 2.39 366.40 2.70
16: 413.20 3.11 412.40 2.41
32: 616.40 2.61 620.20 2.05
64: 1266.00 6.48 1255.80 3.90
fs/mount.h | 7 +++
fs/namespace.c | 118 +++++++++++++++++++++++++++++++++----------------
2 files changed, 86 insertions(+), 39 deletions(-)
diff --git a/fs/mount.h b/fs/mount.h
index c7abb7b394d8..627e635cd7d1 100644
--- a/fs/mount.h
+++ b/fs/mount.h
@@ -28,6 +28,12 @@ struct mnt_namespace {
struct mnt_pcp {
int mnt_count;
int mnt_writers;
+ /*
+ * If holding multiple instances of this lock, they
+ * must be ordered by cpu number.
+ */
+ spinlock_t mnt_writers_lock;
+ struct lock_class_key mnt_writers_lock_key;
};
struct mountpoint {
@@ -51,6 +57,7 @@ struct mount {
#else
int mnt_count;
int mnt_writers;
+ spinlock_t mnt_writers_lock;
#endif
struct list_head mnt_mounts; /* list of children, anchored here */
struct list_head mnt_child; /* and going through their mnt_child */
diff --git a/fs/namespace.c b/fs/namespace.c
index f30ed401cc6d..e292c91f966d 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -177,6 +177,7 @@ static struct mount *alloc_vfsmnt(const char *name)
struct mount *mnt = kmem_cache_zalloc(mnt_cache, GFP_KERNEL);
if (mnt) {
int err;
+ int cpu;
err = mnt_alloc_id(mnt);
if (err)
@@ -194,9 +195,21 @@ static struct mount *alloc_vfsmnt(const char *name)
goto out_free_devname;
this_cpu_add(mnt->mnt_pcp->mnt_count, 1);
+
+ for_each_possible_cpu(cpu) {
+ struct mnt_pcp *writer =
+ per_cpu_ptr(mnt->mnt_pcp, cpu);
+
+ lockdep_register_key(&writer->mnt_writers_lock_key);
+ spin_lock_init(&writer->mnt_writers_lock);
+ lockdep_set_class(&writer->mnt_writers_lock,
+ &writer->mnt_writers_lock_key);
+ }
#else
+ (void)cpu;
mnt->mnt_count = 1;
mnt->mnt_writers = 0;
+ spin_lock_init(&mnt->mnt_writers_lock);
#endif
INIT_HLIST_NODE(&mnt->mnt_hash);
@@ -311,29 +324,26 @@ static int mnt_is_readonly(struct vfsmount *mnt)
int __mnt_want_write(struct vfsmount *m)
{
struct mount *mnt = real_mount(m);
+ spinlock_t *lock;
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);
+#ifdef CONFIG_SMP
+ migrate_disable();
+ lock = &this_cpu_ptr(mnt->mnt_pcp)->mnt_writers_lock;
+#else
+ lock = &mnt->mnt_writers_lock;
+#endif
+
+ spin_lock(lock);
+ if (mnt_is_readonly(m))
ret = -EROFS;
- }
- preempt_enable();
+ else
+ mnt_inc_writers(mnt);
+ spin_unlock(lock);
+
+#ifdef CONFIG_SMP
+ migrate_enable();
+#endif
return ret;
}
@@ -459,17 +469,39 @@ void mnt_drop_write_file(struct file *file)
}
EXPORT_SYMBOL(mnt_drop_write_file);
+static void mnt_lock_writers(struct mount *mnt)
+{
+#ifdef CONFIG_SMP
+ int cpu;
+
+ for_each_possible_cpu(cpu) {
+ spin_lock(&per_cpu_ptr(mnt->mnt_pcp,
+ cpu)->mnt_writers_lock);
+ }
+#else
+ spin_lock(&mnt->mnt_writers_lock);
+#endif
+}
+
+static void mnt_unlock_writers(struct mount *mnt)
+{
+#ifdef CONFIG_SMP
+ int cpu;
+
+ for_each_possible_cpu(cpu) {
+ spin_unlock(&per_cpu_ptr(mnt->mnt_pcp,
+ cpu)->mnt_writers_lock);
+ }
+#else
+ spin_unlock(&mnt->mnt_writers_lock);
+#endif
+}
+
static int mnt_make_readonly(struct mount *mnt)
{
int ret = 0;
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
@@ -482,21 +514,17 @@ static int mnt_make_readonly(struct mount *mnt)
* 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.
+ * mnt_writers_lock protects against this scenario, because all CPUs
+ * are prevented from incrementing the counter until the summation of
+ * all CPU counters is complete.
*/
+ mnt_lock_writers(mnt);
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;
+ mnt_unlock_writers(mnt);
+
unlock_mount_hash();
return ret;
}
@@ -514,15 +542,15 @@ 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_lock. */
if (atomic_long_read(&sb->s_remove_count))
return -EBUSY;
lock_mount_hash();
list_for_each_entry(mnt, &sb->s_mounts, mnt_instance) {
if (!(mnt->mnt.mnt_flags & MNT_READONLY)) {
+ mnt_lock_writers(mnt);
mnt->mnt.mnt_flags |= MNT_WRITE_HOLD;
- smp_mb();
if (mnt_get_writers(mnt) > 0) {
err = -EBUSY;
break;
@@ -537,8 +565,10 @@ int sb_prepare_remount_readonly(struct super_block *sb)
smp_wmb();
}
list_for_each_entry(mnt, &sb->s_mounts, mnt_instance) {
- if (mnt->mnt.mnt_flags & MNT_WRITE_HOLD)
+ if (mnt->mnt.mnt_flags & MNT_WRITE_HOLD) {
mnt->mnt.mnt_flags &= ~MNT_WRITE_HOLD;
+ mnt_unlock_writers(mnt);
+ }
}
unlock_mount_hash();
@@ -1099,6 +1129,7 @@ static void cleanup_mnt(struct mount *mnt)
{
struct hlist_node *p;
struct mount *m;
+ int cpu;
/*
* The warning here probably indicates that somebody messed
* up a mnt_want/drop_write() pair. If this happens, the
@@ -1117,6 +1148,15 @@ static void cleanup_mnt(struct mount *mnt)
dput(mnt->mnt.mnt_root);
deactivate_super(mnt->mnt.mnt_sb);
mnt_free_id(mnt);
+#ifdef CONFIG_SMP
+ for_each_possible_cpu(cpu) {
+ struct mnt_pcp *writer = per_cpu_ptr(mnt->mnt_pcp, cpu);
+
+ lockdep_unregister_key(&writer->mnt_writers_lock_key);
+ }
+#else
+ (void)cpu;
+#endif
call_rcu(&mnt->mnt_rcu, delayed_free_vfsmnt);
}
--
2.20.1