[PATCH v9 08/17] gfs2: Introduce flag for glock holder auto-demotion

From: Andreas Gruenbacher
Date: Tue Nov 02 2021 - 08:30:58 EST

From: Bob Peterson <rpeterso@xxxxxxxxxx>

This patch introduces a new HIF_MAY_DEMOTE flag and infrastructure that
will allow glocks to be demoted automatically on locking conflicts.
When a locking request comes in that isn't compatible with the locking
state of an active holder and that holder has the HIF_MAY_DEMOTE flag
set, the holder will be demoted before the incoming locking request is
granted.

Note that this mechanism demotes active holders (with the HIF_HOLDER
flag set), while before we were only demoting glocks without any active
holders. This allows processes to keep hold of locks that may form a
cyclic locking dependency; the core glock logic will then break those
dependencies in case a conflicting locking request occurs. We'll use
this to avoid giving up the inode glock proactively before faulting in
pages.

Processes that allow a glock holder to be taken away indicate this by
calling gfs2_holder_allow_demote(), which sets the HIF_MAY_DEMOTE flag.
Later, they call gfs2_holder_disallow_demote() to clear the flag again,
and then they check if their holder is still queued: if it is, they are
still holding the glock; if it isn't, they can re-acquire the glock (or
abort).

Signed-off-by: Bob Peterson <rpeterso@xxxxxxxxxx>
Signed-off-by: Andreas Gruenbacher <agruenba@xxxxxxxxxx>
---
fs/gfs2/glock.c | 215 +++++++++++++++++++++++++++++++++++++++--------
fs/gfs2/glock.h | 20 +++++
fs/gfs2/incore.h | 1 +
3 files changed, 200 insertions(+), 36 deletions(-)

diff --git a/fs/gfs2/glock.c b/fs/gfs2/glock.c
index bffd9c20f2de..f686083d0250 100644
--- a/fs/gfs2/glock.c
+++ b/fs/gfs2/glock.c
@@ -58,6 +58,7 @@ struct gfs2_glock_iter {
typedef void (*glock_examiner) (struct gfs2_glock * gl);

static void do_xmote(struct gfs2_glock *gl, struct gfs2_holder *gh, unsigned int target);
+static void __gfs2_glock_dq(struct gfs2_holder *gh);

static struct dentry *gfs2_root;
static struct workqueue_struct *glock_workqueue;
@@ -197,6 +198,12 @@ static int demote_ok(const struct gfs2_glock *gl)

if (gl->gl_state == LM_ST_UNLOCKED)
return 0;
+ /*
+ * Note that demote_ok is used for the lru process of disposing of
+ * glocks. For this purpose, we don't care if the glock's holders
+ * have the HIF_MAY_DEMOTE flag set or not. If someone is using
+ * them, don't demote.
+ */
if (!list_empty(&gl->gl_holders))
return 0;
if (glops->go_demote_ok)
@@ -379,7 +386,7 @@ static void do_error(struct gfs2_glock *gl, const int ret)
struct gfs2_holder *gh, *tmp;

list_for_each_entry_safe(gh, tmp, &gl->gl_holders, gh_list) {
- if (test_bit(HIF_HOLDER, &gh->gh_iflags))
+ if (!test_bit(HIF_WAIT, &gh->gh_iflags))
continue;
if (ret & LM_OUT_ERROR)
gh->gh_error = -EIO;
@@ -393,6 +400,40 @@ static void do_error(struct gfs2_glock *gl, const int ret)
}
}

+/**
+ * demote_incompat_holders - demote incompatible demoteable holders
+ * @gl: the glock we want to promote
+ * @new_gh: the new holder to be promoted
+ */
+static void demote_incompat_holders(struct gfs2_glock *gl,
+ struct gfs2_holder *new_gh)
+{
+ struct gfs2_holder *gh;
+
+ /*
+ * Demote incompatible holders before we make ourselves eligible.
+ * (This holder may or may not allow auto-demoting, but we don't want
+ * to demote the new holder before it's even granted.)
+ */
+ list_for_each_entry(gh, &gl->gl_holders, gh_list) {
+ /*
+ * Since holders are at the front of the list, we stop when we
+ * find the first non-holder.
+ */
+ if (!test_bit(HIF_HOLDER, &gh->gh_iflags))
+ return;
+ if (test_bit(HIF_MAY_DEMOTE, &gh->gh_iflags) &&
+ !may_grant(gl, new_gh, gh)) {
+ /*
+ * We should not recurse into do_promote because
+ * __gfs2_glock_dq only calls handle_callback,
+ * gfs2_glock_add_to_lru and __gfs2_glock_queue_work.
+ */
+ __gfs2_glock_dq(gh);
+ }
+ }
+}
+
/**
* find_first_holder - find the first "holder" gh
* @gl: the glock
@@ -411,6 +452,26 @@ static inline struct gfs2_holder *find_first_holder(const struct gfs2_glock *gl)
return NULL;
}

+/**
+ * find_first_strong_holder - find the first non-demoteable holder
+ * @gl: the glock
+ *
+ * Find the first holder that doesn't have the HIF_MAY_DEMOTE flag set.
+ */
+static inline struct gfs2_holder *
+find_first_strong_holder(struct gfs2_glock *gl)
+{
+ struct gfs2_holder *gh;
+
+ list_for_each_entry(gh, &gl->gl_holders, gh_list) {
+ if (!test_bit(HIF_HOLDER, &gh->gh_iflags))
+ return NULL;
+ if (!test_bit(HIF_MAY_DEMOTE, &gh->gh_iflags))
+ return gh;
+ }
+ return NULL;
+}
+
/**
* do_promote - promote as many requests as possible on the current queue
* @gl: The glock
@@ -425,14 +486,20 @@ __acquires(&gl->gl_lockref.lock)
{
const struct gfs2_glock_operations *glops = gl->gl_ops;
struct gfs2_holder *gh, *tmp, *first_gh;
+ bool incompat_holders_demoted = false;
int ret;

restart:
- first_gh = find_first_holder(gl);
+ first_gh = find_first_strong_holder(gl);
list_for_each_entry_safe(gh, tmp, &gl->gl_holders, gh_list) {
- if (test_bit(HIF_HOLDER, &gh->gh_iflags))
+ if (!test_bit(HIF_WAIT, &gh->gh_iflags))
continue;
if (may_grant(gl, first_gh, gh)) {
+ if (!incompat_holders_demoted) {
+ demote_incompat_holders(gl, first_gh);
+ incompat_holders_demoted = true;
+ first_gh = gh;
+ }
if (gh->gh_list.prev == &gl->gl_holders &&
glops->go_lock) {
spin_unlock(&gl->gl_lockref.lock);
@@ -458,6 +525,11 @@ __acquires(&gl->gl_lockref.lock)
gfs2_holder_wake(gh);
continue;
}
+ /*
+ * If we get here, it means we may not grant this holder for
+ * some reason. If this holder is the head of the list, it
+ * means we have a blocked holder at the head, so return 1.
+ */
if (gh->gh_list.prev == &gl->gl_holders)
return 1;
do_error(gl, 0);
@@ -1372,7 +1444,7 @@ __acquires(&gl->gl_lockref.lock)
if (test_bit(GLF_LOCK, &gl->gl_flags)) {
struct gfs2_holder *first_gh;

- first_gh = find_first_holder(gl);
+ first_gh = find_first_strong_holder(gl);
try_futile = !may_grant(gl, first_gh, gh);
}
if (test_bit(GLF_INVALIDATE_IN_PROGRESS, &gl->gl_flags))
@@ -1381,7 +1453,8 @@ __acquires(&gl->gl_lockref.lock)

list_for_each_entry(gh2, &gl->gl_holders, gh_list) {
if (unlikely(gh2->gh_owner_pid == gh->gh_owner_pid &&
- (gh->gh_gl->gl_ops->go_type != LM_TYPE_FLOCK)))
+ (gh->gh_gl->gl_ops->go_type != LM_TYPE_FLOCK) &&
+ !test_bit(HIF_MAY_DEMOTE, &gh2->gh_iflags)))
goto trap_recursive;
if (try_futile &&
!(gh2->gh_flags & (LM_FLAG_TRY | LM_FLAG_TRY_1CB))) {
@@ -1477,51 +1550,83 @@ int gfs2_glock_poll(struct gfs2_holder *gh)
return test_bit(HIF_WAIT, &gh->gh_iflags) ? 0 : 1;
}

-/**
- * gfs2_glock_dq - dequeue a struct gfs2_holder from a glock (release a glock)
- * @gh: the glock holder
- *
- */
+static inline bool needs_demote(struct gfs2_glock *gl)
+{
+ return (test_bit(GLF_DEMOTE, &gl->gl_flags) ||
+ test_bit(GLF_PENDING_DEMOTE, &gl->gl_flags));
+}

-void gfs2_glock_dq(struct gfs2_holder *gh)
+static void __gfs2_glock_dq(struct gfs2_holder *gh)
{
struct gfs2_glock *gl = gh->gh_gl;
struct gfs2_sbd *sdp = gl->gl_name.ln_sbd;
unsigned delay = 0;
int fast_path = 0;

- spin_lock(&gl->gl_lockref.lock);
/*
- * If we're in the process of file system withdraw, we cannot just
- * dequeue any glocks until our journal is recovered, lest we
- * introduce file system corruption. We need two exceptions to this
- * rule: We need to allow unlocking of nondisk glocks and the glock
- * for our own journal that needs recovery.
+ * This while loop is similar to function demote_incompat_holders:
+ * If the glock is due to be demoted (which may be from another node
+ * or even if this holder is GL_NOCACHE), the weak holders are
+ * demoted as well, allowing the glock to be demoted.
*/
- if (test_bit(SDF_WITHDRAW_RECOVERY, &sdp->sd_flags) &&
- glock_blocked_by_withdraw(gl) &&
- gh->gh_gl != sdp->sd_jinode_gl) {
- sdp->sd_glock_dqs_held++;
- spin_unlock(&gl->gl_lockref.lock);
- might_sleep();
- wait_on_bit(&sdp->sd_flags, SDF_WITHDRAW_RECOVERY,
- TASK_UNINTERRUPTIBLE);
- spin_lock(&gl->gl_lockref.lock);
- }
- if (gh->gh_flags & GL_NOCACHE)
- handle_callback(gl, LM_ST_UNLOCKED, 0, false);
+ while (gh) {
+ /*
+ * If we're in the process of file system withdraw, we cannot
+ * just dequeue any glocks until our journal is recovered, lest
+ * we introduce file system corruption. We need two exceptions
+ * to this rule: We need to allow unlocking of nondisk glocks
+ * and the glock for our own journal that needs recovery.
+ */
+ if (test_bit(SDF_WITHDRAW_RECOVERY, &sdp->sd_flags) &&
+ glock_blocked_by_withdraw(gl) &&
+ gh->gh_gl != sdp->sd_jinode_gl) {
+ sdp->sd_glock_dqs_held++;
+ spin_unlock(&gl->gl_lockref.lock);
+ might_sleep();
+ wait_on_bit(&sdp->sd_flags, SDF_WITHDRAW_RECOVERY,
+ TASK_UNINTERRUPTIBLE);
+ spin_lock(&gl->gl_lockref.lock);
+ }
+
+ /*
+ * This holder should not be cached, so mark it for demote.
+ * Note: this should be done before the check for needs_demote
+ * below.
+ */
+ if (gh->gh_flags & GL_NOCACHE)
+ handle_callback(gl, LM_ST_UNLOCKED, 0, false);
+
+ list_del_init(&gh->gh_list);
+ clear_bit(HIF_HOLDER, &gh->gh_iflags);
+ trace_gfs2_glock_queue(gh, 0);
+
+ /*
+ * If there hasn't been a demote request we are done.
+ * (Let the remaining holders, if any, keep holding it.)
+ */
+ if (!needs_demote(gl)) {
+ if (list_empty(&gl->gl_holders))
+ fast_path = 1;
+ break;
+ }
+ /*
+ * If we have another strong holder (we cannot auto-demote)
+ * we are done. It keeps holding it until it is done.
+ */
+ if (find_first_strong_holder(gl))
+ break;

- list_del_init(&gh->gh_list);
- clear_bit(HIF_HOLDER, &gh->gh_iflags);
- if (list_empty(&gl->gl_holders) &&
- !test_bit(GLF_PENDING_DEMOTE, &gl->gl_flags) &&
- !test_bit(GLF_DEMOTE, &gl->gl_flags))
- fast_path = 1;
+ /*
+ * If we have a weak holder at the head of the list, it
+ * (and all others like it) must be auto-demoted. If there
+ * are no more weak holders, we exit the while loop.
+ */
+ gh = find_first_holder(gl);
+ }

if (!test_bit(GLF_LFLUSH, &gl->gl_flags) && demote_ok(gl))
gfs2_glock_add_to_lru(gl);

- trace_gfs2_glock_queue(gh, 0);
if (unlikely(!fast_path)) {
gl->gl_lockref.count++;
if (test_bit(GLF_PENDING_DEMOTE, &gl->gl_flags) &&
@@ -1530,6 +1635,19 @@ void gfs2_glock_dq(struct gfs2_holder *gh)
delay = gl->gl_hold_time;
__gfs2_glock_queue_work(gl, delay);
}
+}
+
+/**
+ * gfs2_glock_dq - dequeue a struct gfs2_holder from a glock (release a glock)
+ * @gh: the glock holder
+ *
+ */
+void gfs2_glock_dq(struct gfs2_holder *gh)
+{
+ struct gfs2_glock *gl = gh->gh_gl;
+
+ spin_lock(&gl->gl_lockref.lock);
+ __gfs2_glock_dq(gh);
spin_unlock(&gl->gl_lockref.lock);
}

@@ -1692,6 +1810,7 @@ void gfs2_glock_dq_m(unsigned int num_gh, struct gfs2_holder *ghs)

void gfs2_glock_cb(struct gfs2_glock *gl, unsigned int state)
{
+ struct gfs2_holder mock_gh = { .gh_gl = gl, .gh_state = state, };
unsigned long delay = 0;
unsigned long holdtime;
unsigned long now = jiffies;
@@ -1706,6 +1825,28 @@ void gfs2_glock_cb(struct gfs2_glock *gl, unsigned int state)
if (test_bit(GLF_REPLY_PENDING, &gl->gl_flags))
delay = gl->gl_hold_time;
}
+ /*
+ * Note 1: We cannot call demote_incompat_holders from handle_callback
+ * or gfs2_set_demote due to recursion problems like: gfs2_glock_dq ->
+ * handle_callback -> demote_incompat_holders -> gfs2_glock_dq
+ * Plus, we only want to demote the holders if the request comes from
+ * a remote cluster node because local holder conflicts are resolved
+ * elsewhere.
+ *
+ * Note 2: if a remote node wants this glock in EX mode, lock_dlm will
+ * request that we set our state to UNLOCKED. Here we mock up a holder
+ * to make it look like someone wants the lock EX locally. Any SH
+ * and DF requests should be able to share the lock without demoting.
+ *
+ * Note 3: We only want to demote the demoteable holders when there
+ * are no more strong holders. The demoteable holders might as well
+ * keep the glock until the last strong holder is done with it.
+ */
+ if (!find_first_strong_holder(gl)) {
+ if (state == LM_ST_UNLOCKED)
+ mock_gh.gh_state = LM_ST_EXCLUSIVE;
+ demote_incompat_holders(gl, &mock_gh);
+ }
handle_callback(gl, state, delay, true);
__gfs2_glock_queue_work(gl, delay);
spin_unlock(&gl->gl_lockref.lock);
@@ -2095,6 +2236,8 @@ static const char *hflags2str(char *buf, u16 flags, unsigned long iflags)
*p++ = 'H';
if (test_bit(HIF_WAIT, &iflags))
*p++ = 'W';
+ if (test_bit(HIF_MAY_DEMOTE, &iflags))
+ *p++ = 'D';
*p = 0;
return buf;
}
diff --git a/fs/gfs2/glock.h b/fs/gfs2/glock.h
index 31a8f2f649b5..9012487da4c6 100644
--- a/fs/gfs2/glock.h
+++ b/fs/gfs2/glock.h
@@ -150,6 +150,8 @@ static inline struct gfs2_holder *gfs2_glock_is_locked_by_me(struct gfs2_glock *
list_for_each_entry(gh, &gl->gl_holders, gh_list) {
if (!test_bit(HIF_HOLDER, &gh->gh_iflags))
break;
+ if (test_bit(HIF_MAY_DEMOTE, &gh->gh_iflags))
+ continue;
if (gh->gh_owner_pid == pid)
goto out;
}
@@ -325,6 +327,24 @@ static inline void glock_clear_object(struct gfs2_glock *gl, void *object)
spin_unlock(&gl->gl_lockref.lock);
}

+static inline void gfs2_holder_allow_demote(struct gfs2_holder *gh)
+{
+ struct gfs2_glock *gl = gh->gh_gl;
+
+ spin_lock(&gl->gl_lockref.lock);
+ set_bit(HIF_MAY_DEMOTE, &gh->gh_iflags);
+ spin_unlock(&gl->gl_lockref.lock);
+}
+
+static inline void gfs2_holder_disallow_demote(struct gfs2_holder *gh)
+{
+ struct gfs2_glock *gl = gh->gh_gl;
+
+ spin_lock(&gl->gl_lockref.lock);
+ clear_bit(HIF_MAY_DEMOTE, &gh->gh_iflags);
+ spin_unlock(&gl->gl_lockref.lock);
+}
+
extern void gfs2_inode_remember_delete(struct gfs2_glock *gl, u64 generation);
extern bool gfs2_inode_already_deleted(struct gfs2_glock *gl, u64 generation);

diff --git a/fs/gfs2/incore.h b/fs/gfs2/incore.h
index 0fe49770166e..58b7bac501e4 100644
--- a/fs/gfs2/incore.h
+++ b/fs/gfs2/incore.h
@@ -252,6 +252,7 @@ struct gfs2_lkstats {

enum {
/* States */
+ HIF_MAY_DEMOTE = 1,
HIF_HOLDER = 6, /* Set for gh that "holds" the glock */
HIF_WAIT = 10,
};
--
2.31.1