[PATCH 2/4] [RFC] seqcount: Add lockdep functionality to seqcount/seqlock structures

From: John Stultz
Date: Mon Oct 07 2013 - 18:52:43 EST

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Currently seqlocks and seqcounts don't support lockdep.

After running across a seqcount related deadlock in the timekeeping
code, I used a less-refined and more focused variant of this patch
to narrow down the cause of the issue.

This is a first-pass attempt to properly enable lockdep functionality
on seqlocks and seqcounts.

Since seqcounts are used in the vdso gettimeofday code, I've provided
non-lockdep accessors for those needs.

I've also handled one case where there were nested seqlock writers
and there may be more edge cases.

Comments and feedback would be appreciated!

Cc: Eric Dumazet <eric.dumazet@xxxxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: Li Zefan <lizefan@xxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
---
arch/x86/vdso/vclock_gettime.c | 8 ++---
fs/dcache.c | 4 +--
fs/fs_struct.c | 2 +-
include/linux/init_task.h | 8 ++---
include/linux/lockdep.h | 8 +++--
include/linux/seqlock.h | 79 ++++++++++++++++++++++++++++++++++++++----
mm/filemap_xip.c | 2 +-
7 files changed, 90 insertions(+), 21 deletions(-)

diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c
index 72074d5..2ada505 100644
--- a/arch/x86/vdso/vclock_gettime.c
+++ b/arch/x86/vdso/vclock_gettime.c
@@ -178,7 +178,7 @@ notrace static int __always_inline do_realtime(struct timespec *ts)

ts->tv_nsec = 0;
do {
- seq = read_seqcount_begin(&gtod->seq);
+ seq = read_seqcount_begin_no_lockdep(&gtod->seq);
mode = gtod->clock.vclock_mode;
ts->tv_sec = gtod->wall_time_sec;
ns = gtod->wall_time_snsec;
@@ -198,7 +198,7 @@ notrace static int do_monotonic(struct timespec *ts)

ts->tv_nsec = 0;
do {
- seq = read_seqcount_begin(&gtod->seq);
+ seq = read_seqcount_begin_no_lockdep(&gtod->seq);
mode = gtod->clock.vclock_mode;
ts->tv_sec = gtod->monotonic_time_sec;
ns = gtod->monotonic_time_snsec;
@@ -214,7 +214,7 @@ notrace static int do_realtime_coarse(struct timespec *ts)
{
unsigned long seq;
do {
- seq = read_seqcount_begin(&gtod->seq);
+ seq = read_seqcount_begin_no_lockdep(&gtod->seq);
ts->tv_sec = gtod->wall_time_coarse.tv_sec;
ts->tv_nsec = gtod->wall_time_coarse.tv_nsec;
} while (unlikely(read_seqcount_retry(&gtod->seq, seq)));
@@ -225,7 +225,7 @@ notrace static int do_monotonic_coarse(struct timespec *ts)
{
unsigned long seq;
do {
- seq = read_seqcount_begin(&gtod->seq);
+ seq = read_seqcount_begin_no_lockdep(&gtod->seq);
ts->tv_sec = gtod->monotonic_time_coarse.tv_sec;
ts->tv_nsec = gtod->monotonic_time_coarse.tv_nsec;
} while (unlikely(read_seqcount_retry(&gtod->seq, seq)));
diff --git a/fs/dcache.c b/fs/dcache.c
index 4100030..2f39b81 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2574,7 +2574,7 @@ static void __d_move(struct dentry * dentry, struct dentry * target)
dentry_lock_for_move(dentry, target);

write_seqcount_begin(&dentry->d_seq);
- write_seqcount_begin(&target->d_seq);
+ write_seqcount_begin_nested(&target->d_seq, DENTRY_D_LOCK_NESTED);

/* __d_drop does write_seqcount_barrier, but they're OK to nest. */

@@ -2706,7 +2706,7 @@ static void __d_materialise_dentry(struct dentry *dentry, struct dentry *anon)
dentry_lock_for_move(anon, dentry);

write_seqcount_begin(&dentry->d_seq);
- write_seqcount_begin(&anon->d_seq);
+ write_seqcount_begin_nested(&anon->d_seq, DENTRY_D_LOCK_NESTED);

dparent = dentry->d_parent;

diff --git a/fs/fs_struct.c b/fs/fs_struct.c
index d8ac61d..7dca743 100644
--- a/fs/fs_struct.c
+++ b/fs/fs_struct.c
@@ -161,6 +161,6 @@ EXPORT_SYMBOL(current_umask);
struct fs_struct init_fs = {
.users = 1,
.lock = __SPIN_LOCK_UNLOCKED(init_fs.lock),
- .seq = SEQCNT_ZERO,
+ .seq = SEQCNT_ZERO(init_fs.seq),
.umask = 0022,
};
diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index 5cd0f09..b0ed422 100644
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -32,10 +32,10 @@ extern struct fs_struct init_fs;
#endif

#ifdef CONFIG_CPUSETS
-#define INIT_CPUSET_SEQ \
- .mems_allowed_seq = SEQCNT_ZERO,
+#define INIT_CPUSET_SEQ(tsk) \
+ .mems_allowed_seq = SEQCNT_ZERO(tsk.mems_allowed_seq),
#else
-#define INIT_CPUSET_SEQ
+#define INIT_CPUSET_SEQ(tsk)
#endif

#define INIT_SIGNALS(sig) { \
@@ -220,7 +220,7 @@ extern struct task_group root_task_group;
INIT_FTRACE_GRAPH \
INIT_TRACE_RECURSION \
INIT_TASK_RCU_PREEMPT(tsk) \
- INIT_CPUSET_SEQ \
+ INIT_CPUSET_SEQ(tsk) \
INIT_VTIME(tsk) \
}

diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index cfc2f11..92b1bfc 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -497,6 +497,10 @@ static inline void print_irqtrace_events(struct task_struct *curr)
#define rwlock_acquire_read(l, s, t, i) lock_acquire_shared_recursive(l, s, t, NULL, i)
#define rwlock_release(l, n, i) lock_release(l, n, i)

+#define seqcount_acquire(l, s, t, i) lock_acquire_exclusive(l, s, t, NULL, i)
+#define seqcount_acquire_read(l, s, t, i) lock_acquire_shared_recursive(l, s, t, NULL, i)
+#define seqcount_release(l, n, i) lock_release(l, n, i)
+
#define mutex_acquire(l, s, t, i) lock_acquire_exclusive(l, s, t, NULL, i)
#define mutex_acquire_nest(l, s, t, n, i) lock_acquire_exclusive(l, s, t, n, i)
#define mutex_release(l, n, i) lock_release(l, n, i)
@@ -504,11 +508,11 @@ static inline void print_irqtrace_events(struct task_struct *curr)
#define rwsem_acquire(l, s, t, i) lock_acquire_exclusive(l, s, t, NULL, i)
#define rwsem_acquire_nest(l, s, t, n, i) lock_acquire_exclusive(l, s, t, n, i)
#define rwsem_acquire_read(l, s, t, i) lock_acquire_shared(l, s, t, NULL, i)
-# define rwsem_release(l, n, i) lock_release(l, n, i)
+#define rwsem_release(l, n, i) lock_release(l, n, i)

#define lock_map_acquire(l) lock_acquire_exclusive(l, 0, 0, NULL, _THIS_IP_)
#define lock_map_acquire_read(l) lock_acquire_shared_recursive(l, 0, 0, NULL, _THIS_IP_)
-# define lock_map_release(l) lock_release(l, 1, _THIS_IP_)
+#define lock_map_release(l) lock_release(l, 1, _THIS_IP_)

#ifdef CONFIG_PROVE_LOCKING
# define might_lock(lock) \
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 21a2093..1e8a8b6 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -34,6 +34,7 @@

#include <linux/spinlock.h>
#include <linux/preempt.h>
+#include <linux/lockdep.h>
#include <asm/processor.h>

/*
@@ -44,10 +45,50 @@
*/
typedef struct seqcount {
unsigned sequence;
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+ struct lockdep_map dep_map;
+#endif
} seqcount_t;

-#define SEQCNT_ZERO { 0 }
-#define seqcount_init(x) do { *(x) = (seqcount_t) SEQCNT_ZERO; } while (0)
+static inline void __seqcount_init(seqcount_t *s, const char *name,
+ struct lock_class_key *key)
+{
+ /*
+ * Make sure we are not reinitializing a held lock:
+ */
+ lockdep_init_map(&s->dep_map, name, key, 0);
+ s->sequence = 0;
+}
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+# define SEQCOUNT_DEP_MAP_INIT(lockname) \
+ .dep_map = { .name = #lockname } \
+
+# define seqcount_init(s) \
+ do { \
+ static struct lock_class_key __key; \
+ __seqcount_init((s), #s, &__key); \
+ } while (0)
+
+static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
+{
+ seqcount_t *l = (seqcount_t *)s;
+ unsigned long flags;
+
+ local_irq_save(flags);
+ seqcount_acquire_read(&l->dep_map, 0, 0, _RET_IP_);
+ seqcount_release(&l->dep_map, 1, _RET_IP_);
+ local_irq_restore(flags);
+}
+
+#else
+# define SEQCOUNT_DEP_MAP_INIT(lockname)
+# define seqcount_init(s) __seqcount_init(s, NULL, NULL)
+# define seqcount_lockdep_reader_access(x)
+#endif
+
+#define SEQCNT_ZERO(lockname) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(lockname)}
+

/**
* __read_seqcount_begin - begin a seq-read critical section (without barrier)
@@ -76,6 +117,22 @@ repeat:
}

/**
+ * read_seqcount_begin_no_lockdep - start seq-read critical section w/o lockdep
+ * @s: pointer to seqcount_t
+ * Returns: count to be passed to read_seqcount_retry
+ *
+ * read_seqcount_begin_no_lockdep opens a read critical section of the given
+ * seqcount, but without any lockdep checking. Validity of the critical
+ * section is tested by checking read_seqcount_retry function.
+ */
+static inline unsigned read_seqcount_begin_no_lockdep(const seqcount_t *s)
+{
+ unsigned ret = __read_seqcount_begin(s);
+ smp_rmb();
+ return ret;
+}
+
+/**
* read_seqcount_begin - begin a seq-read critical section
* @s: pointer to seqcount_t
* Returns: count to be passed to read_seqcount_retry
@@ -86,9 +143,8 @@ repeat:
*/
static inline unsigned read_seqcount_begin(const seqcount_t *s)
{
- unsigned ret = __read_seqcount_begin(s);
- smp_rmb();
- return ret;
+ seqcount_lockdep_reader_access(s);
+ return read_seqcount_begin_no_lockdep(s);
}

/**
@@ -108,6 +164,8 @@ static inline unsigned read_seqcount_begin(const seqcount_t *s)
static inline unsigned raw_seqcount_begin(const seqcount_t *s)
{
unsigned ret = ACCESS_ONCE(s->sequence);
+
+ seqcount_lockdep_reader_access(s);
smp_rmb();
return ret & ~1;
}
@@ -152,14 +210,21 @@ static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
* Sequence counter only version assumes that callers are using their
* own mutexing.
*/
-static inline void write_seqcount_begin(seqcount_t *s)
+static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
{
s->sequence++;
smp_wmb();
+ seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
+}
+
+static inline void write_seqcount_begin(seqcount_t *s)
+{
+ write_seqcount_begin_nested(s, 0);
}

static inline void write_seqcount_end(seqcount_t *s)
{
+ seqcount_release(&s->dep_map, 1, _RET_IP_);
smp_wmb();
s->sequence++;
}
@@ -188,7 +253,7 @@ typedef struct {
*/
#define __SEQLOCK_UNLOCKED(lockname) \
{ \
- .seqcount = SEQCNT_ZERO, \
+ .seqcount = SEQCNT_ZERO(lockname), \
.lock = __SPIN_LOCK_UNLOCKED(lockname) \
}

diff --git a/mm/filemap_xip.c b/mm/filemap_xip.c
index 28fe26b..d8d9fe3 100644
--- a/mm/filemap_xip.c
+++ b/mm/filemap_xip.c
@@ -26,7 +26,7 @@
* of ZERO_PAGE(), such as /dev/zero
*/
static DEFINE_MUTEX(xip_sparse_mutex);
-static seqcount_t xip_sparse_seq = SEQCNT_ZERO;
+static seqcount_t xip_sparse_seq = SEQCNT_ZERO(xip_sparse_seq);
static struct page *__xip_sparse_page;

/* called under xip_sparse_mutex */
--
1.8.1.2

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Next message: John Stultz: "[PATCH 4/4] [RFC] ipv6: Fix for possible ipv6 seqlock deadlock"
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