Re: [PATCH v8 06/14] lockdep: Detect and handle hist_lock ring buffer overwrite

Peter Zijlstra
2017-08-09T14:16:22Z

On Mon, Aug 07, 2017 at 04:12:53PM +0900, Byungchul Park wrote:
> @@ -4773,14 +4784,28 @@ void lockdep_rcu_suspicious(const char *file, const int line, const char *s)
> */
> void crossrelease_hist_start(enum context_t c)
> {
> - if (current->xhlocks)
> - current->xhlock_idx_hist[c] = current->xhlock_idx;
> + struct task_struct *cur = current;
> +
> + if (cur->xhlocks) {
> + cur->xhlock_idx_hist[c] = cur->xhlock_idx;
> + cur->hist_id_save[c] = cur->hist_id;
> + }
> }
>
> void crossrelease_hist_end(enum context_t c)
> {
> - if (current->xhlocks)
> - current->xhlock_idx = current->xhlock_idx_hist[c];
> + struct task_struct *cur = current;
> +
> + if (cur->xhlocks) {
> + unsigned int idx = cur->xhlock_idx_hist[c];
> + struct hist_lock *h = &xhlock(idx);
> +
> + cur->xhlock_idx = idx;
> +
> + /* Check if the ring was overwritten. */
> + if (h->hist_id != cur->hist_id_save[c])
> + invalidate_xhlock(h);
> + }
> }
>
> static int cross_lock(struct lockdep_map *lock)
> @@ -4826,6 +4851,7 @@ static inline int depend_after(struct held_lock *hlock)
> * Check if the xhlock is valid, which would be false if,
> *
> * 1. Has not used after initializaion yet.
> + * 2. Got invalidated.
> *
> * Remind hist_lock is implemented as a ring buffer.
> */
> @@ -4857,6 +4883,7 @@ static void add_xhlock(struct held_lock *hlock)
>
> /* Initialize hist_lock's members */
> xhlock->hlock = *hlock;
> + xhlock->hist_id = current->hist_id++;
>
> xhlock->trace.nr_entries = 0;
> xhlock->trace.max_entries = MAX_XHLOCK_TRACE_ENTRIES;

Hehe, _another_ scheme...

Yes I think this works.. but I had just sort of understood the last one.

How about I do this on top? That I think is a combination of what I
proposed last and your single invalidate thing. Combined they solve the
problem with the least amount of extra storage (a single int).

---
Subject: lockdep: Simplify xhlock ring buffer invalidation
From: Peter Zijlstra <peterz@infradead.org>
Date: Wed Aug 9 15:31:27 CEST 2017

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
include/linux/lockdep.h | 20 -----------
include/linux/sched.h | 4 --
kernel/locking/lockdep.c | 82 ++++++++++++++++++++++++++++++-----------------
3 files changed, 54 insertions(+), 52 deletions(-)

--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -284,26 +284,6 @@ struct held_lock {
*/
struct hist_lock {
/*
- * Id for each entry in the ring buffer. This is used to
- * decide whether the ring buffer was overwritten or not.
- *
- * For example,
- *
- * |<----------- hist_lock ring buffer size ------->|
- * pppppppppppppppppppppiiiiiiiiiiiiiiiiiiiiiiiiiiiii
- * wrapped > iiiiiiiiiiiiiiiiiiiiiiiiiii.......................
- *
- * where 'p' represents an acquisition in process
- * context, 'i' represents an acquisition in irq
- * context.
- *
- * In this example, the ring buffer was overwritten by
- * acquisitions in irq context, that should be detected on
- * rollback or commit.
- */
- unsigned int hist_id;
-
- /*
* Seperate stack_trace data. This will be used at commit step.
*/
struct stack_trace trace;
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -855,9 +855,7 @@ struct task_struct {
unsigned int xhlock_idx;
/* For restoring at history boundaries */
unsigned int xhlock_idx_hist[XHLOCK_NR];
- unsigned int hist_id;
- /* For overwrite check at each context exit */
- unsigned int hist_id_save[XHLOCK_NR];
+ unsigned int xhlock_idx_max;
#endif

#ifdef CONFIG_UBSAN
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -4818,26 +4818,65 @@ void crossrelease_hist_start(enum contex
{
struct task_struct *cur = current;

- if (cur->xhlocks) {
+ if (cur->xhlocks)
cur->xhlock_idx_hist[c] = cur->xhlock_idx;
- cur->hist_id_save[c] = cur->hist_id;
- }
}

void crossrelease_hist_end(enum context_t c)
{
struct task_struct *cur = current;
+ unsigned int idx;

- if (cur->xhlocks) {
- unsigned int idx = cur->xhlock_idx_hist[c];
- struct hist_lock *h = &xhlock(idx);
-
- cur->xhlock_idx = idx;
-
- /* Check if the ring was overwritten. */
- if (h->hist_id != cur->hist_id_save[c])
- invalidate_xhlock(h);
- }
+ if (!cur->xhlocks)
+ return;
+
+ idx = cur->xhlock_idx_hist[c];
+ cur->xhlock_idx = idx;
+
+ /*
+ * A bit of magic here.. this deals with rewinding the (cyclic) history
+ * array further than its size. IOW. looses the complete history.
+ *
+ * We detect this by tracking the previous oldest entry we've (over)
+ * written in @xhlock_idx_max, this means the next entry is the oldest
+ * entry still in the buffer, ie. its tail.
+ *
+ * So when we restore an @xhlock_idx that is at least MAX_XHLOCKS_NR
+ * older than @xhlock_idx_max we know we've just wiped the entire
+ * history.
+ */
+ if ((cur->xhlock_idx_max - idx) < MAX_XHLOCKS_NR)
+ return;
+
+ /*
+ * Now that we know the buffer is effectively empty, reset our state
+ * such that it appears empty (without in fact clearing the entire
+ * buffer).
+ *
+ * Pick @idx as the 'new' beginning, (re)set all save-points to not
+ * rewind past it and reset the max. Then invalidate this idx such that
+ * commit_xhlocks() will never rewind past it. Since xhlock_idx_inc()
+ * will return the _next_ entry, we'll not overwrite this invalid entry
+ * until the entire buffer is full again.
+ */
+ for (c = 0; c < XHLOCK_NR; c++)
+ cur->xhlock_idx_hist[c] = idx;
+ cur->xhlock_idx_max = idx;
+ invalidate_xhlock(&xhlock(idx));
+}
+
+static inline unsigned int xhlock_idx_inc(void)
+{
+ struct task_struct *cur = current;
+ unsigned int idx = ++cur->xhlock_idx;
+
+ /*
+ * As per the requirement in crossrelease_hist_end(), track the tail.
+ */
+ if ((int)(cur->xhlock_idx_max - idx) < 0)
+ cur->xhlock_idx_max = idx;
+
+ return idx;
}

static int cross_lock(struct lockdep_map *lock)
@@ -4902,7 +4941,7 @@ static inline int xhlock_valid(struct hi
*/
static void add_xhlock(struct held_lock *hlock)
{
- unsigned int idx = ++current->xhlock_idx;
+ unsigned int idx = xhlock_idx_inc();
struct hist_lock *xhlock = &xhlock(idx);

#ifdef CONFIG_DEBUG_LOCKDEP
@@ -4915,7 +4954,6 @@ static void add_xhlock(struct held_lock

/* Initialize hist_lock's members */
xhlock->hlock = *hlock;
- xhlock->hist_id = current->hist_id++;

xhlock->trace.nr_entries = 0;
xhlock->trace.max_entries = MAX_XHLOCK_TRACE_ENTRIES;
@@ -5071,7 +5109,6 @@ static int commit_xhlock(struct cross_lo
static void commit_xhlocks(struct cross_lock *xlock)
{
unsigned int cur = current->xhlock_idx;
- unsigned int prev_hist_id = xhlock(cur).hist_id;
unsigned int i;

if (!graph_lock())
@@ -5091,17 +5128,6 @@ static void commit_xhlocks(struct cross_
break;

/*
- * Filter out the cases that the ring buffer was
- * overwritten and the previous entry has a bigger
- * hist_id than the following one, which is impossible
- * otherwise.
- */
- if (unlikely(before(xhlock->hist_id, prev_hist_id)))
- break;
-
- prev_hist_id = xhlock->hist_id;
-
- /*
* commit_xhlock() returns 0 with graph_lock already
* released if fail.
*/
@@ -5186,11 +5212,9 @@ void lockdep_init_task(struct task_struc
int i;

task->xhlock_idx = UINT_MAX;
- task->hist_id = 0;

for (i = 0; i < XHLOCK_NR; i++) {
task->xhlock_idx_hist[i] = UINT_MAX;
- task->hist_id_save[i] = 0;
}

task->xhlocks = kzalloc(sizeof(struct hist_lock) * MAX_XHLOCKS_NR,