unimore/kernel-hacking-2024-linux-steffo
unimore/kernel-hacking-2024-linux-steffo
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bcachefs: Print deadlock cycle in debugfs

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In the event that we're not finished debugging the cycle detector, this
adds a new file to debugfs that shows what the cycle detector finds, if
anything. By comparing this with btree_transactions, which shows held
locks for every btree_transaction, we'll be able to determine if it's
the cycle detector that's buggy or something else.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
This commit is contained in:
Kent Overstreet 2022-08-22 23:12:11 -04:00
parent 33bd5d0686
commit 96d994b37c
4 changed files with 69 additions and 21 deletions
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2
fs/bcachefs/btree_iter.c
View file

@ -2992,7 +2992,6 @@ bch2_btree_path_node_to_text(struct printbuf *out,
c.n[0], c.n[1], c.n[2], pid);
}
#ifdef CONFIG_BCACHEFS_DEBUG_TRANSACTIONS
void bch2_btree_trans_to_text(struct printbuf *out, struct btree_trans *trans)
{
struct btree_path *path;
@ -3041,7 +3040,6 @@ void bch2_btree_trans_to_text(struct printbuf *out, struct btree_trans *trans)
prt_printf(out, "\n");
}
}
#endif
void bch2_fs_btree_iter_exit(struct bch_fs *c)
{

44
fs/bcachefs/btree_locking.c
View file

@ -76,6 +76,17 @@ static void lock_graph_pop(struct lock_graph *g)
closure_put(&g->g[--g->nr].trans->ref);
}
static noinline void print_cycle(struct printbuf *out, struct lock_graph *g)
{
struct trans_waiting_for_lock *i;
prt_printf(out, "Found lock cycle (%u entries):", g->nr);
prt_newline(out);
for (i = g->g; i < g->g + g->nr; i++)
bch2_btree_trans_to_text(out, i->trans);
}
static int abort_lock(struct lock_graph *g, struct trans_waiting_for_lock *i)
{
int ret;
@ -122,7 +133,8 @@ static noinline int break_cycle(struct lock_graph *g)
BUG();
}
static int lock_graph_descend(struct lock_graph *g, struct btree_trans *trans)
static int lock_graph_descend(struct lock_graph *g, struct btree_trans *trans,
struct printbuf *cycle)
{
struct btree_trans *orig_trans = g->g->trans;
struct trans_waiting_for_lock *i;
@ -136,7 +148,14 @@ static int lock_graph_descend(struct lock_graph *g, struct btree_trans *trans)
}
if (i->trans == trans) {
ret = break_cycle(g);
if (cycle) {
/* Only checking: */
print_cycle(cycle, g);
ret = -1;
} else {
ret = break_cycle(g);
}
if (ret)
goto deadlock;
/*
@ -170,19 +189,6 @@ static int lock_graph_descend(struct lock_graph *g, struct btree_trans *trans)
return ret;
}
#if 0
static void print_cycle(struct printbuf *out, struct lock_graph *g)
{
struct trans_waiting_for_lock *i;
prt_str(out, "Found lock cycle:");
prt_newline(out);
for (i = g->g; i < g->g + g->nr; i++)
bch2_btree_trans_to_text(out, i->trans);
}
#endif
static noinline void lock_graph_remove_non_waiters(struct lock_graph *g)
{
struct trans_waiting_for_lock *i;
@ -202,7 +208,7 @@ static bool lock_type_conflicts(enum six_lock_type t1, enum six_lock_type t2)
return t1 + t2 > 1;
}
static int check_for_deadlock(struct btree_trans *trans)
int bch2_check_for_deadlock(struct btree_trans *trans, struct printbuf *cycle)
{
struct lock_graph g;
struct trans_waiting_for_lock *top;
@ -214,7 +220,7 @@ static int check_for_deadlock(struct btree_trans *trans)
return btree_trans_restart(trans, BCH_ERR_transaction_restart_would_deadlock);
g.nr = 0;
ret = lock_graph_descend(&g, trans);
ret = lock_graph_descend(&g, trans, cycle);
BUG_ON(ret);
next:
if (!g.nr)
@ -265,7 +271,7 @@ static int check_for_deadlock(struct btree_trans *trans)
!lock_type_conflicts(lock_held, trans->locking_wait.lock_want))
continue;
ret = lock_graph_descend(&g, trans);
ret = lock_graph_descend(&g, trans, cycle);
raw_spin_unlock(&b->lock.wait_lock);
if (ret)
@ -285,7 +291,7 @@ int bch2_six_check_for_deadlock(struct six_lock *lock, void *p)
{
struct btree_trans *trans = p;
return check_for_deadlock(trans);
return bch2_check_for_deadlock(trans, NULL);
}
int __bch2_btree_node_lock_write(struct btree_trans *trans,

1
fs/bcachefs/btree_locking.h
View file

@ -426,6 +426,7 @@ struct six_lock_count bch2_btree_node_lock_counts(struct btree_trans *,
struct btree_bkey_cached_common *b,
unsigned);
int bch2_check_for_deadlock(struct btree_trans *, struct printbuf *);
#ifdef CONFIG_BCACHEFS_DEBUG
void bch2_btree_path_verify_locks(struct btree_path *);

43
fs/bcachefs/debug.c
View file

@ -11,6 +11,7 @@
#include "btree_cache.h"
#include "btree_io.h"
#include "btree_iter.h"
#include "btree_locking.h"
#include "btree_update.h"
#include "buckets.h"
#include "debug.h"
@ -708,6 +709,45 @@ static const struct file_operations lock_held_stats_op = {
.read = lock_held_stats_read,
};
static ssize_t bch2_btree_deadlock_read(struct file *file, char __user *buf,
size_t size, loff_t *ppos)
{
struct dump_iter *i = file->private_data;
struct bch_fs *c = i->c;
struct btree_trans *trans;
ssize_t ret = 0;
i->ubuf = buf;
i->size = size;
i->ret = 0;
if (i->iter)
goto out;
mutex_lock(&c->btree_trans_lock);
list_for_each_entry(trans, &c->btree_trans_list, list)
if (bch2_check_for_deadlock(trans, &i->buf)) {
i->iter = 1;
break;
}
mutex_unlock(&c->btree_trans_lock);
out:
if (i->buf.allocation_failure)
ret = -ENOMEM;
if (!ret)
ret = flush_buf(i);
return ret ?: i->ret;
}
static const struct file_operations btree_deadlock_ops = {
.owner = THIS_MODULE,
.open = bch2_dump_open,
.release = bch2_dump_release,
.read = bch2_btree_deadlock_read,
};
void bch2_fs_debug_exit(struct bch_fs *c)
{
if (!IS_ERR_OR_NULL(c->fs_debug_dir))
@ -741,6 +781,9 @@ void bch2_fs_debug_init(struct bch_fs *c)
debugfs_create_file("btree_transaction_stats", 0400, c->fs_debug_dir,
c, &lock_held_stats_op);
debugfs_create_file("btree_deadlock", 0400, c->fs_debug_dir,
c->btree_debug, &btree_deadlock_ops);
c->btree_debug_dir = debugfs_create_dir("btrees", c->fs_debug_dir);
if (IS_ERR_OR_NULL(c->btree_debug_dir))
return;