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bcachefs: Fix race between trans_put() and btree_transactions_read()

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debug.c was using closure_get() on a different thread's closure where
the we don't know if the object being refcounted is alive.

We keep btree_trans objects on a list so they can be printed by debug
code, and because it is cost prohibitive to touch the btree_trans list
every time we allocate and free btree_trans objects, cached objects are
also on this list.

However, we do not want the debug code to see cached but not in use
btree_trans objects - critically because the btree_paths array will have
been freed (if it was reallocated).

closure_get() is also incorrect to use when that get may race with it
hitting zero, i.e. we must already have a ref on the object or know the
ref can't currently hit 0 for other reasons (as used in the cycle
detector).

to fix this, use the previously introduced closure_get_not_zero(),
closure_return_sync(), and closure_init_stack_release(); the debug code
now can only take a ref on a trans object if it's alive and in use.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
This commit is contained in:
Kent Overstreet 2024-06-22 22:02:09 -04:00
parent 06efa5f30c
commit de611ab6fc
2 changed files with 13 additions and 16 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

10
fs/bcachefs/btree_iter.c
View File

@ -3130,7 +3130,6 @@ struct btree_trans *__bch2_trans_get(struct bch_fs *c, unsigned fn_idx)
trans = mempool_alloc(&c->btree_trans_pool, GFP_NOFS);
memset(trans, 0, sizeof(*trans));
closure_init_stack(&trans->ref);
seqmutex_lock(&c->btree_trans_lock);
if (IS_ENABLED(CONFIG_BCACHEFS_DEBUG)) {
@ -3161,7 +3160,6 @@ struct btree_trans *__bch2_trans_get(struct bch_fs *c, unsigned fn_idx)
list_add_done:
seqmutex_unlock(&c->btree_trans_lock);
got_trans:
trans->ref.closure_get_happened = false;
trans->c = c;
trans->last_begin_time = local_clock();
trans->fn_idx = fn_idx;
@ -3200,6 +3198,8 @@ got_trans:
trans->srcu_idx = srcu_read_lock(&c->btree_trans_barrier);
trans->srcu_lock_time = jiffies;
trans->srcu_held = true;
closure_init_stack_release(&trans->ref);
return trans;
}
@ -3257,10 +3257,10 @@ void bch2_trans_put(struct btree_trans *trans)
bch2_journal_keys_put(c);
/*
* trans->ref protects trans->locking_wait.task, btree_paths arary; used
* trans->ref protects trans->locking_wait.task, btree_paths array; used
* by cycle detector
*/
closure_sync(&trans->ref);
closure_return_sync(&trans->ref);
trans->locking_wait.task = NULL;
unsigned long *paths_allocated = trans->paths_allocated;
@ -3385,8 +3385,6 @@ void bch2_fs_btree_iter_exit(struct bch_fs *c)
per_cpu_ptr(c->btree_trans_bufs, cpu)->trans;
if (trans) {
closure_sync(&trans->ref);
seqmutex_lock(&c->btree_trans_lock);
list_del(&trans->list);
seqmutex_unlock(&c->btree_trans_lock);

19
fs/bcachefs/debug.c
View File

@ -587,14 +587,10 @@ restart:
if (!task || task->pid <= i->iter)
continue;
closure_get(&trans->ref);
u32 seq = seqmutex_unlock(&c->btree_trans_lock);
if (!closure_get_not_zero(&trans->ref))
continue;
ret = flush_buf(i);
if (ret) {
closure_put(&trans->ref);
goto unlocked;
}
u32 seq = seqmutex_unlock(&c->btree_trans_lock);
bch2_btree_trans_to_text(&i->buf, trans);
@ -604,10 +600,12 @@ restart:
printbuf_indent_sub(&i->buf, 2);
prt_newline(&i->buf);
i->iter = task->pid;
closure_put(&trans->ref);
ret = flush_buf(i);
if (ret)
goto unlocked;
if (!seqmutex_relock(&c->btree_trans_lock, seq))
goto restart;
}
@ -817,7 +815,8 @@ restart:
iter = task->pid;
closure_get(&trans->ref);
if (!closure_get_not_zero(&trans->ref))
continue;
u32 seq = seqmutex_unlock(&c->btree_trans_lock);