bcachefs: Fix bch2_btree_transactions_read() synchronization

Since we're accessing btree_trans objects owned by another thread, we
need to guard against using pointers to freed key cache entries: we need
our own srcu read lock, and we should skip a btree_trans if it didn't
hold the srcu lock (and thus it might have pointers to freed key cache
entries).

00693 Mem abort info:
00693   ESR = 0x0000000096000005
00693   EC = 0x25: DABT (current EL), IL = 32 bits
00693   SET = 0, FnV = 0
00693   EA = 0, S1PTW = 0
00693   FSC = 0x05: level 1 translation fault
00693 Data abort info:
00693   ISV = 0, ISS = 0x00000005, ISS2 = 0x00000000
00693   CM = 0, WnR = 0, TnD = 0, TagAccess = 0
00693   GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
00693 user pgtable: 4k pages, 39-bit VAs, pgdp=000000012e650000
00693 [000000008fb96218] pgd=0000000000000000, p4d=0000000000000000, pud=0000000000000000
00693 Internal error: Oops: 0000000096000005 [kernel-patches#1]  SMP
00693 Modules linked in:
00693 CPU: 0 UID: 0 PID: 4307 Comm: cat Not tainted 6.16.0-rc2-ktest-g9e15af94fd86 #27578 NONE
00693 Hardware name: linux,dummy-virt (DT)
00693 pstate: 60001005 (nZCv daif -PAN -UAO -TCO -DIT +SSBS BTYPE=--)
00693 pc : six_lock_counts+0x20/0xe8
00693 lr : bch2_btree_bkey_cached_common_to_text+0x38/0x130
00693 sp : ffffff80ca98bb60
00693 x29: ffffff80ca98bb60 x28: 000000008fb96200 x27: 0000000000000007
00693 x26: ffffff80eafd06b8 x25: 0000000000000000 x24: ffffffc080d75a60
00693 x23: ffffff80eafd0000 x22: ffffffc080bdfcc0 x21: ffffff80eafd0210
00693 x20: ffffff80c192ff08 x19: 000000008fb96200 x18: 00000000ffffffff
00693 x17: 0000000000000000 x16: 0000000000000000 x15: 00000000ffffffff
00693 x14: 0000000000000000 x13: ffffff80ceb5a29a x12: 20796220646c6568
00693 x11: 72205d3e303c5b20 x10: 0000000000000020 x9 : ffffffc0805fb6b0
00693 x8 : 0000000000000020 x7 : 0000000000000000 x6 : 0000000000000020
00693 x5 : ffffff80ceb5a29c x4 : 0000000000000001 x3 : 000000000000029c
00693 x2 : 0000000000000000 x1 : ffffff80ef66c000 x0 : 000000008fb96200
00693 Call trace:
00693  six_lock_counts+0x20/0xe8 (P)
00693  bch2_btree_bkey_cached_common_to_text+0x38/0x130
00693  bch2_btree_trans_to_text+0x260/0x2a8
00693  bch2_btree_transactions_read+0xac/0x1e8
00693  full_proxy_read+0x74/0xd8
00693  vfs_read+0x90/0x300
00693  ksys_read+0x6c/0x108
00693  __arm64_sys_read+0x20/0x30
00693  invoke_syscall.constprop.0+0x54/0xe8
00693  do_el0_svc+0x44/0xc8
00693  el0_svc+0x18/0x58
00693  el0t_64_sync_handler+0x104/0x130
00693  el0t_64_sync+0x154/0x158
00693 Code: 910003fd f9423c22 f90017e2 d2800002 (f9400c01)
00693 ---[ end trace 0000000000000000 ]---

Signed-off-by: Kent Overstreet <[email protected]>

Author: Kent Overstreet

fs/bcachefs/debug.c

@@ -584,6 +584,8 @@ static ssize_t bch2_btree_transactions_read(struct file *file, char __user *buf,
 	i->ubuf = buf;
 	i->size	= size;
 	i->ret	= 0;
+
+	int srcu_idx = srcu_read_lock(&c->btree_trans_barrier);
 restart:
 	seqmutex_lock(&c->btree_trans_lock);
 	list_sort(&c->btree_trans_list, list_ptr_order_cmp);
@@ -597,6 +599,11 @@ static ssize_t bch2_btree_transactions_read(struct file *file, char __user *buf,
 		if (!closure_get_not_zero(&trans->ref))
 			continue;
 
+		if (!trans->srcu_held) {
+			closure_put(&trans->ref);
+			continue;
+		}
+
 		u32 seq = seqmutex_unlock(&c->btree_trans_lock);
 
 		bch2_btree_trans_to_text(&i->buf, trans);
@@ -618,6 +625,8 @@ static ssize_t bch2_btree_transactions_read(struct file *file, char __user *buf,
 	}
 	seqmutex_unlock(&c->btree_trans_lock);
 unlocked:
+	srcu_read_unlock(&c->btree_trans_barrier, srcu_idx);
+
 	if (i->buf.allocation_failure)
 		ret = -ENOMEM;