bcachefs: Fix race in print_chain()

00636 Unable to handle kernel NULL pointer dereference at virtual address 00000000000000b0
00636 Mem abort info:
00636   ESR = 0x0000000096000005
00636   EC = 0x25: DABT (current EL), IL = 32 bits
00636   SET = 0, FnV = 0
00636   EA = 0, S1PTW = 0
00636   FSC = 0x05: level 1 translation fault
00636 Data abort info:
00636   ISV = 0, ISS = 0x00000005, ISS2 = 0x00000000
00636   CM = 0, WnR = 0, TnD = 0, TagAccess = 0
00636   GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
00636 user pgtable: 4k pages, 39-bit VAs, pgdp=0000000101b10000
00636 [00000000000000b0] pgd=0000000000000000, p4d=0000000000000000, pud=0000000000000000
00636 Internal error: Oops: 0000000096000005 [#1] SMP
00636 Modules linked in:
00636 CPU: 12 UID: 0 PID: 79369 Comm: cat Not tainted 6.14.0-rc6-ktest-g3783b8973ab7 #17757
00636 Hardware name: linux,dummy-virt (DT)
00636 pstate: 20001005 (nzCv daif -PAN -UAO -TCO -DIT +SSBS BTYPE=--)
00636 pc : print_chain+0xb8/0x170
00636 lr : print_chain+0xa0/0x170
00636 sp : ffffff80d9c1bbb0
00636 x29: ffffff80d9c1bbb0 x28: 0000000000000002 x27: ffffff80c1be8250
00636 x26: ffffff80dd9b0000 x25: 0000000000000020 x24: 000000000000002d
00636 x23: 000000000000003c x22: ffffffc080a54518 x21: ffffff80da6e00d0
00636 x20: ffffff80da6e0170 x19: ffffff80c1a1d240 x18: 00000000ffffffff
00636 x17: 3535303937202d3c x16: 203139202d3c2035 x15: 00000000ffffffff
00636 x14: 0000000000000000 x13: ffffff80d71b63f1 x12: 0000000000000006
00636 x11: ffffffc080beb1c0 x10: 0000000000000020 x9 : 00000000000134cc
00636 x8 : 0000000000000020 x7 : 0000000000000004 x6 : 0000000000000020
00636 x5 : ffffff80d71b63f7 x4 : ffffffc080a5451b x3 : 0000000000000000
00636 x2 : 0000000000000000 x1 : 0000000000000000 x0 : 0000000000000000
00636 Call trace:
00636  print_chain+0xb8/0x170 (P)
00636  bch2_check_for_deadlock+0x444/0x5a0
00636  bch2_btree_deadlock_read+0xb4/0x1c8
00636  full_proxy_read+0x74/0xd8
00636  vfs_read+0x90/0x300

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

Author: Kent Overstreet

fs/bcachefs/btree_locking.c

@@ -91,10 +91,10 @@ static noinline void print_chain(struct printbuf *out, struct lock_graph *g)
 	struct trans_waiting_for_lock *i;
 
 	for (i = g->g; i != g->g + g->nr; i++) {
-		struct task_struct *task = i->trans->locking_wait.task;
+		struct task_struct *task = READ_ONCE(i->trans->locking_wait.task);
 		if (i != g->g)
 			prt_str(out, "<- ");
-		prt_printf(out, "%u ", task ?task->pid : 0);
+		prt_printf(out, "%u ", task ? task->pid : 0);
 	}
 	prt_newline(out);
 }