NFS: Fix nfs_netfs_issue_read() xarray locking for writeback interrupt

The loop inside nfs_netfs_issue_read() currently does not disable
interrupts while iterating through pages in the xarray to submit
for NFS read. This is not safe though since after taking xa_lock,
another page in the mapping could be processed for writeback inside
an interrupt, and deadlock can occur. The fix is simple and clean
if we use xa_for_each_range(), which handles the iteration with RCU
while reducing code complexity.

The problem is easily reproduced with the following test:
mount -o vers=3,fsc 127.0.0.1:/export /mnt/nfs
dd if=/dev/zero of=/mnt/nfs/file1.bin bs=4096 count=1
echo 3 > /proc/sys/vm/drop_caches
dd if=/mnt/nfs/file1.bin of=/dev/null
umount /mnt/nfs

On the console with a lockdep-enabled kernel a message similar to
the following will be seen:

================================
WARNING: inconsistent lock state
6.7.0-lockdbg+ #10 Not tainted
--------------------------------
inconsistent {IN-SOFTIRQ-W} -> {SOFTIRQ-ON-W} usage.
test5/1708 [HC0[0]:SC0[0]:HE1:SE1] takes:
ffff888127baa598 (&xa->xa_lock#4){+.?.}-{3:3}, at:
nfs_netfs_issue_read+0x1b2/0x4b0 [nfs]
{IN-SOFTIRQ-W} state was registered at:
lock_acquire+0x144/0x380
_raw_spin_lock_irqsave+0x4e/0xa0
__folio_end_writeback+0x17e/0x5c0
folio_end_writeback+0x93/0x1b0
iomap_finish_ioend+0xeb/0x6a0
blk_update_request+0x204/0x7f0
blk_mq_end_request+0x30/0x1c0
blk_complete_reqs+0x7e/0xa0
__do_softirq+0x113/0x544
__irq_exit_rcu+0xfe/0x120
irq_exit_rcu+0xe/0x20
sysvec_call_function_single+0x6f/0x90
asm_sysvec_call_function_single+0x1a/0x20
pv_native_safe_halt+0xf/0x20
default_idle+0x9/0x20
default_idle_call+0x67/0xa0
do_idle+0x2b5/0x300
cpu_startup_entry+0x34/0x40
start_secondary+0x19d/0x1c0
secondary_startup_64_no_verify+0x18f/0x19b
irq event stamp: 176891
hardirqs last enabled at (176891): [<ffffffffa67a0be4>]
_raw_spin_unlock_irqrestore+0x44/0x60
hardirqs last disabled at (176890): [<ffffffffa67a0899>]
_raw_spin_lock_irqsave+0x79/0xa0
softirqs last enabled at (176646): [<ffffffffa515d91e>]
__irq_exit_rcu+0xfe/0x120
softirqs last disabled at (176633): [<ffffffffa515d91e>]
__irq_exit_rcu+0xfe/0x120

other info that might help us debug this:
Possible unsafe locking scenario:

CPU0
----
lock(&xa->xa_lock#4);
<Interrupt>
lock(&xa->xa_lock#4);

*** DEADLOCK ***

2 locks held by test5/1708:
#0: ffff888127baa498 (&sb->s_type->i_mutex_key#22){++++}-{4:4}, at:
nfs_start_io_read+0x28/0x90 [nfs]
#1: ffff888127baa650 (mapping.invalidate_lock#3){.+.+}-{4:4}, at:
page_cache_ra_unbounded+0xa4/0x280

stack backtrace:
CPU: 6 PID: 1708 Comm: test5 Kdump: loaded Not tainted 6.7.0-lockdbg+
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-1.fc39
04/01/2014
Call Trace:
dump_stack_lvl+0x5b/0x90
mark_lock+0xb3f/0xd20
__lock_acquire+0x77b/0x3360
_raw_spin_lock+0x34/0x80
nfs_netfs_issue_read+0x1b2/0x4b0 [nfs]
netfs_begin_read+0x77f/0x980 [netfs]
nfs_netfs_readahead+0x45/0x60 [nfs]
nfs_readahead+0x323/0x5a0 [nfs]
read_pages+0xf3/0x5c0
page_cache_ra_unbounded+0x1c8/0x280
filemap_get_pages+0x38c/0xae0
filemap_read+0x206/0x5e0
nfs_file_read+0xb7/0x140 [nfs]
vfs_read+0x2a9/0x460
ksys_read+0xb7/0x140

Fixes: 000dbe0bec05 ("NFS: Convert buffered read paths to use netfs when fscache is enabled")
Suggested-by: Jeff Layton <[e-mail]>
Signed-off-by: Dave Wysochanski <[e-mail]>
Reviewed-by: Jeff Layton <[e-mail]>
Reviewed-by: David Howells <[e-mail]>
Signed-off-by: Trond Myklebust <[e-mail]>

fs/nfs/fscache.c

···
 	struct inode *inode = sreq->rreq->inode;
 	struct nfs_open_context *ctx = sreq->rreq->netfs_priv;
 	struct page *page;
+	unsigned long idx;
 	int err;
 	pgoff_t start = (sreq->start + sreq->transferred) >> PAGE_SHIFT;
 	pgoff_t last = ((sreq->start + sreq->len -
 			 sreq->transferred - 1) >> PAGE_SHIFT);
-	XA_STATE(xas, &sreq->rreq->mapping->i_pages, start);
 
 	nfs_pageio_init_read(&pgio, inode, false,
 			     &nfs_async_read_completion_ops);
···
 
 	pgio.pg_netfs = netfs; /* used in completion */
 
-	xas_lock(&xas);
-	xas_for_each(&xas, page, last) {
+	xa_for_each_range(&sreq->rreq->mapping->i_pages, idx, page, start, last) {
 		/* nfs_read_add_folio() may schedule() due to pNFS layout and other RPCs  */
-		xas_pause(&xas);
-		xas_unlock(&xas);
 		err = nfs_read_add_folio(&pgio, ctx, page_folio(page));
 		if (err < 0) {
 			netfs->error = err;
 			goto out;
 		}
-		xas_lock(&xas);
 	}
-	xas_unlock(&xas);
 out:
 	nfs_pageio_complete_read(&pgio);
 	nfs_netfs_put(netfs);