On Wed, 2024-01-31 at 11:10 -0500, Dave Wysochanski wrote:
> 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 <jlayton@xxxxxxxxxx>
> Signed-off-by: Dave Wysochanski <dwysocha@xxxxxxxxxx>
> ---
> fs/nfs/fscache.c | 9 ++-------
> 1 file changed, 2 insertions(+), 7 deletions(-)
>
> diff --git a/fs/nfs/fscache.c b/fs/nfs/fscache.c
> index b05717fe0d4e..60a3c28784e0 100644
> --- a/fs/nfs/fscache.c
> +++ b/fs/nfs/fscache.c
> @@ -307,11 +307,11 @@ static void nfs_netfs_issue_read(struct netfs_io_subrequest *sreq)
> 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);
> @@ -322,19 +322,14 @@ static void nfs_netfs_issue_read(struct netfs_io_subrequest *sreq)
>
> 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);
Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>
