Revision a9ce385344f916cd1c36a33905e564f5581beae9 authored by Jens Axboe on 15 September 2023, 19:14:23 UTC, committed by Mike Snitzer on 15 September 2023, 19:39:59 UTC
dm looks up the table for IO based on the request type, with an assumption that if the request is marked REQ_NOWAIT, it's fine to attempt to submit that IO while under RCU read lock protection. This is not OK, as REQ_NOWAIT just means that we should not be sleeping waiting on other IO, it does not mean that we can't potentially schedule. A simple test case demonstrates this quite nicely: int main(int argc, char *argv[]) { struct iovec iov; int fd; fd = open("/dev/dm-0", O_RDONLY | O_DIRECT); posix_memalign(&iov.iov_base, 4096, 4096); iov.iov_len = 4096; preadv2(fd, &iov, 1, 0, RWF_NOWAIT); return 0; } which will instantly spew: BUG: sleeping function called from invalid context at include/linux/sched/mm.h:306 in_atomic(): 0, irqs_disabled(): 0, non_block: 0, pid: 5580, name: dm-nowait preempt_count: 0, expected: 0 RCU nest depth: 1, expected: 0 INFO: lockdep is turned off. CPU: 7 PID: 5580 Comm: dm-nowait Not tainted 6.6.0-rc1-g39956d2dcd81 #132 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014 Call Trace: <TASK> dump_stack_lvl+0x11d/0x1b0 __might_resched+0x3c3/0x5e0 ? preempt_count_sub+0x150/0x150 mempool_alloc+0x1e2/0x390 ? mempool_resize+0x7d0/0x7d0 ? lock_sync+0x190/0x190 ? lock_release+0x4b7/0x670 ? internal_get_user_pages_fast+0x868/0x2d40 bio_alloc_bioset+0x417/0x8c0 ? bvec_alloc+0x200/0x200 ? internal_get_user_pages_fast+0xb8c/0x2d40 bio_alloc_clone+0x53/0x100 dm_submit_bio+0x27f/0x1a20 ? lock_release+0x4b7/0x670 ? blk_try_enter_queue+0x1a0/0x4d0 ? dm_dax_direct_access+0x260/0x260 ? rcu_is_watching+0x12/0xb0 ? blk_try_enter_queue+0x1cc/0x4d0 __submit_bio+0x239/0x310 ? __bio_queue_enter+0x700/0x700 ? kvm_clock_get_cycles+0x40/0x60 ? ktime_get+0x285/0x470 submit_bio_noacct_nocheck+0x4d9/0xb80 ? should_fail_request+0x80/0x80 ? preempt_count_sub+0x150/0x150 ? lock_release+0x4b7/0x670 ? __bio_add_page+0x143/0x2d0 ? iov_iter_revert+0x27/0x360 submit_bio_noacct+0x53e/0x1b30 submit_bio_wait+0x10a/0x230 ? submit_bio_wait_endio+0x40/0x40 __blkdev_direct_IO_simple+0x4f8/0x780 ? blkdev_bio_end_io+0x4c0/0x4c0 ? stack_trace_save+0x90/0xc0 ? __bio_clone+0x3c0/0x3c0 ? lock_release+0x4b7/0x670 ? lock_sync+0x190/0x190 ? atime_needs_update+0x3bf/0x7e0 ? timestamp_truncate+0x21b/0x2d0 ? inode_owner_or_capable+0x240/0x240 blkdev_direct_IO.part.0+0x84a/0x1810 ? rcu_is_watching+0x12/0xb0 ? lock_release+0x4b7/0x670 ? blkdev_read_iter+0x40d/0x530 ? reacquire_held_locks+0x4e0/0x4e0 ? __blkdev_direct_IO_simple+0x780/0x780 ? rcu_is_watching+0x12/0xb0 ? __mark_inode_dirty+0x297/0xd50 ? preempt_count_add+0x72/0x140 blkdev_read_iter+0x2a4/0x530 do_iter_readv_writev+0x2f2/0x3c0 ? generic_copy_file_range+0x1d0/0x1d0 ? fsnotify_perm.part.0+0x25d/0x630 ? security_file_permission+0xd8/0x100 do_iter_read+0x31b/0x880 ? import_iovec+0x10b/0x140 vfs_readv+0x12d/0x1a0 ? vfs_iter_read+0xb0/0xb0 ? rcu_is_watching+0x12/0xb0 ? rcu_is_watching+0x12/0xb0 ? lock_release+0x4b7/0x670 do_preadv+0x1b3/0x260 ? do_readv+0x370/0x370 __x64_sys_preadv2+0xef/0x150 do_syscall_64+0x39/0xb0 entry_SYSCALL_64_after_hwframe+0x63/0xcd RIP: 0033:0x7f5af41ad806 Code: 41 54 41 89 fc 55 44 89 c5 53 48 89 cb 48 83 ec 18 80 3d e4 dd 0d 00 00 74 7a 45 89 c1 49 89 ca 45 31 c0 b8 47 01 00 00 0f 05 <48> 3d 00 f0 ff ff 0f 87 be 00 00 00 48 85 c0 79 4a 48 8b 0d da 55 RSP: 002b:00007ffd3145c7f0 EFLAGS: 00000246 ORIG_RAX: 0000000000000147 RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f5af41ad806 RDX: 0000000000000001 RSI: 00007ffd3145c850 RDI: 0000000000000003 RBP: 0000000000000008 R08: 0000000000000000 R09: 0000000000000008 R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000003 R13: 00007ffd3145c850 R14: 000055f5f0431dd8 R15: 0000000000000001 </TASK> where in fact it is dm itself that attempts to allocate a bio clone with GFP_NOIO under the rcu read lock, regardless of the request type. Fix this by getting rid of the special casing for REQ_NOWAIT, and just use the normal SRCU protected table lookup. Get rid of the bio based table locking helpers at the same time, as they are now unused. Cc: stable@vger.kernel.org Fixes: 563a225c9fd2 ("dm: introduce dm_{get,put}_live_table_bio called from dm_submit_bio") Signed-off-by: Jens Axboe <axboe@kernel.dk> Signed-off-by: Mike Snitzer <snitzer@kernel.org>
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hidreport-parsing.rst
.. SPDX-License-Identifier: GPL-2.0
========================================
Manual parsing of HID report descriptors
========================================
Consider again the mouse HID report descriptor
introduced in Documentation/hid/hidintro.rst::
$ hexdump -C /sys/bus/hid/devices/0003\:093A\:2510.0002/report_descriptor
00000000 05 01 09 02 a1 01 09 01 a1 00 05 09 19 01 29 03 |..............).|
00000010 15 00 25 01 75 01 95 03 81 02 75 05 95 01 81 01 |..%.u.....u.....|
00000020 05 01 09 30 09 31 09 38 15 81 25 7f 75 08 95 03 |...0.1.8..%.u...|
00000030 81 06 c0 c0 |....|
00000034
and try to parse it by hand.
Start with the first number, 0x05: it carries 2 bits for the
length of the item, 2 bits for the type of the item and 4 bits for the
function::
+----------+
| 00000101 |
+----------+
^^
---- Length of data (see HID spec 6.2.2.2)
^^
------ Type of the item (see HID spec 6.2.2.2, then jump to 6.2.2.7)
^^^^
--------- Function of the item (see HID spec 6.2.2.7, then HUT Sec 3)
In our case, the length is 1 byte, the type is ``Global`` and the
function is ``Usage Page``, thus for parsing the value 0x01 in the second byte
we need to refer to HUT Sec 3.
The second number is the actual data, and its meaning can be found in
the HUT. We have a ``Usage Page``, thus we need to refer to HUT
Sec. 3, "Usage Pages"; from there, one sees that ``0x01`` stands for
``Generic Desktop Page``.
Moving now to the second two bytes, and following the same scheme,
``0x09`` (i.e. ``00001001``) will be followed by one byte (``01``)
and is a ``Local`` item (``10``). Thus, the meaning of the remaining four bits
(``0000``) is given in the HID spec Sec. 6.2.2.8 "Local Items", so that
we have a ``Usage``. From HUT, Sec. 4, "Generic Desktop Page", we see that
0x02 stands for ``Mouse``.
The following numbers can be parsed in the same way.
![swh spinner](/static/img/swh-spinner.gif)
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