From: David Howells dhowells@redhat.com
[ Upstream commit 598ad0bd09329818ee041cb3e4b60ba0a70cb1ee ]
Taking sb_writers whilst holding mmap_lock isn't allowed and will result in a lockdep warning like that below. The problem comes from cachefiles needing to take the sb_writers lock in order to do a write to the cache, but being asked to do this by netfslib called from readpage, readahead or write_begin[1].
Fix this by always offloading the write to the cache off to a worker thread. The main thread doesn't need to wait for it, so deadlock can be avoided.
This can be tested by running the quick xfstests on something like afs or ceph with lockdep enabled.
WARNING: possible circular locking dependency detected 5.15.0-rc1-build2+ #292 Not tainted ------------------------------------------------------ holetest/65517 is trying to acquire lock: ffff88810c81d730 (mapping.invalidate_lock#3){.+.+}-{3:3}, at: filemap_fault+0x276/0x7a5
but task is already holding lock: ffff8881595b53e8 (&mm->mmap_lock#2){++++}-{3:3}, at: do_user_addr_fault+0x28d/0x59c
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #2 (&mm->mmap_lock#2){++++}-{3:3}: validate_chain+0x3c4/0x4a8 __lock_acquire+0x89d/0x949 lock_acquire+0x2dc/0x34b __might_fault+0x87/0xb1 strncpy_from_user+0x25/0x18c removexattr+0x7c/0xe5 __do_sys_fremovexattr+0x73/0x96 do_syscall_64+0x67/0x7a entry_SYSCALL_64_after_hwframe+0x44/0xae
-> #1 (sb_writers#10){.+.+}-{0:0}: validate_chain+0x3c4/0x4a8 __lock_acquire+0x89d/0x949 lock_acquire+0x2dc/0x34b cachefiles_write+0x2b3/0x4bb netfs_rreq_do_write_to_cache+0x3b5/0x432 netfs_readpage+0x2de/0x39d filemap_read_page+0x51/0x94 filemap_get_pages+0x26f/0x413 filemap_read+0x182/0x427 new_sync_read+0xf0/0x161 vfs_read+0x118/0x16e ksys_read+0xb8/0x12e do_syscall_64+0x67/0x7a entry_SYSCALL_64_after_hwframe+0x44/0xae
-> #0 (mapping.invalidate_lock#3){.+.+}-{3:3}: check_noncircular+0xe4/0x129 check_prev_add+0x16b/0x3a4 validate_chain+0x3c4/0x4a8 __lock_acquire+0x89d/0x949 lock_acquire+0x2dc/0x34b down_read+0x40/0x4a filemap_fault+0x276/0x7a5 __do_fault+0x96/0xbf do_fault+0x262/0x35a __handle_mm_fault+0x171/0x1b5 handle_mm_fault+0x12a/0x233 do_user_addr_fault+0x3d2/0x59c exc_page_fault+0x85/0xa5 asm_exc_page_fault+0x1e/0x30
other info that might help us debug this:
Chain exists of: mapping.invalidate_lock#3 --> sb_writers#10 --> &mm->mmap_lock#2
Possible unsafe locking scenario:
CPU0 CPU1 ---- ---- lock(&mm->mmap_lock#2); lock(sb_writers#10); lock(&mm->mmap_lock#2); lock(mapping.invalidate_lock#3);
*** DEADLOCK ***
1 lock held by holetest/65517: #0: ffff8881595b53e8 (&mm->mmap_lock#2){++++}-{3:3}, at: do_user_addr_fault+0x28d/0x59c
stack backtrace: CPU: 0 PID: 65517 Comm: holetest Not tainted 5.15.0-rc1-build2+ #292 Hardware name: ASUS All Series/H97-PLUS, BIOS 2306 10/09/2014 Call Trace: dump_stack_lvl+0x45/0x59 check_noncircular+0xe4/0x129 ? print_circular_bug+0x207/0x207 ? validate_chain+0x461/0x4a8 ? add_chain_block+0x88/0xd9 ? hlist_add_head_rcu+0x49/0x53 check_prev_add+0x16b/0x3a4 validate_chain+0x3c4/0x4a8 ? check_prev_add+0x3a4/0x3a4 ? mark_lock+0xa5/0x1c6 __lock_acquire+0x89d/0x949 lock_acquire+0x2dc/0x34b ? filemap_fault+0x276/0x7a5 ? rcu_read_unlock+0x59/0x59 ? add_to_page_cache_lru+0x13c/0x13c ? lock_is_held_type+0x7b/0xd3 down_read+0x40/0x4a ? filemap_fault+0x276/0x7a5 filemap_fault+0x276/0x7a5 ? pagecache_get_page+0x2dd/0x2dd ? __lock_acquire+0x8bc/0x949 ? pte_offset_kernel.isra.0+0x6d/0xc3 __do_fault+0x96/0xbf ? do_fault+0x124/0x35a do_fault+0x262/0x35a ? handle_pte_fault+0x1c1/0x20d __handle_mm_fault+0x171/0x1b5 ? handle_pte_fault+0x20d/0x20d ? __lock_release+0x151/0x254 ? mark_held_locks+0x1f/0x78 ? rcu_read_unlock+0x3a/0x59 handle_mm_fault+0x12a/0x233 do_user_addr_fault+0x3d2/0x59c ? pgtable_bad+0x70/0x70 ? rcu_read_lock_bh_held+0xab/0xab exc_page_fault+0x85/0xa5 ? asm_exc_page_fault+0x8/0x30 asm_exc_page_fault+0x1e/0x30 RIP: 0033:0x40192f Code: ff 48 89 c3 48 8b 05 50 28 00 00 48 85 ed 7e 23 31 d2 4b 8d 0c 2f eb 0a 0f 1f 00 48 8b 05 39 28 00 00 48 0f af c2 48 83 c2 01 <48> 89 1c 01 48 39 d5 7f e8 8b 0d f2 27 00 00 31 c0 85 c9 74 0e 8b RSP: 002b:00007f9931867eb0 EFLAGS: 00010202 RAX: 0000000000000000 RBX: 00007f9931868700 RCX: 00007f993206ac00 RDX: 0000000000000001 RSI: 0000000000000000 RDI: 00007ffc13e06ee0 RBP: 0000000000000100 R08: 0000000000000000 R09: 00007f9931868700 R10: 00007f99318689d0 R11: 0000000000000202 R12: 00007ffc13e06ee0 R13: 0000000000000c00 R14: 00007ffc13e06e00 R15: 00007f993206a000
Fixes: 726218fdc22c ("netfs: Define an interface to talk to a cache") Signed-off-by: David Howells dhowells@redhat.com Tested-by: Jeff Layton jlayton@kernel.org cc: Jan Kara jack@suse.cz cc: linux-cachefs@redhat.com cc: linux-fsdevel@vger.kernel.org Link: https://lore.kernel.org/r/20210922110420.GA21576@quack2.suse.cz/ [1] Link: https://lore.kernel.org/r/163887597541.1596626.2668163316598972956.stgit@war... # v1 Signed-off-by: Sasha Levin sashal@kernel.org --- fs/netfs/read_helper.c | 15 +++++---------- 1 file changed, 5 insertions(+), 10 deletions(-)
diff --git a/fs/netfs/read_helper.c b/fs/netfs/read_helper.c index 994ec22d40402..4b54529f8176b 100644 --- a/fs/netfs/read_helper.c +++ b/fs/netfs/read_helper.c @@ -354,16 +354,11 @@ static void netfs_rreq_write_to_cache_work(struct work_struct *work) netfs_rreq_do_write_to_cache(rreq); }
-static void netfs_rreq_write_to_cache(struct netfs_read_request *rreq, - bool was_async) +static void netfs_rreq_write_to_cache(struct netfs_read_request *rreq) { - if (was_async) { - rreq->work.func = netfs_rreq_write_to_cache_work; - if (!queue_work(system_unbound_wq, &rreq->work)) - BUG(); - } else { - netfs_rreq_do_write_to_cache(rreq); - } + rreq->work.func = netfs_rreq_write_to_cache_work; + if (!queue_work(system_unbound_wq, &rreq->work)) + BUG(); }
/* @@ -560,7 +555,7 @@ static void netfs_rreq_assess(struct netfs_read_request *rreq, bool was_async) wake_up_bit(&rreq->flags, NETFS_RREQ_IN_PROGRESS);
if (test_bit(NETFS_RREQ_WRITE_TO_CACHE, &rreq->flags)) - return netfs_rreq_write_to_cache(rreq, was_async); + return netfs_rreq_write_to_cache(rreq);
netfs_rreq_completed(rreq, was_async); }