From: Rabin Vincent <rabinv@xxxxxxxx>
When a CIFS filesystem is mounted with the forcemand option and the
following command is run on it, lockdep warns about a circular locking
dependency between CifsInodeInfo::lock_sem and the inode lock.
while echo foo > hello; do :; done & while touch -c hello; do :; done
cifs_writev() takes the locks in the wrong order, but note that we can't
only flip the order around because it releases the inode lock before the
call to generic_write_sync() while it holds the lock_sem across that
call.
But, AFAICS, there is no need to hold the CifsInodeInfo::lock_sem across
the generic_write_sync() call either, so we can release both the locks
before generic_write_sync(), and change the order.
======================================================
WARNING: possible circular locking dependency detected
4.12.0-rc7+ #9 Not tainted
------------------------------------------------------
touch/487 is trying to acquire lock:
(&cifsi->lock_sem){++++..}, at: cifsFileInfo_put+0x88f/0x16a0
but task is already holding lock:
(&sb->s_type->i_mutex_key#11){+.+.+.}, at: utimes_common+0x3ad/0x870
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #1 (&sb->s_type->i_mutex_key#11){+.+.+.}:
__lock_acquire+0x1f74/0x38f0
lock_acquire+0x1cc/0x600
down_write+0x74/0x110
cifs_strict_writev+0x3cb/0x8c0
__vfs_write+0x4c1/0x930
vfs_write+0x14c/0x2d0
SyS_write+0xf7/0x240
entry_SYSCALL_64_fastpath+0x1f/0xbe
-> #0 (&cifsi->lock_sem){++++..}:
check_prevs_add+0xfa0/0x1d10
__lock_acquire+0x1f74/0x38f0
lock_acquire+0x1cc/0x600
down_write+0x74/0x110
cifsFileInfo_put+0x88f/0x16a0
cifs_setattr+0x992/0x1680
notify_change+0x61a/0xa80
utimes_common+0x3d4/0x870
do_utimes+0x1c1/0x220
SyS_utimensat+0x84/0x1a0
entry_SYSCALL_64_fastpath+0x1f/0xbe
other info that might help us debug this:
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock(&sb->s_type->i_mutex_key#11);
lock(&cifsi->lock_sem);
lock(&sb->s_type->i_mutex_key#11);
lock(&cifsi->lock_sem);
*** DEADLOCK ***
2 locks held by touch/487:
#0: (sb_writers#10){.+.+.+}, at: mnt_want_write+0x41/0xb0
#1: (&sb->s_type->i_mutex_key#11){+.+.+.}, at: utimes_common+0x3ad/0x870
stack backtrace:
CPU: 0 PID: 487 Comm: touch Not tainted 4.12.0-rc7+ #9
Call Trace:
dump_stack+0xdb/0x185
print_circular_bug+0x45b/0x790
__lock_acquire+0x1f74/0x38f0
lock_acquire+0x1cc/0x600
down_write+0x74/0x110
cifsFileInfo_put+0x88f/0x16a0
cifs_setattr+0x992/0x1680
notify_change+0x61a/0xa80
utimes_common+0x3d4/0x870
do_utimes+0x1c1/0x220
SyS_utimensat+0x84/0x1a0
entry_SYSCALL_64_fastpath+0x1f/0xbe
Fixes: 19dfc1f5f2ef03a52 ("cifs: fix the race in cifs_writev()")
Signed-off-by: Rabin Vincent <rabinv@xxxxxxxx>
---
fs/cifs/file.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/fs/cifs/file.c b/fs/cifs/file.c
index fcef706..d16fa55 100644
--- a/fs/cifs/file.c
+++ b/fs/cifs/file.c
@@ -2810,12 +2810,12 @@ cifs_writev(struct kiocb *iocb, struct iov_iter *from)
struct TCP_Server_Info *server = tlink_tcon(cfile->tlink)->ses->server;
ssize_t rc;
+ inode_lock(inode);
/*
* We need to hold the sem to be sure nobody modifies lock list
* with a brlock that prevents writing.
*/
down_read(&cinode->lock_sem);
- inode_lock(inode);
rc = generic_write_checks(iocb, from);
if (rc <= 0)
@@ -2828,11 +2828,11 @@ cifs_writev(struct kiocb *iocb, struct iov_iter *from)
else
rc = -EACCES;
out:
+ up_read(&cinode->lock_sem);
inode_unlock(inode);
if (rc > 0)
rc = generic_write_sync(iocb, rc);
- up_read(&cinode->lock_sem);
return rc;
}
--
2.1.4
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