----- Original Message -----
> From: "Dave Wysochanski" <dwysocha@xxxxxxxxxx>
> To: linux-cifs@xxxxxxxxxxxxxxx
> Sent: Wednesday, 23 October, 2019 6:33:43 AM
> Subject: [RFC PATCH] cifs: Fix cifsInodeInfo lock_sem deadlock with multiple readers
>
> NOTE: I have verified this fixes the problem but have not run
> locking tests yet.
>
> There's a deadlock that is possible that can easily be seen with
> multiple readers open/read/close of the same file. The deadlock
> is due to a reader calling down_read(lock_sem) and holding
> it across the full IO, even if a network or server disruption
> occurs and the session has to be reconnected. Upon reconnect,
> cifs_relock_file is called where down_read(lock_sem) is called
> a second time. Normally this is not a problem, but if there is
> another process that calls down_write(lock_sem) in between the
> first and second reader call to down_read(lock_sem), this will
> cause a deadlock. The caller of down_write (often either
> _cifsFileInfo_put that is just removing and freeing cifsLockInfo
> structures from the list of locks, or cifs_new_fileinfo, which
> is just attaching cifs_fid_locks to cifsInodeInfo->llist), will
> block due to the reader's first down_read(lock_sem) that obtains
> the semaphore (read IO in flight). And then when the server
> comes back up, the reader that holds calls down_read(lock_sem)
> a second time, and this time is blocked too because of the
> blocked in down_write (rw_semaphores would starve writers if
> this was not the case). Interestingly enough, the callers of
> down_write in the simple test case was not adding a
> conflicting lock at all, just either opening or closing the
> file, and modifying the list of locks attached to cifsInodeInfo,
> this ends up tripping up the reader process and causing the
> deadlock.
>
> The root of the problem is that lock_sem both protects the
> cifsInodeInfo fields (such as the lllist - the list of locks),
> but is also being re-used to avoid a conflicting lock coming
> in while IO is in flight. Add a new semaphore that tracks
> just the IO in flight, and must be obtained before adding
> a new lock. While this does add another layer of complexity
> and a semaphore ordering that must be obeyed to avoid new
> deadlocks, it does clealy solve the underlying problem.
The patch is hard to read(not your fault) since "patch" decided that almost all changes are in cifs_closedir() :-(
You are reverting 560d388950. it is unfortunate because I think we should make either cifs_reopen_file() or cifs_strict_readv()
using down_read_nested() to suppress the warnings from the validator or else we will get a lot of these log entries in dmesg
(almost) everytime we get a reconnect.
A different approach, could be to change _cifsFileInfo_put() to use a down_write_trylock()-sleep() loop instead of a blocking down_write() call.
I.e. something like this ?
(and then the same at the other places where we have a deadlock vulnerable down_write() call)
diff --git a/fs/cifs/file.c b/fs/cifs/file.c
index 936e03892e2a..530af080dc61 100644
--- a/fs/cifs/file.c
+++ b/fs/cifs/file.c
@@ -464,7 +464,8 @@ void _cifsFileInfo_put(struct cifsFileInfo *cifs_file, bool wait_oplock_handler)
* Delete any outstanding lock records. We'll lose them when the file
* is closed anyway.
*/
- down_write(&cifsi->lock_sem);
+ while (!down_write_trylock(&cifsi->lock_sem))
+ msleep(125);
list_for_each_entry_safe(li, tmp, &cifs_file->llist->locks, llist) {
list_del(&li->llist);
cifs_del_lock_waiters(li);
>
> Signed-off-by: Dave Wysochanski <dwysocha@xxxxxxxxxx>
> ---
> fs/cifs/cifsfs.c | 1 +
> fs/cifs/cifsglob.h | 1 +
> fs/cifs/file.c | 24 +++++++++++++++++++-----
> 3 files changed, 21 insertions(+), 5 deletions(-)
>
> diff --git a/fs/cifs/cifsfs.c b/fs/cifs/cifsfs.c
> index c049c7b3aa87..10f614324e4e 100644
> --- a/fs/cifs/cifsfs.c
> +++ b/fs/cifs/cifsfs.c
> @@ -1336,6 +1336,7 @@ static ssize_t cifs_copy_file_range(struct file
> *src_file, loff_t off,
>
> inode_init_once(&cifsi->vfs_inode);
> init_rwsem(&cifsi->lock_sem);
> + init_rwsem(&cifsi->io_inflight_sem);
> }
>
> static int __init
> diff --git a/fs/cifs/cifsglob.h b/fs/cifs/cifsglob.h
> index 50dfd9049370..40e8358dc1cc 100644
> --- a/fs/cifs/cifsglob.h
> +++ b/fs/cifs/cifsglob.h
> @@ -1392,6 +1392,7 @@ struct cifsInodeInfo {
> bool can_cache_brlcks;
> struct list_head llist; /* locks helb by this inode */
> struct rw_semaphore lock_sem; /* protect the fields above */
> + struct rw_semaphore io_inflight_sem; /* Used to avoid lock conflicts */
> /* BB add in lists for dirty pages i.e. write caching info for oplock */
> struct list_head openFileList;
> spinlock_t open_file_lock; /* protects openFileList */
> diff --git a/fs/cifs/file.c b/fs/cifs/file.c
> index 5ad15de2bb4f..417baa7f5dd3 100644
> --- a/fs/cifs/file.c
> +++ b/fs/cifs/file.c
> @@ -621,7 +621,7 @@ int cifs_open(struct inode *inode, struct file *file)
> struct cifs_tcon *tcon = tlink_tcon(cfile->tlink);
> int rc = 0;
>
> - down_read_nested(&cinode->lock_sem, SINGLE_DEPTH_NESTING);
> + down_read(&cinode->lock_sem);
> if (cinode->can_cache_brlcks) {
> /* can cache locks - no need to relock */
> up_read(&cinode->lock_sem);
> @@ -973,6 +973,7 @@ int cifs_closedir(struct inode *inode, struct file *file)
> struct cifs_fid_locks *cur;
> struct cifsInodeInfo *cinode = CIFS_I(d_inode(cfile->dentry));
>
> + down_read(&cinode->lock_sem);
> list_for_each_entry(cur, &cinode->llist, llist) {
> rc = cifs_find_fid_lock_conflict(cur, offset, length, type,
> flags, cfile, conf_lock,
> @@ -980,6 +981,7 @@ int cifs_closedir(struct inode *inode, struct file *file)
> if (rc)
> break;
> }
> + up_read(&cinode->lock_sem);
>
> return rc;
> }
> @@ -1027,9 +1029,11 @@ int cifs_closedir(struct inode *inode, struct file
> *file)
> cifs_lock_add(struct cifsFileInfo *cfile, struct cifsLockInfo *lock)
> {
> struct cifsInodeInfo *cinode = CIFS_I(d_inode(cfile->dentry));
> + down_write(&cinode->io_inflight_sem);
> down_write(&cinode->lock_sem);
> list_add_tail(&lock->llist, &cfile->llist->locks);
> up_write(&cinode->lock_sem);
> + up_write(&cinode->io_inflight_sem);
> }
>
> /*
> @@ -1049,6 +1053,7 @@ int cifs_closedir(struct inode *inode, struct file
> *file)
>
> try_again:
> exist = false;
> + down_write(&cinode->io_inflight_sem);
> down_write(&cinode->lock_sem);
>
> exist = cifs_find_lock_conflict(cfile, lock->offset, lock->length,
> @@ -1057,6 +1062,7 @@ int cifs_closedir(struct inode *inode, struct file
> *file)
> if (!exist && cinode->can_cache_brlcks) {
> list_add_tail(&lock->llist, &cfile->llist->locks);
> up_write(&cinode->lock_sem);
> + up_write(&cinode->io_inflight_sem);
> return rc;
> }
>
> @@ -1077,6 +1083,7 @@ int cifs_closedir(struct inode *inode, struct file
> *file)
> }
>
> up_write(&cinode->lock_sem);
> + up_write(&cinode->io_inflight_sem);
> return rc;
> }
>
> @@ -1125,14 +1132,17 @@ int cifs_closedir(struct inode *inode, struct file
> *file)
> return rc;
>
> try_again:
> + down_write(&cinode->io_inflight_sem);
> down_write(&cinode->lock_sem);
> if (!cinode->can_cache_brlcks) {
> up_write(&cinode->lock_sem);
> + down_write(&cinode->io_inflight_sem);
> return rc;
> }
>
> rc = posix_lock_file(file, flock, NULL);
> up_write(&cinode->lock_sem);
> + up_write(&cinode->io_inflight_sem);
> if (rc == FILE_LOCK_DEFERRED) {
> rc = wait_event_interruptible(flock->fl_wait, !flock->fl_blocker);
> if (!rc)
> @@ -1331,6 +1341,7 @@ struct lock_to_push {
> int rc = 0;
>
> /* we are going to update can_cache_brlcks here - need a write access */
> + down_write(&cinode->io_inflight_sem);
> down_write(&cinode->lock_sem);
> if (!cinode->can_cache_brlcks) {
> up_write(&cinode->lock_sem);
> @@ -1346,6 +1357,7 @@ struct lock_to_push {
>
> cinode->can_cache_brlcks = false;
> up_write(&cinode->lock_sem);
> + up_write(&cinode->io_inflight_sem);
> return rc;
> }
>
> @@ -1522,6 +1534,7 @@ struct lock_to_push {
> if (!buf)
> return -ENOMEM;
>
> + down_write(&cinode->io_inflight_sem);
> down_write(&cinode->lock_sem);
> for (i = 0; i < 2; i++) {
> cur = buf;
> @@ -1593,6 +1606,7 @@ struct lock_to_push {
> }
>
> up_write(&cinode->lock_sem);
> + up_write(&cinode->io_inflight_sem);
> kfree(buf);
> return rc;
> }
> @@ -3148,7 +3162,7 @@ ssize_t cifs_user_writev(struct kiocb *iocb, struct
> iov_iter *from)
> * We need to hold the sem to be sure nobody modifies lock list
> * with a brlock that prevents writing.
> */
> - down_read(&cinode->lock_sem);
> + down_read(&cinode->io_inflight_sem);
>
> rc = generic_write_checks(iocb, from);
> if (rc <= 0)
> @@ -3161,7 +3175,7 @@ ssize_t cifs_user_writev(struct kiocb *iocb, struct
> iov_iter *from)
> else
> rc = -EACCES;
> out:
> - up_read(&cinode->lock_sem);
> + up_read(&cinode->io_inflight_sem);
> inode_unlock(inode);
>
> if (rc > 0)
> @@ -3887,12 +3901,12 @@ ssize_t cifs_user_readv(struct kiocb *iocb, struct
> iov_iter *to)
> * We need to hold the sem to be sure nobody modifies lock list
> * with a brlock that prevents reading.
> */
> - down_read(&cinode->lock_sem);
> + down_read(&cinode->io_inflight_sem);
> if (!cifs_find_lock_conflict(cfile, iocb->ki_pos, iov_iter_count(to),
> tcon->ses->server->vals->shared_lock_type,
> 0, NULL, CIFS_READ_OP))
> rc = generic_file_read_iter(iocb, to);
> - up_read(&cinode->lock_sem);
> + up_read(&cinode->io_inflight_sem);
> return rc;
> }
>
> --
> 1.8.3.1
>
>
