On Thu, Oct 24, 2019 at 7:51 PM Ronnie Sahlberg <lsahlber@xxxxxxxxxx> wrote:
>
> From: Dave Wysochanski <dwysocha@xxxxxxxxxx>
>
> There's a deadlock that is possible and can easily be seen with
> a test where multiple readers open/read/close of the same file
> and a disruption occurs causing reconnect. The deadlock is due
> a reader thread inside cifs_strict_readv calling down_read and
> obtaining lock_sem, and then after reconnect inside
> cifs_reopen_file calling down_read a second time. If in
> between the two down_read calls, a down_write comes from
> another process, deadlock occurs.
>
> CPU0 CPU1
> ---- ----
> cifs_strict_readv()
> down_read(&cifsi->lock_sem);
> _cifsFileInfo_put
> OR
> cifs_new_fileinfo
> down_write(&cifsi->lock_sem);
> cifs_reopen_file()
> down_read(&cifsi->lock_sem);
>
> Fix the above by changing all down_write(lock_sem) calls to
> down_write_trylock(lock_sem)/msleep() loop, which in turn
> makes the second down_read call benign since it will never
> block behind the writer while holding lock_sem.
>
> Signed-off-by: Dave Wysochanski <dwysocha@xxxxxxxxxx>
> Suggested-by: Ronnie Sahlberg <lsahlber@xxxxxxxxxx>
> ---
> fs/cifs/cifsglob.h | 5 +++++
> fs/cifs/cifsproto.h | 1 +
> fs/cifs/file.c | 23 +++++++++++++++--------
> fs/cifs/smb2file.c | 2 +-
> 4 files changed, 22 insertions(+), 9 deletions(-)
>
> diff --git a/fs/cifs/cifsglob.h b/fs/cifs/cifsglob.h
> index 50dfd9049370..d78bfcc19156 100644
> --- a/fs/cifs/cifsglob.h
> +++ b/fs/cifs/cifsglob.h
> @@ -1391,6 +1391,11 @@ void cifsFileInfo_put(struct cifsFileInfo *cifs_file);
> struct cifsInodeInfo {
> bool can_cache_brlcks;
> struct list_head llist; /* locks helb by this inode */
> + /*
> + * NOTE: Some code paths call down_read(lock_sem) twice, so
> + * we must always use use cifs_down_write() instead of down_write()
> + * for this semaphore to avoid deadlocks.
> + */
> struct rw_semaphore lock_sem; /* protect the fields above */
> /* BB add in lists for dirty pages i.e. write caching info for oplock */
> struct list_head openFileList;
> diff --git a/fs/cifs/cifsproto.h b/fs/cifs/cifsproto.h
> index e53e9f62b87b..fe597d3d5208 100644
> --- a/fs/cifs/cifsproto.h
> +++ b/fs/cifs/cifsproto.h
> @@ -170,6 +170,7 @@ extern int cifs_unlock_range(struct cifsFileInfo *cfile,
> struct file_lock *flock, const unsigned int xid);
> extern int cifs_push_mandatory_locks(struct cifsFileInfo *cfile);
>
> +extern void cifs_down_write(struct rw_semaphore *sem);
> extern struct cifsFileInfo *cifs_new_fileinfo(struct cifs_fid *fid,
> struct file *file,
> struct tcon_link *tlink,
> diff --git a/fs/cifs/file.c b/fs/cifs/file.c
> index 0e0217641de1..a80ec683ea32 100644
> --- a/fs/cifs/file.c
> +++ b/fs/cifs/file.c
> @@ -281,6 +281,13 @@ cifs_has_mand_locks(struct cifsInodeInfo *cinode)
> return has_locks;
> }
>
> +void
> +cifs_down_write(struct rw_semaphore *sem)
> +{
> + while (!down_write_trylock(sem))
> + msleep(10);
> +}
> +
> struct cifsFileInfo *
> cifs_new_fileinfo(struct cifs_fid *fid, struct file *file,
> struct tcon_link *tlink, __u32 oplock)
> @@ -306,7 +313,7 @@ cifs_new_fileinfo(struct cifs_fid *fid, struct file *file,
> INIT_LIST_HEAD(&fdlocks->locks);
> fdlocks->cfile = cfile;
> cfile->llist = fdlocks;
> - down_write(&cinode->lock_sem);
> + cifs_down_write(&cinode->lock_sem);
> list_add(&fdlocks->llist, &cinode->llist);
> up_write(&cinode->lock_sem);
>
> @@ -464,7 +471,7 @@ 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);
> + cifs_down_write(&cifsi->lock_sem);
> list_for_each_entry_safe(li, tmp, &cifs_file->llist->locks, llist) {
> list_del(&li->llist);
> cifs_del_lock_waiters(li);
> @@ -1027,7 +1034,7 @@ static void
> cifs_lock_add(struct cifsFileInfo *cfile, struct cifsLockInfo *lock)
> {
> struct cifsInodeInfo *cinode = CIFS_I(d_inode(cfile->dentry));
> - down_write(&cinode->lock_sem);
> + cifs_down_write(&cinode->lock_sem);
> list_add_tail(&lock->llist, &cfile->llist->locks);
> up_write(&cinode->lock_sem);
> }
> @@ -1049,7 +1056,7 @@ cifs_lock_add_if(struct cifsFileInfo *cfile, struct cifsLockInfo *lock,
>
> try_again:
> exist = false;
> - down_write(&cinode->lock_sem);
> + cifs_down_write(&cinode->lock_sem);
>
> exist = cifs_find_lock_conflict(cfile, lock->offset, lock->length,
> lock->type, lock->flags, &conf_lock,
> @@ -1072,7 +1079,7 @@ cifs_lock_add_if(struct cifsFileInfo *cfile, struct cifsLockInfo *lock,
> (lock->blist.next == &lock->blist));
> if (!rc)
> goto try_again;
> - down_write(&cinode->lock_sem);
> + cifs_down_write(&cinode->lock_sem);
> list_del_init(&lock->blist);
> }
>
> @@ -1125,7 +1132,7 @@ cifs_posix_lock_set(struct file *file, struct file_lock *flock)
> return rc;
>
> try_again:
> - down_write(&cinode->lock_sem);
> + cifs_down_write(&cinode->lock_sem);
> if (!cinode->can_cache_brlcks) {
> up_write(&cinode->lock_sem);
> return rc;
> @@ -1331,7 +1338,7 @@ cifs_push_locks(struct cifsFileInfo *cfile)
> int rc = 0;
>
> /* we are going to update can_cache_brlcks here - need a write access */
> - down_write(&cinode->lock_sem);
> + cifs_down_write(&cinode->lock_sem);
> if (!cinode->can_cache_brlcks) {
> up_write(&cinode->lock_sem);
> return rc;
> @@ -1522,7 +1529,7 @@ cifs_unlock_range(struct cifsFileInfo *cfile, struct file_lock *flock,
> if (!buf)
> return -ENOMEM;
>
> - down_write(&cinode->lock_sem);
> + cifs_down_write(&cinode->lock_sem);
> for (i = 0; i < 2; i++) {
> cur = buf;
> num = 0;
> diff --git a/fs/cifs/smb2file.c b/fs/cifs/smb2file.c
> index e6a1fc72018f..8b0b512c5792 100644
> --- a/fs/cifs/smb2file.c
> +++ b/fs/cifs/smb2file.c
> @@ -145,7 +145,7 @@ smb2_unlock_range(struct cifsFileInfo *cfile, struct file_lock *flock,
>
> cur = buf;
>
> - down_write(&cinode->lock_sem);
> + cifs_down_write(&cinode->lock_sem);
> list_for_each_entry_safe(li, tmp, &cfile->llist->locks, llist) {
> if (flock->fl_start > li->offset ||
> (flock->fl_start + length) <
> --
> 2.13.6
>
Thanks for doing this! Looks fine to me.
I'm not loving this approach to solve this problem but I understand
why it may be preferred right now.
I note that trylock with a sleep / retry loop is not often used in
filesystems that I could tell (though it is used in drivers), so maybe
this can be improved another day.
