Re: [PATCH] cifs: Fix cifsInodeInfo lock_sem deadlock when reconnect occurs

[Pavel Shilovsky <piastryyy@gmail.com>]

чт, 24 окт. 2019 г. в 04:04, Dave Wysochanski <dwysocha@redhat.com>:
>
> 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@redhat.com>
> Suggested-by: Ronnie Sahlberg <lsahlber@redhat.com>
> ---
>  fs/cifs/cifsglob.h |  5 +++++
>  fs/cifs/file.c     | 24 ++++++++++++++++--------
>  fs/cifs/smb2file.c |  3 ++-
>  3 files changed, 23 insertions(+), 9 deletions(-)
>
> diff --git a/fs/cifs/cifsglob.h b/fs/cifs/cifsglob.h
> index 50dfd9049370..2c4a7adbcb4e 100644
> --- a/fs/cifs/cifsglob.h
> +++ b/fs/cifs/cifsglob.h
> @@ -1391,6 +1391,11 @@ struct cifs_writedata {
>  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 down_write_trylock()/msleep() loop
> +        * to avoid deadlock.
> +        */
>         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/file.c b/fs/cifs/file.c
> index 5ad15de2bb4f..52454df5ae39 100644
> --- a/fs/cifs/file.c
> +++ b/fs/cifs/file.c
> @@ -306,7 +306,8 @@ struct cifsFileInfo *
>         INIT_LIST_HEAD(&fdlocks->locks);
>         fdlocks->cfile = cfile;
>         cfile->llist = fdlocks;
> -       down_write(&cinode->lock_sem);
> +       while (!down_write_trylock(&cinode->lock_sem))
> +               msleep(125);
>         list_add(&fdlocks->llist, &cinode->llist);
>         up_write(&cinode->lock_sem);
>
> @@ -464,7 +465,8 @@ 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);

Please wrap the above code into a helper function e.g.
cifs_acquire_lock_sem(struct cifsInodeInfo *cinode) or any other name
you like.

Other than that it looks good.

--
Best regards,
Pavel Shilovsky