Re: [RFC PATCH] cifs: Fix cifsInodeInfo lock_sem deadlock with multiple readers

[David Wysochanski <dwysocha@redhat.com>]

On Wed, Oct 23, 2019 at 12:50 AM Ronnie Sahlberg <lsahlber@redhat.com> wrote:
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> ----- Original Message -----
> > From: "Dave Wysochanski" <dwysocha@redhat.com>
> > To: linux-cifs@vger.kernel.org
> > 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() :-(
>

Sorry I should have also said in the header that the patch was built
on top of 5.4-rc4 plus Pavel's patch:
"CIFS: Fix retry mid list corruption on reconnects"

Also, there is an obvious bug in this patch where I am taking lock_sem
inside cifs_find_lock_conflict but that
does not work for obvious reasons - needs to be moved back to callers.
FWIW, I have a v2 patch that fixes
that and I have started some locking tests on.

>
> 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.
>

I think you misunderstood the patch or the header didn't explain that
part.  It is actually removing the top level holding of
lock_sem across the IO, because that leads to the deadlock - the root
is that one process calls down_read twice, which
IMO is erroneous.

Instead of leaving the code paths where down_read can be called twice,
there is a new rw_semaphore, inflight_io_sem, that
protects the IO from someone adding a brlock that would restrict the
IO.  The lock_sem still protects the same fields from
modification, it just does not cover "preventing someone from adding a
conflicting brlock during IO".


> 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)
>

Yes your approach is less lines way to go about preventing the
deadlock.  The biggest downsides I see is that it does not
remove the code paths which call down_read twice (which is dangerous
for the reason here), and it does not eliminate
(but does reduce) the lock contention due to multi-use nature of
lock_sem (not only does it protect modification to the
fields, it also doubles as preventing the conflicting brlock during
IO).  I agree it is simpler and avoids adding another
semaphore so it may be a better approach.

For your suggested approach, I'm not sure how many places there are
that really matter as far as calls to down_write.
Looks like there are 9 different calls to down_write(cifsi->lock_sem),
and I know in the reproducer at least 2 of them matter.
I would say we should do all 9 to be thorough, not leave anything to
chance, and have consistent handling of lock_sem.
What do you think?


> 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@redhat.com>
> > ---
> >  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
> >
> >