From: Johannes Berg johannes.berg@intel.com

Ben Greear further reports deadlocks during concurrent debugfs remove while files are being accessed, even though the code in question now uses debugfs cancellations. Turns out that despite all the review on the locking, we missed completely that the logic is wrong: if the refcount hits zero we can finish (and need not wait for the completion), but if it doesn't we have to trigger all the cancellations. As written, we can _never_ get into the loop triggering the cancellations. Fix this, and explain it better while at it.

Cc: stable@vger.kernel.org Fixes: 8c88a474357e ("debugfs: add API to allow debugfs operations cancellation") Reported-by: Ben Greear greearb@candelatech.com Closes: https://lore.kernel.org/r/1c9fa9e5-09f1-0522-fdbc-dbcef4d255ca@candelatech.c... Tested-by: Madhan Sai madhan.singaraju@candelatech.com Signed-off-by: Johannes Berg johannes.berg@intel.com --- fs/debugfs/inode.c | 25 ++++++++++++++++++++----- 1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c index 034a617cb1a5..a40da0065433 100644 --- a/fs/debugfs/inode.c +++ b/fs/debugfs/inode.c @@ -751,13 +751,28 @@ static void __debugfs_file_removed(struct dentry *dentry) if ((unsigned long)fsd & DEBUGFS_FSDATA_IS_REAL_FOPS_BIT) return;

- /* if we hit zero, just wait for all to finish */ - if (!refcount_dec_and_test(&fsd->active_users)) { - wait_for_completion(&fsd->active_users_drained); + /* if this was the last reference, we're done */ + if (refcount_dec_and_test(&fsd->active_users)) return; - }

- /* if we didn't hit zero, try to cancel any we can */ + /* + * If there's still a reference, the code that obtained it can + * be in different states: + * - The common case of not using cancellations, or already + * after debugfs_leave_cancellation(), where we just need + * to wait for debugfs_file_put() which signals the completion; + * - inside a cancellation section, i.e. between + * debugfs_enter_cancellation() and debugfs_leave_cancellation(), + * in which case we need to trigger the ->cancel() function, + * and then wait for debugfs_file_put() just like in the + * previous case; + * - before debugfs_enter_cancellation() (but obviously after + * debugfs_file_get()), in which case we may not see the + * cancellation in the list on the first round of the loop, + * but debugfs_enter_cancellation() signals the completion + * after adding it, so this code gets woken up to call the + * ->cancel() function. + */ while (refcount_read(&fsd->active_users)) { struct debugfs_cancellation *c;