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3e55189b50
bcachefs freeze testing via fstests generic/390 occasionally reproduces the following BUG from bch2_fs_read_only(): BUG_ON(atomic_long_read(&c->btree_key_cache.nr_dirty)); This indicates that one or more dirty key cache keys still exist after the attempt to flush and quiesce the fs. The sequence that leads to this problem actually occurs on unfreeze (ro->rw), and looks something like the following: - Task A begins a transaction commit and acquires journal_res for the current seq. This transaction intends to perform key cache insertion. - Task B begins a bch2_journal_flush() via bch2_sync_fs(). This ends up in journal_entry_want_write(), which closes the current journal entry and drops the reference to the pin list created on entry open. The pin put pops the front of the journal via fast reclaim since the reference count has dropped to 0. - Task A attempts to set the journal pin for the associated cached key, but bch2_journal_pin_set() skips the pin insert because the seq of the transaction reservation is behind the front of the pin list fifo. The end result is that the pin associated with the cached key is not added, which prevents a subsequent reclaim from processing the key and thus leaves it dangling at freeze time. The fundamental cause of this problem is that the front of the journal is allowed to pop before a transaction with outstanding reservation on the associated journal seq is able to add a pin. The count for the pin list associated with the seq drops to zero and is prematurely reclaimed as a result. The logical fix for this problem lies in how the journal buffer is managed in similar scenarios where the entry might have been closed before a transaction with outstanding reservations happens to be committed. When a journal entry is opened, the current sequence number is bumped, the associated pin list is initialized with a reference count of 1, and the journal buffer reference count is bumped (via journal_state_inc()). When a journal reservation is acquired, the reservation also acquires a reference on the associated buffer. If the journal entry is closed in the meantime, it drops both the pin and buffer references held by the open entry, but the buffer still has references held by outstanding reservation. After the associated transaction commits, the reservation release drops the associated buffer references and the buffer is written out once the reference count has dropped to zero. The fundamental problem here is that the lifecycle of the pin list reference held by an open journal entry is too short to cover the processing of transactions with outstanding reservations. The simplest way to address this is to expand the pin list reference to the lifecycle of the buffer vs. the shorter lifecycle of the open journal entry. This ensures the pin list for a seq with outstanding reservation cannot be popped and reclaimed before all outstanding reservations have been released, even if the associated journal entry has been closed for further reservations. Move the pin put from journal entry close to where final processing of the journal buffer occurs. Create a duplicate helper to cover the case where the caller doesn't already hold the journal lock. This allows generic/390 to pass reliably. Signed-off-by: Brian Foster <bfoster@redhat.com> Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.