Dave Chinner wrote:
> When we release the iclog, we do an atomic_dec_and_lock to determine
> if we are the last reference to enable update of log headers and
> writeout. however, in xlog_state_get_iclog_space() we need to check
> if we have the last reference count there. if we do, we release the
> log buffer, otherwise we decrement the reference count.
>
> The issue is that the compare and decrement in
> xlog_state_get_iclog_space() is not atomic, so both places can see a
> reference count of 2 and neither will release the iclog. That leads
> to a filesystem hang.
>
> Close the hole replacing the compare and decrement with
> atomic_add_unless() to ensure that they are executed atomically.
>
> Signed-off-by: Dave Chinner <david@xxxxxxxxxxxxx>
Tested-by: Eric Sandeen <sandeen@xxxxxxxxxxx>
Passes the fs_mark testcase I hit this on, 18 million inodes & counting.
Thanks,
-Eric
> ---
> fs/xfs/xfs_log.c | 15 +++++++++++----
> 1 files changed, 11 insertions(+), 4 deletions(-)
>
> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> index 760d543..0816c5d 100644
> --- a/fs/xfs/xfs_log.c
> +++ b/fs/xfs/xfs_log.c
> @@ -2425,13 +2425,20 @@ restart:
> if (iclog->ic_size - iclog->ic_offset < 2*sizeof(xlog_op_header_t)) {
> xlog_state_switch_iclogs(log, iclog, iclog->ic_size);
>
> - /* If I'm the only one writing to this iclog, sync it to disk */
> - if (atomic_read(&iclog->ic_refcnt) == 1) {
> + /*
> + * If I'm the only one writing to this iclog, sync it to disk.
> + * We need to do an atomic compare and decrement here to avoid
> + * racing with concurrent atomic_dec_and_lock() calls in
> + * xlog_state_release_iclog() when there is more than one
> + * reference to the iclog.
> + */
> + if (!atomic_add_unless(&iclog->ic_refcnt, -1, 1)) {
> + /* we are the only one */
> spin_unlock(&log->l_icloglock);
> - if ((error = xlog_state_release_iclog(log, iclog)))
> + error = xlog_state_release_iclog(log, iclog);
> + if (error)
> return error;
> } else {
> - atomic_dec(&iclog->ic_refcnt);
> spin_unlock(&log->l_icloglock);
> }
> goto restart;
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