[PATCH v5 1/2] xfs: add a XFS_TRANS_WRITECOUNT_TRYLOCK flag

[Aditya Srivastava <aditya.ansh182@gmail.com>]

From: Aditya Prakash Srivastava <aditya.ansh182@gmail.com>

Introduce a new transaction allocation flag, XFS_TRANS_WRITECOUNT_TRYLOCK.
When this flag is specified, __xfs_trans_alloc() attempts to obtain
freeze protection using sb_start_intwrite_trylock() instead of blocking
indefinitely on sb_start_intwrite().

If the trylock fails, the allocation is aborted gracefully: the freshly
allocated transaction handle is freed, and the function returns the
appropriate error pointer ERR_PTR(-EAGAIN), which is then propagated
to the caller by xfs_trans_alloc().

Also add an assertion in __xfs_trans_alloc() to ensure that both
XFS_TRANS_NO_WRITECOUNT and XFS_TRANS_WRITECOUNT_TRYLOCK are never
specified at the same time, as they are mutually exclusive.

Suggested-by: Christoph Hellwig <hch@infradead.org>
Reviewed-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Aditya Prakash Srivastava <aditya.ansh182@gmail.com>
---
 fs/xfs/libxfs/xfs_shared.h |  3 +++
 fs/xfs/xfs_trans.c         | 12 +++++++++++-
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/libxfs/xfs_shared.h b/fs/xfs/libxfs/xfs_shared.h
index b1e0d9bc1f7d..68d22b6cddd3 100644
--- a/fs/xfs/libxfs/xfs_shared.h
+++ b/fs/xfs/libxfs/xfs_shared.h
@@ -164,6 +164,9 @@ void	xfs_log_get_max_trans_res(struct xfs_mount *mp,
 /* Transaction has locked the rtbitmap and rtsum inodes */
 #define XFS_TRANS_RTBITMAP_LOCKED	(1u << 9)
 
+/* Try lock filesystem superblock for freeze protection */
+#define XFS_TRANS_WRITECOUNT_TRYLOCK	(1u << 10)
+
 /*
  * Field values for xfs_trans_mod_sb.
  */
diff --git a/fs/xfs/xfs_trans.c b/fs/xfs/xfs_trans.c
index 148cc32449c1..3860e44d6439 100644
--- a/fs/xfs/xfs_trans.c
+++ b/fs/xfs/xfs_trans.c
@@ -216,10 +216,18 @@ __xfs_trans_alloc(
 	struct xfs_trans	*tp;
 
 	ASSERT(!(flags & XFS_TRANS_RES_FDBLKS) || xfs_has_lazysbcount(mp));
+	ASSERT(!((flags & XFS_TRANS_NO_WRITECOUNT) &&
+		 (flags & XFS_TRANS_WRITECOUNT_TRYLOCK)));
 
 	tp = kmem_cache_zalloc(xfs_trans_cache, GFP_KERNEL | __GFP_NOFAIL);
-	if (!(flags & XFS_TRANS_NO_WRITECOUNT))
+	if (flags & XFS_TRANS_WRITECOUNT_TRYLOCK) {
+		if (!sb_start_intwrite_trylock(mp->m_super)) {
+			kmem_cache_free(xfs_trans_cache, tp);
+			return ERR_PTR(-EAGAIN);
+		}
+	} else if (!(flags & XFS_TRANS_NO_WRITECOUNT)) {
 		sb_start_intwrite(mp->m_super);
+	}
 	xfs_trans_set_context(tp);
 	tp->t_flags = flags;
 	tp->t_mountp = mp;
@@ -252,6 +260,8 @@ xfs_trans_alloc(
 	 */
 retry:
 	tp = __xfs_trans_alloc(mp, flags);
+	if (IS_ERR(tp))
+		return PTR_ERR(tp);
 	WARN_ON(mp->m_super->s_writers.frozen == SB_FREEZE_COMPLETE);
 	error = xfs_trans_reserve(tp, resp, blocks, rtextents);
 	if (error == -ENOSPC && want_retry) {
-- 
2.47.3