Re: [PATCH] xfs: using mutex instead of semaphore for xfs_buf_lock()

Darrick J. Wong
2024-12-19T17:36:15Z

On Fri, Dec 20, 2024 at 01:16:29AM +0800, Jinliang Zheng wrote:
> xfs_buf uses a semaphore for mutual exclusion, and its count value
> is initialized to 1, which is equivalent to a mutex.
>
> However, mutex->owner can provide more information when analyzing
> vmcore, making it easier for us to identify which task currently
> holds the lock.

Does XFS pass buffers between tasks? xfs_btree_split has that whole
blob of ugly code where it can pass a locked inode and transaction to a
workqueue function to avoid overrunning the kernel stack.

--D

> Signed-off-by: Jinliang Zheng <alexjlzheng@tencent.com>
> ---
> fs/xfs/xfs_buf.c | 9 +++++----
> fs/xfs/xfs_buf.h | 4 ++--
> fs/xfs/xfs_trace.h | 25 +++++--------------------
> 3 files changed, 12 insertions(+), 26 deletions(-)
>
> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index aa4dbda7b536..7c59d7905ea1 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -243,7 +243,8 @@ _xfs_buf_alloc(
> INIT_LIST_HEAD(&bp->b_lru);
> INIT_LIST_HEAD(&bp->b_list);
> INIT_LIST_HEAD(&bp->b_li_list);
> - sema_init(&bp->b_sema, 0); /* held, no waiters */
> + mutex_init(&bp->b_mutex);
> + mutex_lock(&bp->b_mutex); /* held, no waiters */
> spin_lock_init(&bp->b_lock);
> bp->b_target = target;
> bp->b_mount = target->bt_mount;
> @@ -1168,7 +1169,7 @@ xfs_buf_trylock(
> {
> int locked;
>
> - locked = down_trylock(&bp->b_sema) == 0;
> + locked = mutex_trylock(&bp->b_mutex);
> if (locked)
> trace_xfs_buf_trylock(bp, _RET_IP_);
> else
> @@ -1193,7 +1194,7 @@ xfs_buf_lock(
>
> if (atomic_read(&bp->b_pin_count) && (bp->b_flags & XBF_STALE))
> xfs_log_force(bp->b_mount, 0);
> - down(&bp->b_sema);
> + mutex_lock(&bp->b_mutex);
>
> trace_xfs_buf_lock_done(bp, _RET_IP_);
> }
> @@ -1204,7 +1205,7 @@ xfs_buf_unlock(
> {
> ASSERT(xfs_buf_islocked(bp));
>
> - up(&bp->b_sema);
> + mutex_unlock(&bp->b_mutex);
> trace_xfs_buf_unlock(bp, _RET_IP_);
> }
>
> diff --git a/fs/xfs/xfs_buf.h b/fs/xfs/xfs_buf.h
> index b1580644501f..2c48e388d451 100644
> --- a/fs/xfs/xfs_buf.h
> +++ b/fs/xfs/xfs_buf.h
> @@ -171,7 +171,7 @@ struct xfs_buf {
> atomic_t b_hold; /* reference count */
> atomic_t b_lru_ref; /* lru reclaim ref count */
> xfs_buf_flags_t b_flags; /* status flags */
> - struct semaphore b_sema; /* semaphore for lockables */
> + struct mutex b_mutex; /* mutex for lockables */
>
> /*
> * concurrent access to b_lru and b_lru_flags are protected by
> @@ -304,7 +304,7 @@ extern int xfs_buf_trylock(struct xfs_buf *);
> extern void xfs_buf_lock(struct xfs_buf *);
> extern void xfs_buf_unlock(struct xfs_buf *);
> #define xfs_buf_islocked(bp) \
> - ((bp)->b_sema.count <= 0)
> + mutex_is_locked(&(bp)->b_mutex)
>
> static inline void xfs_buf_relse(struct xfs_buf *bp)
> {
> diff --git a/fs/xfs/xfs_trace.h b/fs/xfs/xfs_trace.h
> index 180ce697305a..ba6c003b82af 100644
> --- a/fs/xfs/xfs_trace.h
> +++ b/fs/xfs/xfs_trace.h
> @@ -443,7 +443,6 @@ DECLARE_EVENT_CLASS(xfs_buf_class,
> __field(int, nblks)
> __field(int, hold)
> __field(int, pincount)
> - __field(unsigned, lockval)
> __field(unsigned, flags)
> __field(unsigned long, caller_ip)
> __field(const void *, buf_ops)
> @@ -454,19 +453,17 @@ DECLARE_EVENT_CLASS(xfs_buf_class,
> __entry->nblks = bp->b_length;
> __entry->hold = atomic_read(&bp->b_hold);
> __entry->pincount = atomic_read(&bp->b_pin_count);
> - __entry->lockval = bp->b_sema.count;
> __entry->flags = bp->b_flags;
> __entry->caller_ip = caller_ip;
> __entry->buf_ops = bp->b_ops;
> ),
> TP_printk("dev %d:%d daddr 0x%llx bbcount 0x%x hold %d pincount %d "
> - "lock %d flags %s bufops %pS caller %pS",
> + "flags %s bufops %pS caller %pS",
> MAJOR(__entry->dev), MINOR(__entry->dev),
> (unsigned long long)__entry->bno,
> __entry->nblks,
> __entry->hold,
> __entry->pincount,
> - __entry->lockval,
> __print_flags(__entry->flags, "|", XFS_BUF_FLAGS),
> __entry->buf_ops,
> (void *)__entry->caller_ip)
> @@ -514,7 +511,6 @@ DECLARE_EVENT_CLASS(xfs_buf_flags_class,
> __field(unsigned int, length)
> __field(int, hold)
> __field(int, pincount)
> - __field(unsigned, lockval)
> __field(unsigned, flags)
> __field(unsigned long, caller_ip)
> ),
> @@ -525,17 +521,15 @@ DECLARE_EVENT_CLASS(xfs_buf_flags_class,
> __entry->flags = flags;
> __entry->hold = atomic_read(&bp->b_hold);
> __entry->pincount = atomic_read(&bp->b_pin_count);
> - __entry->lockval = bp->b_sema.count;
> __entry->caller_ip = caller_ip;
> ),
> TP_printk("dev %d:%d daddr 0x%llx bbcount 0x%x hold %d pincount %d "
> - "lock %d flags %s caller %pS",
> + "flags %s caller %pS",
> MAJOR(__entry->dev), MINOR(__entry->dev),
> (unsigned long long)__entry->bno,
> __entry->length,
> __entry->hold,
> __entry->pincount,
> - __entry->lockval,
> __print_flags(__entry->flags, "|", XFS_BUF_FLAGS),
> (void *)__entry->caller_ip)
> )
> @@ -558,7 +552,6 @@ TRACE_EVENT(xfs_buf_ioerror,
> __field(unsigned, flags)
> __field(int, hold)
> __field(int, pincount)
> - __field(unsigned, lockval)
> __field(int, error)
> __field(xfs_failaddr_t, caller_ip)
> ),
> @@ -568,19 +561,17 @@ TRACE_EVENT(xfs_buf_ioerror,
> __entry->length = bp->b_length;
> __entry->hold = atomic_read(&bp->b_hold);
> __entry->pincount = atomic_read(&bp->b_pin_count);
> - __entry->lockval = bp->b_sema.count;
> __entry->error = error;
> __entry->flags = bp->b_flags;
> __entry->caller_ip = caller_ip;
> ),
> TP_printk("dev %d:%d daddr 0x%llx bbcount 0x%x hold %d pincount %d "
> - "lock %d error %d flags %s caller %pS",
> + "error %d flags %s caller %pS",
> MAJOR(__entry->dev), MINOR(__entry->dev),
> (unsigned long long)__entry->bno,
> __entry->length,
> __entry->hold,
> __entry->pincount,
> - __entry->lockval,
> __entry->error,
> __print_flags(__entry->flags, "|", XFS_BUF_FLAGS),
> (void *)__entry->caller_ip)
> @@ -595,7 +586,6 @@ DECLARE_EVENT_CLASS(xfs_buf_item_class,
> __field(unsigned int, buf_len)
> __field(int, buf_hold)
> __field(int, buf_pincount)
> - __field(int, buf_lockval)
> __field(unsigned, buf_flags)
> __field(unsigned, bli_recur)
> __field(int, bli_refcount)
> @@ -612,18 +602,16 @@ DECLARE_EVENT_CLASS(xfs_buf_item_class,
> __entry->buf_flags = bip->bli_buf->b_flags;
> __entry->buf_hold = atomic_read(&bip->bli_buf->b_hold);
> __entry->buf_pincount = atomic_read(&bip->bli_buf->b_pin_count);
> - __entry->buf_lockval = bip->bli_buf->b_sema.count;
> __entry->li_flags = bip->bli_item.li_flags;
> ),
> TP_printk("dev %d:%d daddr 0x%llx bbcount 0x%x hold %d pincount %d "
> - "lock %d flags %s recur %d refcount %d bliflags %s "
> + "flags %s recur %d refcount %d bliflags %s "
> "liflags %s",
> MAJOR(__entry->dev), MINOR(__entry->dev),
> (unsigned long long)__entry->buf_bno,
> __entry->buf_len,
> __entry->buf_hold,
> __entry->buf_pincount,
> - __entry->buf_lockval,
> __print_flags(__entry->buf_flags, "|", XFS_BUF_FLAGS),
> __entry->bli_recur,
> __entry->bli_refcount,
> @@ -4802,7 +4790,6 @@ DECLARE_EVENT_CLASS(xfbtree_buf_class,
> __field(int, nblks)
> __field(int, hold)
> __field(int, pincount)
> - __field(unsigned int, lockval)
> __field(unsigned int, flags)
> ),
> TP_fast_assign(
> @@ -4811,16 +4798,14 @@ DECLARE_EVENT_CLASS(xfbtree_buf_class,
> __entry->nblks = bp->b_length;
> __entry->hold = atomic_read(&bp->b_hold);
> __entry->pincount = atomic_read(&bp->b_pin_count);
> - __entry->lockval = bp->b_sema.count;
> __entry->flags = bp->b_flags;
> ),
> - TP_printk("xfino 0x%lx daddr 0x%llx bbcount 0x%x hold %d pincount %d lock %d flags %s",
> + TP_printk("xfino 0x%lx daddr 0x%llx bbcount 0x%x hold %d pincount %d flags %s",
> __entry->xfino,
> (unsigned long long)__entry->bno,
> __entry->nblks,
> __entry->hold,
> __entry->pincount,
> - __entry->lockval,
> __print_flags(__entry->flags, "|", XFS_BUF_FLAGS))
> )
>
> --
> 2.41.1
>
>