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

Darrick J. Wong
2024-12-19T18:35:20Z

On Fri, Dec 20, 2024 at 01:51:49AM +0800, Jinliang Zheng wrote:
> On Thu, 19 Dec 2024 09:36:15 -0800, Darrick J. Wong wrote:
> > 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.
>
> When xfs_buf_lock() causes a hung task, we need to know which task
> currently holds the lock.
>
> However, sometimes the command 'search -t ' may
> not be effective, such as when the stack frame of xfs_buf_lock()
> has been popped.
>
> Replacing the semaphore with a mutex for xfs_buf has no negative
> functional impact, but in certain situations, it indeed facilitates
> our debugging.

Oh? What will the users of mutex::owner react when the xfs_buf
protected by the mutex is accessed by the workqueue thread? What
happens when the buffers locked by the btree split worker are passed
back to the original thread when the worker completes?

Also, notice how lockdep doesn't track semaphores but it does track
mutexes? Will that cause problems with lockdep?

--D

> Thank you,
> Jinliang Zheng
>
> >
> > --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
> > >
> > >
>