Re: [PATCH] locking/rwsem: Remove unnessary check in rwsem_down_read_slowpath()

[Waiman Long]

On 11/10/23 05:29, Haifeng Xu wrote:
> On 2023/11/10 14:54, Tang Yizhou wrote:
> > On Thu, Nov 9, 2023 at 11:17 AM Haifeng Xu <haifeng.xu@xxxxxxxxxx> wrote:
> > > reader          writer                                  reader
> > >
> > > acquire
> > > release
> > >                  rwsem_write_trylock
> > >                          set RWSEM_WRITER_LOCKED
> > >                                                          rwsem_down_read_slowpath
> > >                          set owner
> > >
> > > If prev lock holder is a reader, when it releases the lock, the owner isn't cleared(CONFIG_DEBUG_RWSEMS isn't enabled).
> > > A writer comes and can set the RWSEM_WRITER_LOCKED bit succsessfully, then a new reader run into slow path, before
> > > the writer set the owner, the new reader will see that both the RWSEM_READER_OWNED bit and RWSEM_WRITER_LOCKED bit are
> > > set.
> > For the above example, it won't cause a problem. When the writer
> > successfully sets RWSEM_WRITER_LOCKED, the reader, when reading rcnt
> > through rwsem_down_read_slowpath(), will see that rcnt is 0 and will
> > jump to the queue label.
> >
> > Thanks,
> > Tang
> Yes, so if rcnt > 1, the RWSEM_WRITER_LOCKED bit couldn't be set?

No. The way readers acquire the lock is via 
atomic_long_add_return_acquire() without looking at current state of the 
rwsem (write-locked or not). So rcnt can be greater than 0 and the rwsem 
is still writer-owned.

Because of that atomic_long_add_return_acquire() primitive, rcnt 
includes its reader count. The lock may be read-locked if only if there 
is at least one other reader present. So rcnt must be bigger than 1.

Cheers,
Longman