lkml.org 
[lkml]   [2019]   [Apr]   [16]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v4 07/16] locking/rwsem: Implement lock handoff to prevent lock starvation
On Tue, Apr 16, 2019 at 05:49:37PM +0200, Peter Zijlstra wrote:
> See, if you first write that function in the form:
>
> long new;
>
> do {
> new = count | RWSEM_WRITER_LOCKED;
>
> if (count & RWSEM_LOCK_MASK)
> return false;
>
> if (list_is_singular(&sem->wait_list))
> new &= ~RWSEM_FLAG_WAITERS;
>
> } while (atomic_long_try_cmpxchg_acquire(&sem->count, &count, new));
>
> rwsem_set_owner(sem);
> return true;
>
> And then add the HANDOFF bits like:
>
> long new;
>
> do {
> + bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);
>
> + new = (count | RWSEM_WRITER_LOCKED) & ~RWSEM_FLAG_HANDOFF;
>
> if (count & RWSEM_LOCK_MASK) {
> + if (has_handoff && wstate != WRITER_HANDOFF)
> + return false;
> new |= RWSEM_FLAG_HANDOFF;
> }
>
> + if (has_handoff && wstate == WRITER_NOT_FIRST)
> + return false;
>
> if (list_is_singular(&sem->wait_list))
> new &= ~RWSEM_FLAG_WAITERS;
>
> } while (atomic_long_try_cmpxchg_acquire(&sem->count, &count, new));

obviously that should be !

>
> rwsem_set_owner(sem);
> return true;
>
> it almost looks like sensible code.

\
 
 \ /
  Last update: 2019-04-16 18:16    [W:0.174 / U:12.392 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site