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Re: __mutex_lock_common() unlikely very likely

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On Wed, Jan 18, 2017 at 03:58:24PM -0500, Steven Rostedt wrote:
> Chris,
> 
> My branch tracer flagged the unlikely in __mutex_lock_common() as
> always hit. That's the:
> 
> 	if (use_ww_ctx) {
> 		[...]
> 		if (unlikely(ww_ctx == READ_ONCE(ww->ctx)))
> 			return -EALREADY;
> 	}
> 
> This is hit 100% of the time, and its coming from the drm logic:

By design this is an exceptional case. In practice, drm modesetting is a
little slapsidasical when it comes to locking. However, it is the
minority use case, just that on intel, the more prevalent users do not
hit this path - though they will with the ww_mutex refactoring work. ttm
drivers (amdgpu, nouveau etc) will be demonstrating that this is the
unlikely branch.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre

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Thread

__mutex_lock_common() unlikely very likely Steven Rostedt <rostedt@goodmis.org> - 2017-01-18 22:10 +0100
  Re: __mutex_lock_common() unlikely very likely Chris Wilson <chris@chris-wilson.co.uk> - 2017-01-19 10:00 +0100
    Re: __mutex_lock_common() unlikely very likely Steven Rostedt <rostedt@goodmis.org> - 2017-01-19 14:40 +0100
    [PATCH] mutex: Remove ww_ctx unlikely() from __mutex_lock_common() "Steven Rostedt (VMware)" <rostedt@goodmis.org> - 2017-01-19 15:10 +0100
      Re: [PATCH] mutex: Remove ww_ctx unlikely() from  __mutex_lock_common() Steven Rostedt <rostedt@goodmis.org> - 2017-01-19 18:00 +0100
      Re: [PATCH] mutex: Remove ww_ctx unlikely() from  __mutex_lock_common() Peter Zijlstra <peterz@infradead.org> - 2017-01-19 22:00 +0100

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