On 16.12.2016 21:00, Peter Zijlstra wrote:
On Fri, Dec 16, 2016 at 07:11:41PM +0100, Nicolai Hähnle wrote:
mutex_optimistic_spin() already calls __mutex_trylock, and for the no-spin
case, __mutex_unlock_slowpath() only calls wake_up_q() after releasing the
wait_lock.
mutex_optimistic_spin() is a no-op when !CONFIG_MUTEX_SPIN_ON_OWNER
Does this change the conclusion in a meaningful way? I did mention the
no-spin case in the very part you quoted...
Again, AFAIU we're talking about the part of my proposal that turns what
is effectively
__mutex_trylock(lock, ...);
spin_lock_mutex(&lock->wait_lock, flags);
(independent of whether the trylock succeeds or not!) into
spin_lock_mutex(&lock->wait_lock, flags);
__mutex_trylock(lock, ...);
in an effort to streamline the code overall.
Also AFAIU, you're concerned that spin_lock_mutex(...) has to wait for
an unlock from mutex_unlock(), but when does that actually happen with
relevant probability?
When we spin optimistically, that could happen -- except that
__mutex_trylock is already called in mutex_optimistic_spin, so it
doesn't matter. When we don't spin -- whether due to .config or !first
-- then the chance of overlap with mutex_unlock is exceedingly small.
Even if we do overlap, we'll have to wait for mutex_unlock to release
the wait_lock anyway! So what good does acquiring the lock first really do?
Anyway, this is really more of an argument about whether there's really
a good reason to calling __mutex_trylock twice in that loop. I don't
think there is, your arguments certainly haven't been convincing, but
the issue can be side-stepped for this patch by keeping the trylock
calls as they are and just setting first = true unconditionally for
ww_ctx != NULL (but keep the logic for when to set the HANDOFF flag
as-is). Should probably rename the variable s/first/handoff/ then.
Nicolai
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