Messages in this thread | | | Subject | Re: [PATCH v2 05/11] locking/ww_mutex: Add waiters in stamp order | From | Nicolai Hähnle <> | Date | Fri, 16 Dec 2016 23:35:12 +0100 |
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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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