linux-kernel - Re: [PATCH v3 1/2] qspinlock: Introducing a 4-byte queue spinlock implementation

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Message-ID: <20140129025740.17866.qmail@science.horizon.com>
Date:	28 Jan 2014 21:57:40 -0500
From:	"George Spelvin" <linux@...izon.com>
To:	andi@...stfloor.org, Waiman.Long@...com
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Subject: Re: [PATCH v3 1/2] qspinlock: Introducing a 4-byte queue spinlock implementation

> So the 1-2 threads case is the standard case on a small
> system, isn't it? This may well cause regressions.

Well, the common case should be uncontended, which is faster.
But yes, testing would be nice.

>> In the extremely unlikely case that all the queue node entries are
>> used up, the current code will fall back to busy spinning without
>> waiting in a queue with warning message.

> Traditionally we had some code which could take thousands
> of locks in rare cases (e.g. all locks in a hash table or all locks of
> a big reader lock) 

Doesn't apply; the question implies a misunderstanding of what's
happening.  The entry is only needed while spinning waiting for
the lock.  Once the lock has been acquired, it may be recycled.

The thread may *hold* thousands of locks; the entries only apply
to locks being *waited for*.

>From process context a thread may only be waiting for one at a time.
Additional entries are only needed in case a processor takes an interrupt
while spinning, and the interrupt handler wants to take a lock, too.

If that lock also has to be waited for, and during the wait you take a
nested interrupt or NMI, a third level might happen.

The chances of this being nested more than 4 deep seem sufficiently
minute.
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