linux-kernel - Re: [PATCH 3/4] rwsem: implement write lock stealing

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Message-ID: <CANN689F-Gbh56bDFDMSogxsYtUTTqObxyFTzvrz_somvWh-CUw@mail.gmail.com>
Date:	Fri, 8 Feb 2013 23:42:15 -0800
From:	Michel Lespinasse <walken@...gle.com>
To:	Hillf Danton <dhillf@...il.com>
Cc:	linux-kernel@...r.kernel.org, anton@...ba.org, hpa@...or.com,
	mingo@...nel.org, arjan@...ux.intel.com, a.p.zijlstra@...llo.nl,
	torvalds@...ux-foundation.org, alex.shi@...el.com,
	yuanhan.liu@...ux.intel.com, dhowells@...hat.com,
	akpm@...ux-foundation.org
Subject: Re: [PATCH 3/4] rwsem: implement write lock stealing

On Fri, Feb 8, 2013 at 11:30 PM, Hillf Danton <dhillf@...il.com> wrote:
> On Sat, Feb 9, 2013 at 10:45 AM, Michel Lespinasse <walken@...gle.com> wrote:
>> +               if (waiter->type != RWSEM_WAITING_FOR_WRITE) {
>> +                       list_del(&waiter->list);
>> +
>> +                       /* Set RWSEM_WAITING_BIAS before waking the last reader
>> +                        * so we know there will be a remaining active locker.
>> +                        */
>> +                       if (!(--readers) && !list_empty(&sem->wait_list))
>> +                               rwsem_atomic_add(RWSEM_WAITING_BIAS, sem);
>> +
>> +                       tsk = waiter->task;
>> +                       smp_mb();
>
> For what with mb?

This is the same mb that used to be before the readers_only label
before this change.
The intention is that the write to waiter->task must be the last
access to waiter - after that, the reader is granted the lock so it
could do anything, including deallocating the waiter structure as it
gets out of rwsem_down_read_failed() and possibly deallocating the
task structure if it exits

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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