From: Waiman Long <waiman.long@hpe.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
LKML <linux-kernel@vger.kernel.org>,
Ingo Molnar <mingo@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
"H. Peter Anvin" <hpa@zytor.com>,
"David S. Miller" <davem@davemloft.net>,
Tony Luck <tony.luck@intel.com>,
Andrew Morton <akpm@linux-foundation.org>,
Chris Zankel <chris@zankel.net>,
Max Filippov <jcmvbkbc@gmail.com>,
Davidlohr Bueso <dave@stgolabs.net>
Subject: Re: [PATCH 03/11] locking, rwsem: introduce basis for down_write_killable
Date: Thu, 12 May 2016 15:42:20 -0400 [thread overview]
Message-ID: <5734DC9C.9060105@hpe.com> (raw)
In-Reply-To: <20160512121907.GG4200@dhcp22.suse.cz>
On 05/12/2016 08:19 AM, Michal Hocko wrote:
> On Thu 12-05-16 14:12:04, Peter Zijlstra wrote:
>> On Wed, May 11, 2016 at 08:03:46PM +0200, Michal Hocko wrote:
>>> I still cannot say I would understand why the pending
>>> RWSEM_WAITING_BIAS matters but I would probably need to look at the code
>>> again with a clean head, __rwsem_wake is quite tricky...
>> Ah, you're asking why an unconditional __rwsem_wake(ANY) isn't enough?
>>
>> Because; if at that point there's nobody waiting, we're left with an
>> empty list and WAITER_BIAS set. This in turn will make all fast paths
>> fail.
>>
>> Look at rwsem_down_read_failed() for instance; if we enter that we'll
>> unconditionally queue ourself, with nobody left to come wake us.
> This is still not clear to me because rwsem_down_read_failed will call
> __rwsem_do_wake if the count is RWSEM_WAITING_BIAS so we shouldn't go to
> sleep and get the lock. So you are right that we would force everybody
> to the slow path which is not great but shouldn't cause incorrect
> behavior. I guess I must be missing something obvious here...
Because of writer lock stealing, having a count of RWSEM_WAITING_BIAS
doesn't mean the reader can surely get the lock even if it is the first
one in the queue. Calling __rwsem_do_wake() will take care of all the
locking and queue checking work. Yes, I think it is a bit odd for the
possibility that a task may wake up itself. Maybe we can add code like:
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -202,7 +202,8 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum
rwsem_wake_type wake_type)
*/
smp_mb();
waiter->task = NULL;
- wake_up_process(tsk);
+ if (tsk != current)
+ wake_up_process(tsk);
put_task_struct(tsk);
} while (--loop);
Cheers,
Longman
next prev parent reply other threads:[~2016-05-12 19:42 UTC|newest]
Thread overview: 55+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-04-01 11:04 [PATCH 0/11] introduce down_write_killable for rw_semaphore v2 Michal Hocko
2016-04-01 11:04 ` [PATCH 01/11] locking, rwsem: get rid of __down_write_nested Michal Hocko
2016-04-02 0:28 ` Davidlohr Bueso
2016-04-01 11:04 ` [PATCH 02/11] locking, rwsem: drop explicit memory barriers Michal Hocko
2016-04-02 1:17 ` Davidlohr Bueso
2016-04-04 9:03 ` Michal Hocko
2016-04-04 9:06 ` [PATCH 1/2] xtensa, rwsem: drop superfluous arch specific implementation Michal Hocko
2016-04-04 9:06 ` [PATCH 2/2] sh, " Michal Hocko
2016-04-06 9:26 ` Peter Zijlstra
2016-04-06 9:50 ` Geert Uytterhoeven
2016-04-06 10:27 ` Peter Zijlstra
2016-04-04 10:23 ` [PATCH 1/2] xtensa, " Max Filippov
2016-04-06 9:06 ` [PATCH] sparc, " Michal Hocko
2016-04-01 11:04 ` [PATCH 03/11] locking, rwsem: introduce basis for down_write_killable Michal Hocko
2016-04-02 4:41 ` Davidlohr Bueso
2016-04-04 9:17 ` Michal Hocko
2016-04-04 9:21 ` Peter Zijlstra
2016-04-07 6:58 ` Davidlohr Bueso
2016-04-07 7:38 ` Michal Hocko
2016-05-10 10:43 ` Tetsuo Handa
2016-05-10 11:53 ` Michal Hocko
2016-05-10 12:38 ` Peter Zijlstra
2016-05-10 13:57 ` Tetsuo Handa
2016-05-11 7:23 ` Michal Hocko
2016-05-11 8:28 ` Michal Hocko
2016-05-11 8:44 ` Peter Zijlstra
2016-05-11 9:04 ` Michal Hocko
2016-05-11 9:17 ` Peter Zijlstra
2016-05-11 9:31 ` Michal Hocko
2016-05-11 9:41 ` Peter Zijlstra
2016-05-11 13:59 ` Michal Hocko
2016-05-11 18:03 ` Michal Hocko
2016-05-12 11:57 ` [PATCH] locking, rwsem: Fix down_write_killable() Peter Zijlstra
2016-05-12 12:15 ` [tip:locking/rwsem] locking/rwsem: " tip-bot for Peter Zijlstra
2016-05-12 16:59 ` [PATCH] locking, rwsem: " Michal Hocko
2016-05-15 20:57 ` [tip:locking/rwsem] locking/rwsem: " tip-bot for Peter Zijlstra
2016-05-12 12:12 ` [PATCH 03/11] locking, rwsem: introduce basis for down_write_killable Peter Zijlstra
2016-05-12 12:19 ` Michal Hocko
2016-05-12 13:58 ` Peter Zijlstra
2016-05-12 19:42 ` Waiman Long [this message]
2016-05-11 8:35 ` Peter Zijlstra
2016-05-11 9:02 ` Michal Hocko
2016-04-01 11:04 ` [PATCH 04/11] alpha, rwsem: provide __down_write_killable Michal Hocko
2016-04-01 11:04 ` [PATCH 05/11] ia64, " Michal Hocko
2016-04-01 11:04 ` [PATCH 06/11] s390, " Michal Hocko
2016-04-01 11:04 ` [PATCH 07/11] sh, " Michal Hocko
2016-04-01 11:04 ` [PATCH 08/11] sparc, " Michal Hocko
2016-04-01 11:04 ` [PATCH 09/11] xtensa, " Michal Hocko
2016-04-01 11:04 ` [PATCH 10/11] x86, " Michal Hocko
2016-04-06 18:31 ` Peter Zijlstra
2016-04-01 11:04 ` [PATCH 11/11] locking, rwsem: provide down_write_killable Michal Hocko
-- strict thread matches above, loose matches on Subject: below --
2016-02-29 12:58 [PATCH 0/11] introduce down_write_killable for rw_semaphore Michal Hocko
2016-02-29 12:58 ` [PATCH 03/11] locking, rwsem: introduce basis for down_write_killable Michal Hocko
2016-03-30 13:25 ` Peter Zijlstra
2016-03-31 8:33 ` Michal Hocko
2016-03-31 8:44 ` Peter Zijlstra
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