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From: "Alastair D'Silva" <alastair@au1.ibm.com>
To: Frederic Barrat <fbarrat@linux.ibm.com>, linuxppc-dev@lists.ozlabs.org
Cc: linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
	mikey@neuling.org, vaibhav@linux.vnet.ibm.com,
	aneesh.kumar@linux.vnet.ibm.com, malat@debian.org,
	felix@linux.vnet.ibm.com, pombredanne@nexb.com,
	sukadev@linux.vnet.ibm.com, npiggin@gmail.com,
	gregkh@linuxfoundation.org, arnd@arndb.de,
	andrew.donnellan@au1.ibm.com, fbarrat@linux.vnet.ibm.com,
	corbet@lwn.net
Subject: Re: [PATCH v2 3/7] powerpc: use task_pid_nr() for TID allocation
Date: Tue, 08 May 2018 10:40:21 +1000	[thread overview]
Message-ID: <1525740021.7796.45.camel@au1.ibm.com> (raw)
In-Reply-To: <b8aa28c4-9171-cb7f-9a43-22f2f15657b0@linux.ibm.com>

On Mon, 2018-05-07 at 19:37 +0200, Frederic Barrat wrote:
> 
> Le 18/04/2018 à 03:08, Alastair D'Silva a écrit :
> > From: Alastair D'Silva <alastair@d-silva.org>
> > 
> > The current implementation of TID allocation, using a global IDR,
> > may
> > result in an errant process starving the system of available TIDs.
> > Instead, use task_pid_nr(), as mentioned by the original author.
> > The
> > scenario described which prevented it's use is not applicable, as
> > set_thread_tidr can only be called after the task struct has been
> > populated.
> 
> 
> Here is how I understand what's going to happen if 2 threads are
> using 
> the same TIDR value, which is possible with this patch (if unlikely):
> 
> 1. waking up the wrong thread is not really a problem, as threads
> have 
> to handle spurious wake up from the 'wait' instruction anyway, and
> must 
> be using some other condition to know when to loop around the 'wait' 
> instruction.
> 
> 2. missing the right thread: if the wrong thread is on a CPU, and a 
> wake_host_thread/as_notify is sent, the core will see a matching
> thread 
> and will accept the command. The (open)capi adapter won't send an 
> interrupt. The wrong thread is awaken, which is not a problem as 
> discussed above. As the right thread to notify is not running, no
> harm 
> is done either: as soon as the thread runs, it's supposed to check
> its 
> condition (which will be met) or call 'wait', but 'wait' immediately 
> returns when called the first time after a thread is scheduled.
> 
> So I believe we are ok. But I think it requires a huge comment with
> the 
> above (at the minimum) :-)
> 
> With a comment:
> Reviewed-by: Frederic Barrat <fbarrat@linux.vnet.ibm.com>
> 
>    Fred
> 

Good point, I'll add this in the next revision.


-- 
Alastair D'Silva
Open Source Developer
Linux Technology Centre, IBM Australiamob: 0423 762 819

  reply	other threads:[~2018-05-08  0:40 UTC|newest]

Thread overview: 59+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-17  2:09 [PATCH 0/7] ocxl: Implement Power9 as_notify/wait for OpenCAPI Alastair D'Silva
2018-04-17  2:09 ` [PATCH 1/7] powerpc: Add TIDR CPU feature for Power9 Alastair D'Silva
2018-04-17  4:09   ` Andrew Donnellan
2018-04-17  2:09 ` [PATCH 2/7] powerpc: Use TIDR CPU feature to control TIDR allocation Alastair D'Silva
2018-04-17  4:21   ` Andrew Donnellan
2018-04-17  5:31     ` Alastair D'Silva
2018-04-17  2:09 ` [PATCH 3/7] powerpc: use task_pid_nr() for TID allocation Alastair D'Silva
2018-04-17  2:09 ` [PATCH 4/7] ocxl: Rename pnv_ocxl_spa_remove_pe to clarify it's action Alastair D'Silva
2018-04-17  5:37   ` Andrew Donnellan
2018-04-17  2:09 ` [PATCH 5/7] ocxl: Expose the thread_id needed for wait on p9 Alastair D'Silva
2018-04-17  2:09 ` [PATCH 6/7] ocxl: Add an IOCTL so userspace knows which platform the kernel requires Alastair D'Silva
2018-04-17  2:09 ` [PATCH 7/7] ocxl: Document new OCXL IOCTLs Alastair D'Silva
2018-04-17  3:45   ` Andrew Donnellan
2018-04-18  1:08 ` [PATCH v2 0/7] ocxl: Implement Power9 as_notify/wait for OpenCAPI Alastair D'Silva
2018-04-18  1:08   ` [PATCH v2 1/7] powerpc: Add TIDR CPU feature for Power9 Alastair D'Silva
2018-04-18  7:03     ` Andrew Donnellan
2018-05-07 17:17     ` Frederic Barrat
2018-05-08  3:13       ` Alastair D'Silva
2018-04-18  1:08   ` [PATCH v2 2/7] powerpc: Use TIDR CPU feature to control TIDR allocation Alastair D'Silva
2018-04-18  7:13     ` Andrew Donnellan
2018-05-07 17:19     ` Frederic Barrat
2018-04-18  1:08   ` [PATCH v2 3/7] powerpc: use task_pid_nr() for TID allocation Alastair D'Silva
2018-04-20  8:43     ` Andrew Donnellan
2018-04-24 21:12       ` Sukadev Bhattiprolu
2018-04-26  9:25         ` Andrew Donnellan
2018-05-07 17:37     ` Frederic Barrat
2018-05-08  0:40       ` Alastair D'Silva [this message]
2018-04-18  1:08   ` [PATCH v2 4/7] ocxl: Rename pnv_ocxl_spa_remove_pe to clarify it's action Alastair D'Silva
2018-05-07 17:38     ` Frederic Barrat
2018-04-18  1:08   ` [PATCH v2 5/7] ocxl: Expose the thread_id needed for wait on p9 Alastair D'Silva
2018-04-23  7:16     ` Andrew Donnellan
2018-05-07 18:08     ` Frederic Barrat
2018-04-18  1:08   ` [PATCH v2 6/7] ocxl: Add an IOCTL so userspace knows what CPU features are available Alastair D'Silva
2018-04-20  7:25     ` Andrew Donnellan
2018-05-07 18:14     ` Frederic Barrat
2018-05-08  0:41       ` Alastair D'Silva
2018-05-08  3:50         ` Nicholas Piggin
2018-05-08  3:54           ` Alastair D'Silva
2018-04-18  1:08   ` [PATCH v2 7/7] ocxl: Document new OCXL IOCTLs Alastair D'Silva
2018-04-18  7:29     ` Andrew Donnellan
2018-05-07 18:15     ` Frederic Barrat
2018-05-09  0:42 ` [PATCH v3 0/7] ocxl: Implement Power9 as_notify/wait for OpenCAPI Alastair D'Silva
2018-05-09  0:42   ` [PATCH v3 1/7] powerpc: Add TIDR CPU feature for POWER9 Alastair D'Silva
2018-05-09  0:42   ` [PATCH v3 2/7] powerpc: Use TIDR CPU feature to control TIDR allocation Alastair D'Silva
2018-05-09  0:42   ` [PATCH v3 3/7] powerpc: use task_pid_nr() for TID allocation Alastair D'Silva
2018-05-09  0:42   ` [PATCH v3 4/7] ocxl: Rename pnv_ocxl_spa_remove_pe to clarify it's action Alastair D'Silva
2018-05-09  0:42   ` [PATCH v3 5/7] ocxl: Expose the thread_id needed for wait on POWER9 Alastair D'Silva
2018-05-11  5:01     ` Michael Ellerman
2018-05-09  0:42   ` [PATCH v3 6/7] ocxl: Add an IOCTL so userspace knows what OCXL features are available Alastair D'Silva
2018-05-09  0:42   ` [PATCH v3 7/7] ocxl: Document new OCXL IOCTLs Alastair D'Silva
2018-05-09  5:34   ` [PATCH v4 0/7] ocxl: Implement Power9 as_notify/wait for OpenCAPI Alastair D'Silva
2018-05-09  5:35     ` [PATCH v4 1/7] powerpc: Add TIDR CPU feature for POWER9 Alastair D'Silva
2018-05-09  5:35     ` [PATCH v4 2/7] powerpc: Use TIDR CPU feature to control TIDR allocation Alastair D'Silva
2018-05-09  5:35     ` [PATCH v4 3/7] powerpc: use task_pid_nr() for TID allocation Alastair D'Silva
2018-05-09  5:35     ` [PATCH v4 4/7] ocxl: Rename pnv_ocxl_spa_remove_pe to clarify it's action Alastair D'Silva
2018-05-09  5:35     ` [PATCH v4 5/7] ocxl: Expose the thread_id needed for wait on POWER9 Alastair D'Silva
2018-05-09  5:35     ` [PATCH v4 6/7] ocxl: Add an IOCTL so userspace knows what OCXL features are available Alastair D'Silva
2018-05-11  5:20       ` Michael Ellerman
2018-05-09  5:35     ` [PATCH v4 7/7] ocxl: Document new OCXL IOCTLs Alastair D'Silva

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