linux-kernel - efi_pstore: question about how to remove spinlock from a whole process of open/read/close of efi

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Date:	Fri, 24 Aug 2012 19:19:04 +0000
From:	Seiji Aguchi <seiji.aguchi@....com>
To:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Luck, Tony (tony.luck@...el.com)" <tony.luck@...el.com>,
	"Matthew Garrett (mjg@...hat.com)" <mjg@...hat.com>,
	"dzickus@...hat.com" <dzickus@...hat.com>,
	"mikew@...gle.com" <mikew@...gle.com>
CC:	"dle-develop@...ts.sourceforge.net" 
	<dle-develop@...ts.sourceforge.net>,
	Satoru Moriya <satoru.moriya@....com>
Subject: efi_pstore: question about how to remove spinlock from a whole
 process of open/read/close of efi_pstore

Hi,

I'm sending an email to discuss how to remove spinlock from a whole process of open/read/close of efi_pstore.

[Problem]

Current efi_pstore calls kmalloc() in a read callback while holding a spinlock, efivar->lock, in an open callback.
This means efi_pstore may deadlock if it sleeps in kmalloc().

Also, there is a comment that an efivar->lock shouldn't be held when calling kmalloc().

[Idea]

In a process of open/read/close, pstore creates some files in the directory of /dev/pstore.
So, memory allocations like kmalloc() are not avoidable.

My idea fixing this issue is removing spinlock from a whole process of open/read/close of efi_pstore.

Currently, efivar->lock protects two things, efivar->list and ops.[gs]et_variable().
But if we can revise locks of efivar as follows, we can fix this issue.

  - Introduce a new efivar->list_lock to protect efivar->list and RCU is used while searching it.

  - An existing efivar>lock protects just ops.[gs]et_variable() calls.

Any comments are welcome.

Seiji
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