linux-kernel - Re: [PATCH] pstore: avoid recursive spinlocks in the oops_in

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Message-ID: <20120921003706.GB14399@lizard>
Date:	Thu, 20 Sep 2012 17:37:06 -0700
From:	Anton Vorontsov <anton.vorontsov@...aro.org>
To:	"Luck, Tony" <tony.luck@...el.com>
Cc:	"Liu, Chuansheng" <chuansheng.liu@...el.com>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"keescook@...omium.org" <keescook@...omium.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Colin Cross <ccross@...roid.com>
Subject: Re: [PATCH] pstore: avoid recursive spinlocks in the
 oops_in_progress case

On Thu, Sep 20, 2012 at 11:48:32PM +0000, Luck, Tony wrote:
> > True, but the lock is used to protect pstore->buf, I doubt that
> > any backend will actually want to grab it, no?
> 
> The lock is doing double duty to protect the buffer, and the back-end driver.
> 
> But even if we split it into two (one for the buffer, taken by pstore, and one
> internal to the backend to protect interaction with the f/w). Ifwe ignore the
> fact that we can't get the lock that protects  the buffer means it is very likely
> that we corrupt the previous record that was being written by clobbering the
> buffer with the data for this new record.
> 
> I'd prefer to maximize the chances that the earlier record gets written.

Sure, I applied the original patch.

Btw, do you expect that backends protect themselves from concurrent
->write calls, or pstore guarantees to protect backends?

Because the latter is not always possible, for example in tracing: we
won't able to grab locks at all (but not all backends can do tracing
anyway -- they must do things atomically).

Plus, sometimes having the global lock is not "efficient", backends
know better: they might have separate locks per message type.

And my plan was to get rid of the fact that backends touch pstore->buf
directly. Backends would always receive anonymous 'buf' pointer (we
already have write_buf callback that does exactly this), and thus it
would be backends' worry to protect against concurrency. In this
scheme, pstore's console code won't need to grab locks at all: we'll
just pass console string to the backend directly.

And backends, if they can't do writes atomically, will grab their
own locks.

Thanks,
Anton.
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