Re: [PATCH v2 0/2] pstore,efi_pstore: Avoid deadlock in non-blockingpaths

From: Don Zickus
Date: Mon Dec 17 2012 - 16:45:18 EST

On Mon, Dec 17, 2012 at 08:56:27PM +0000, Seiji Aguchi wrote:
> Changelog v1 -> v2
> - Erase a logic checking the number of online cpus.
> - Create a patchset to fix deadlocking issue in both pstore filesystem and
> efi_pstore driver.
> - Introduce a function, is_non_blocking_path(), to check if pstore
> is in panic and emergency-restart paths (PATCH 1/2)
> - Avoid efi_pstore_driver is blocked in non-blocking paths
> such as nmi, panic and emergency-restart paths (PATCH 2/2)

This patch set seems cleaner. Though it seems most of patch2 should have
been in patch1 (except for the efi part).

The only nitpick I have is the name 'is_non_blocking_path'. I don't know
why, but the name doesn't seem exactly right to me. I was thinking
something like 'pstore_cannot_block_path' or something. But it is just a
nitpick, so it doesn't matter either way to me.


> [Issue]
> There are some paths which shouldn't be blocked in kernel, like NMI, panic case after stopping cpus,
> emergency-restart.
> On the other hand, current pstore avoids blocking in a NMI path but it may be blocked in other paths.
> Also, an efi_pstore driver may be blocked in all of those paths because it simply takes a spin lock
> at writing time.
> If they are blocked in those paths, the system will hang up and it has a big impact for users.
> Here is an example scenario which pstore is blocked in panic path.
> - cpuA grabs psinfo->buf_lock
> - cpuB panics and calls smp_send_stop
> - smp_send_stop sends IRQ to cpuA
> after 1 second, cpuB gives up on cpuA and sends an NMI instead
> - cpuA is now in an NMI handler while still holding buf_lock.
> And then, cpuB is deadlocked by taking efi_pstore->lock again.
> This case may happen if a firmware has a bug and cpuA is stuck in it.
> [Solution]
> This patchset avoids that pstore and efi_pstore driver are blocked in the non-blocking paths
> like NMI, panic, and emrgency-restart by introducing a function checking if they are in those paths.
> Please see each patch for detailed explanations.
> Seiji Aguchi (2):
> [PATCH v2 1/2] pstore: Avoid deadlock in panic and emergency-restart path
> [PATCH v2 2/2] efi_pstore: Avoid deadlock in non-blocking paths
> drivers/firmware/efivars.c | 11 ++++++++++-
> fs/pstore/platform.c | 34 ++++++++++++++++++++++++++++------
> include/linux/pstore.h | 6 ++++++
> 3 files changed, 44 insertions(+), 7 deletions(-)
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