linux-kernel - Re: [PATCH] pstore: change mutex locking to spin

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Date:	Thu, 21 Jul 2011 08:40:36 +0800
From:	Huang Ying <ying.huang@...el.com>
To:	Don Zickus <dzickus@...hat.com>
CC:	"Luck, Tony" <tony.luck@...el.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] pstore: change mutex locking to spin_locks

On 07/21/2011 04:56 AM, Don Zickus wrote:
> pstore was using mutex locking to protect read/write access to the
> backend plug-ins.  This causes problems when pstore is executed in
> an NMI context through panic() -> kmsg_dump().
> 
> This patch changes the mutex to a spin_lock_irqsave then also checks to
> see if we are in an NMI context.  If we are in an NMI and can't get the
> lock, just print a message stating that and blow by the locking.
> 
> All this is probably a hack around the bigger locking problem but it
> solves my current situation of trying to sleep in an NMI context.
> 
> Tested by loading the lkdtm module and executing a HARDLOCKUP which
> will cause the machine to panic inside the nmi handler.
> 
> Signed-off-by: Don Zickus <dzickus@...hat.com>
> ---
>  drivers/acpi/apei/erst.c |    2 +-
>  fs/pstore/platform.c     |   28 ++++++++++++++++++++++------
>  include/linux/pstore.h   |    2 +-
>  3 files changed, 24 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/acpi/apei/erst.c b/drivers/acpi/apei/erst.c
> index e6cef8e..3d9c2a3 100644
> --- a/drivers/acpi/apei/erst.c
> +++ b/drivers/acpi/apei/erst.c
> @@ -1155,7 +1155,7 @@ static int __init erst_init(void)
>  		goto err_release_erange;
>  
>  	buf = kmalloc(erst_erange.size, GFP_KERNEL);
> -	mutex_init(&erst_info.buf_mutex);
> +	spin_lock_init(&erst_info.buf_lock);
>  	if (buf) {
>  		erst_info.buf = buf + sizeof(struct cper_pstore_record);
>  		erst_info.bufsize = erst_erange.size -
> diff --git a/fs/pstore/platform.c b/fs/pstore/platform.c
> index f2c3ff2..53ba00b 100644
> --- a/fs/pstore/platform.c
> +++ b/fs/pstore/platform.c
> @@ -27,6 +27,7 @@
>  #include <linux/string.h>
>  #include <linux/slab.h>
>  #include <linux/uaccess.h>
> +#include <linux/hardirq.h>
>  
>  #include "internal.h"
>  
> @@ -68,13 +69,21 @@ static void pstore_dump(struct kmsg_dumper *dumper,
>  	char		*dst, *why;
>  	u64		id;
>  	int		hsize, part = 1;
> +	unsigned long	flags = 0;
> +	int 		is_locked = 0;
>  
>  	if (reason < ARRAY_SIZE(reason_str))
>  		why = reason_str[reason];
>  	else
>  		why = "Unknown";
>  
> -	mutex_lock(&psinfo->buf_mutex);
> +	if (in_nmi()) {
> +		is_locked = spin_trylock(&psinfo->buf_lock);
> +		if (!is_locked)
> +			pr_err("pstore dump routine blocked in NMI, may corrupt error record\n");
> +	} else {
> +		spin_lock_irqsave(&psinfo->buf_lock, flags);
> +	}
>  	oopscount++;
>  	while (total < kmsg_bytes) {
>  		dst = psinfo->buf;
> @@ -103,7 +112,12 @@ static void pstore_dump(struct kmsg_dumper *dumper,
>  		l2 -= l2_cpy;
>  		total += l1_cpy + l2_cpy;
>  	}
> -	mutex_unlock(&psinfo->buf_mutex);
> +	if (in_nmi()) {
> +		if (is_locked)
> +			spin_unlock(&psinfo->buf_lock);
> +	} else {
> +		spin_unlock_irqrestore(&psinfo->buf_lock, flags);
> +	}
>  }
>  
>  static struct kmsg_dumper pstore_dumper = {
> @@ -157,11 +171,12 @@ void pstore_get_records(void)
>  	enum pstore_type_id	type;
>  	struct timespec		time;
>  	int			failed = 0, rc;
> +	unsigned long		flags;
>  
>  	if (!psi)
>  		return;
>  
> -	mutex_lock(&psinfo->buf_mutex);
> +	spin_lock_irqsave(&psinfo->buf_lock, flags);
>  	rc = psi->open(psi);
>  	if (rc)
>  		goto out;
> @@ -173,7 +188,7 @@ void pstore_get_records(void)
>  	}
>  	psi->close(psi);
>  out:
> -	mutex_unlock(&psinfo->buf_mutex);
> +	spin_unlock_irqrestore(&psinfo->buf_lock, flags);
>  
>  	if (failed)
>  		printk(KERN_WARNING "pstore: failed to load %d record(s) from '%s'\n",
> @@ -187,6 +202,7 @@ out:
>  int pstore_write(enum pstore_type_id type, char *buf, size_t size)
>  {
>  	u64	id;
> +	unsigned long flags;
>  
>  	if (!psinfo)
>  		return -ENODEV;
> @@ -194,13 +210,13 @@ int pstore_write(enum pstore_type_id type, char *buf, size_t size)
>  	if (size > psinfo->bufsize)
>  		return -EFBIG;
>  
> -	mutex_lock(&psinfo->buf_mutex);
> +	spin_lock_irqsave(&psinfo->buf_lock, flags);
>  	memcpy(psinfo->buf, buf, size);
>  	id = psinfo->write(type, size);
>  	if (pstore_is_mounted())
>  		pstore_mkfile(PSTORE_TYPE_DMESG, psinfo->name, id, psinfo->buf,
>  			      size, CURRENT_TIME, psinfo->erase);
> -	mutex_unlock(&psinfo->buf_mutex);
> +	spin_unlock_irqrestore(&psinfo->buf_lock, flags);

Is it safe to call pstore_mkfile with IRQ disabled?

pstore_mkfile -> d_alloc_name -> d_alloc -> kmem_cache_alloc(, GFP_KERNEL).

Best Regards,
Huang Ying
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