On Tue, 2011-05-17 at 22:15 -0600, Matthew Wilcox wrote:

On Wed, May 18, 2011 at 09:37:08AM +0530, Desai, Kashyap wrote:

quoted

On Wed, 2011-05-04 at 17:23 +0530, Kashyap, Desai wrote:

quoted

The following code seems to be there in /usr/src/linux/arch/x86/include/asm/io.h.
This is not going to work.

static inline void writeq(__u64 val, volatile void __iomem *addr)
{
        writel(val, addr);
        writel(val >> 32, addr+4);
}

So with this code turned on in the kernel, there is going to be race condition 
where multiple cpus can be writing to the request descriptor at the same time.

Meaning this could happen:
(A) CPU A doest 32bit write
(B) CPU B does 32 bit write
(C) CPU A does 32 bit write
(D) CPU B does 32 bit write

We need the 64 bit completed in one access pci memory write, else spin lock is required.
Since it's going to be difficult to know which writeq was implemented in the kernel, 
the driver is going to have to always acquire a spin lock each time we do 64bit write.

Cc: stable@kernle.org
Signed-off-by: Kashyap Desai <redacted>
---

diff --git a/drivers/scsi/mpt2sas/mpt2sas_base.c b/drivers/scsi/mpt2sas/mpt2sas_base.c
index efa0255..5778334 100644
--- a/drivers/scsi/mpt2sas/mpt2sas_base.c
+++ b/drivers/scsi/mpt2sas/mpt2sas_base.c

@@ -1558,7 +1558,6 @@ mpt2sas_base_free_smid(struct MPT2SAS_ADAPTER *ioc, u16 smid)
  * care of 32 bit environment where its not quarenteed to send the entire word
  * in one transfer.
  */
-#ifndef writeq

Why not make this #ifndef CONFIG_64BIT?  You know that all 64 bit
systems have writeq implemented correctly; you suspect 32 bit systems
don't.

James

James, This issue was observed on PPC64 system. So what you have suggested will not solve this issue.
If we are sure that writeq() is atomic across all architecture, we can use it safely. As we have seen issue on ppc64, we are not confident to use
"writeq" call.

So have you told the powerpc people that they have a broken writeq?

I'm just in the process of finding them now on IRC so I can demand an
explanation: this is a really serious API problem because writeq is
supposed to be atomic on 64 bit.

And why do you obfuscate your report by talking about i386 when it's
really about powerpc64?

James