-----Original Message----- From: James Bottomley [mailto:James.Bottomley@xxxxxxxxxxxxxxxxxxxxx] Sent: Tuesday, May 17, 2011 12:46 PM To: Desai, Kashyap Cc: linux-scsi@xxxxxxxxxxxxxxx; Moore, Eric; Prakash, Sathya Subject: Re: [PATCH 1/3] mpt2sas: remove the use of writeq, since writeq is not atomic On Wed, 2011-05-04 at 17:23 +0530, Kashyap, Desai wrote: > 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@xxxxxxxxxx > Signed-off-by: Kashyap Desai <kashyap.desai@xxxxxxx> > --- > 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. ~ Kashyap > static inline void _base_writeq(__u64 b, volatile void __iomem *addr, > spinlock_t *writeq_lock) > { > @@ -1570,13 +1569,6 @@ static inline void _base_writeq(__u64 b, volatile void __iomem *addr, > writel((u32)(data_out >> 32), (addr + 4)); > spin_unlock_irqrestore(writeq_lock, flags); > } > -#else > -static inline void _base_writeq(__u64 b, volatile void __iomem *addr, > - spinlock_t *writeq_lock) > -{ > - writeq(cpu_to_le64(b), addr); > -} > -#endif > > /** > * mpt2sas_base_put_smid_scsi_io - send SCSI_IO request to firmware > -- > To unsubscribe from this list: send the line "unsubscribe linux-scsi" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html ÿô.nlj·Ÿ®‰†+%ŠË±é¥Šwÿº{.nlj·¥Š{±þÇ‹ø¡Ü}©ž²ÆzÚj:+v‰¨þø®w¥þŠàÞ¨è&¢)ß«a¶Úÿûz¹ÞúŽŠÝjÿŠwèf
