On Tue, 29 Aug 2006, Esben Nielsen wrote:
> > > On Tue, 29 Aug 2006, Li Yang-r58472 wrote: > >> >> > -----Original Message----- >> > From: Esben Nielsen [mailto:nielsen.esben at gogglemail.com] >> > Sent: Tuesday, August 29, 2006 5:57 PM >> > To: Liu Dave-r63238 >> > Cc: Li Yang-r58472; Esben Nielsen; Xupei Liang; >> linuxppc-embedded at ozlabs.org >> > Subject: RE: atomic operations in user space >> > >> > >> > >> > On Tue, 29 Aug 2006, Liu Dave-r63238 wrote: >> > >> > > > > 2) These mutexes are based on futexes which requires atomic >> > > > > operations in userspace. These are available on most >> architectures. >> > > Look at >> > > > > the glibc code in >> > > > > nptl/sysdeps/unix/sysv/linux/powerpc/lowlevellock.h for instance. >> > > > > Use that and your PPC manual to implement your atomic operations. >> > > > >> > > > No matter semaphore or futex, it uses system calls to kernel. >> > >> > There is only a system call if there is congestion - that is the whole >> idea >> > behind the futex. >> > >> > > > And the >> > > > true atomic operation is in kernel not user space. >> > >> > "True" atomic operations are available in user space on most >> architectures. >> > >> > > > Maybe >> > > > it's feasible >> > > > for other architectures to do atomic operations directly in >> > > > user space. >> > > > IMHO, not for powerpc. >> > >> > It is available for PowerPC, but not in POWER and POWER2 >> instructionsets >> > according to http://www.nersc.gov/vendor_docs/ibm/asm/lwarx.htm#idx607 >> > It is the same in the ARM world: Atomic instructions was introduced in >> > ARMv6 I believe. Older ARM processors don't have them. >> >> Yes, I do know there are lwarx and stwrx instructions. But there is >> only one reservation per CPU and reservation can be re-established with >> no difference. >> So there are possibilities reservation is broken and reserved again in >> one atomic block. >> >> Task A Task B >> lwarx >> ...... >> lwarx >> stwrx >> >> ..... >> ..... >> lwarx >> stwrx >> ..... >> stwrx >> >> The addresses of above operations are the same. >> >> In this case Thread A thinks that it is atomic as it holds the same >> reservation, but it is actually broken. Such control flow can't be >> prevented in user space. >> > > So you are saying that futexes on powerpc are broken? Are you sure the reservation isn't automaticly broken due to the task switch? > > Esben > Esben >> > >> > > >> > > Are you meaning that we didn't do atomic operations directly in user >> > > space >> > > on powerpc platform ? >> > > >> > >> > Well, that is not the conclusion I get either when reading the glibc >> code. >> > Try to look at glibc-2.3.5/sysdeps/powerpc/bits/atomic.h. >> > >> > This is by the way probably what the original post in this thread >> wanted >> > in the first place! >> > >> > Esben >> > >> > >> > > -DAve >> >> _______________________________________________ >> Linuxppc-embedded mailing list >> Linuxppc-embedded at ozlabs.org >> https://ozlabs.org/mailman/listinfo/linuxppc-embedded >> > >
