* Re: spin-locks
2002-05-10 13:24 ` spin-locks Keith Owens
@ 2002-05-10 14:06 ` Richard B. Johnson
2002-05-10 14:17 ` spin-locks David Woodhouse
0 siblings, 1 reply; 5+ messages in thread
From: Richard B. Johnson @ 2002-05-10 14:06 UTC (permalink / raw)
To: Keith Owens; +Cc: Linux kernel
On Fri, 10 May 2002, Keith Owens wrote:
> On Fri, 10 May 2002 08:58:47 -0400 (EDT),
> "Richard B. Johnson" <root@chaos.analogic.com> wrote:
> >First, if I create a spin-lock in the ".data" segment it
> >doesn't work on a SMP machine with two CPUs. I know I am
> >supposed to use the macros, but I have some high-speed stuff
> >written in assembly that needs a spin-lock. The 'doesn't work'
> >is that the spin-lock seems to dead-lock, i.e., they loop
> >forever with the interrupts disabled. I think what's really
> >happening is that .data was paged and can't be paged back in
> >with the interrupts off. I don't know. This stuff used to
> >work....
>
> Kernel .data sections are not paged. They are identity[*] mapped along
> with the rest of the kernel text and data and are locked down.
>
> [*] Ignoring NUMA machines which may use non-identity mappings on each
> node.
>
> >In earlier versions of Linux, the locks were in .text_lock.
> >Now they are in : _text_lock_KBUILD_BASENAME
>
> Not quite. They were all in section .text.lock but that broke when
> binutils started detecting dangling references to discarded sections.
>
> The fix was to store the lock code in the same section that called the
> lock, so .text locks are in the .text section, .text.exit locks are in
> the .text.exit section, no dangling references when .text.exit is
> discarded.
>
> The locks are now at the end of the section that references the lock,
> preceded by a label (not a section) of _text_lock_KBUILD_BASENAME.
>
> >So, what is special about this area that allows locks to work?
>
> There is nothing special about the text lock code. It is just moving
> the failure path out of line to speed up the normal case.
>
> >And, what is special about .data that prevents them from working?
>
> Again nothing. The spin lock area goes in .data, the code goes in the
> relevant text section.
>
> >Also, there is a potential bug (ducks and hides under the desk) in
> >the existing spin-lock unlocking. To unlock, the lock is simply
> >set to 1. This works if you have two CPUs, but what about more?
> >
> >Shouldn't the lock/unlock just be incremented/decremented so 'N' CPUs
> >can pound on it?
>
> Spinlocks are single cpu. Only one cpu at a time can modify the data
> that is being protected by obtaining the lock.
>
> >From your description, you are confused about spinlocking. Perhaps if
> you mailed your code instead of assuming where the error was ...
Well, here is code that worked on linux 2.2.17. Same CPUs, same
everything... Just a different version of OS...
In this code, any CPU will modify (increment) the lock so we can
only have 255 CPUs before this fails <grin>...
#-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
#-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
.section .data
lrm_tickl: .long 0
lrm_tickh: .long 0
lockf: .byte 0
.section .text
#-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
#
# This reads the rapidly-changing tick-value and returns it as a
# long long. The output value is in carefully calculated miliseconds.
#
.global lrm_tick
.type lrm_tick,@function
.align 0x04
lrm_tick:
pushf # Restored at the end
cli
lock
incb (lockf) # Bump lock-value
1: cmpb $1,(lockf) # See if we own it
jnz 1b # Nope, spin until we do.
pushl %ebx # Save non-volatile registers
pushl %ecx
pushl %esi
pushl %edi
#
# Locks no longer work so I have to do this hack......
#
2: movl (lrm_tickl), %eax
movl (lrm_tickh), %edx
cmpl (lrm_tickl), %eax
jnz 2b
cmpl (lrm_tickh), %edx
jnz 2b
#
# Okay, we have a stable tick. Now, it's 2048 ticks/second instead
# of 1,000 ticks. Therefore we have to multiply by 1,000 and and
# divide by 2048 to get the millisecond count.
#
xorl %esi, %esi # For overflow
movl %eax, %ebx # Save times 1 low lword
movl %edx, %ecx # Save times 1 high lword
movl %esi, %edi # Save overflow
#
shl $1, %eax # Times 2
adcl $0, %edx # Take care of CY
adcl $0, %esi # Take care of overflow
#
shl $1, %eax # Times 4
adcl $0, %edx # Take care of CY
adcl $0, %esi # Take care of overflow
#
addl %ebx, %eax # Add the times 1 low lword
adcl %ecx, %edx # Add the times 1 high lword
adcl %edi, %esi # Add the times 1 overflow
# Now times 5
#
shl $1, %eax # Times 10
adcl $0, %edx # Take care of CY
adcl $0, %esi # Take care of overflow
#
movl %eax, %ebx # Save times 10 low lword
movl %edx, %ecx # Save times 10 high lword
movl %esi, %edi # Save overflow
#
shl $1, %eax # Times 20
adcl $0, %edx # Take care of CY
adcl $0, %esi # Take care of overflow
#
shl $1, %eax # Times 40
adcl $0, %edx # Take care of CY
adcl $0, %esi # Take care of overflow
#
addl %ebx, %eax # Add the times 10 low lword
adcl %ecx, %edx # Add the times 10 high lword
adcl %edi, %esi # Add the times 10 overflow
# Now times 50
#
shl $1, %eax # Times 100
adcl $0, %edx # Take care of CY
adcl $0, %esi # Take care of overflow
#
movl %eax, %ebx # Save times 100 low lword
movl %edx, %ecx # Save times 100 high lword
movl %esi, %edi # Save overflow
#
shl $1, %eax # Times 200
adcl $0, %edx # Take care of CY
adcl $0, %esi # Take care of overflow
#
shl $1, %eax # Times 400
adcl $0, %edx # Take care of CY
adcl $0, %esi # Take care of overflow
#
addl %ebx, %eax # Add the times 100 low lword
adcl %ecx, %edx # Add the times 100 high lword
adcl %edi, %esi # Add the times 100 overflow
# Now times 500
#
shl $1, %eax # Times 1000
adcl $0, %edx # Take care of CY
adcl $0, %esi # Take care of overflow
#
# Now do the division (2^11) and yes, I do know about loops.
#
shrl $1, %esi
rcrl $1, %edx # Div by 2
rcrl $1, %eax
#
shrl $1, %esi
rcrl $1, %edx # Div by 4
rcrl $1, %eax
#
shrl $1, %esi
rcrl $1, %edx # Div by 8
rcrl $1, %eax
#
shrl $1, %esi
rcrl $1, %edx # Div by 16
rcrl $1, %eax
#
shrl $1, %esi
rcrl $1, %edx # Div by 32
rcrl $1, %eax
#
shrl $1, %esi
rcrl $1, %edx # Div by 64
rcrl $1, %eax
#
shrl $1, %esi
rcrl $1, %edx # Div by 128
rcrl $1, %eax
#
shrl $1, %esi
rcrl $1, %edx # Div by 256
rcrl $1, %eax
#
shrl $1, %esi
rcrl $1, %edx # Div by 512
rcrl $1, %eax
#
shrl $1, %esi
rcrl $1, %edx # Div by 1024
rcrl $1, %eax
#
shrl $1, %esi
rcrl $1, %edx # Div by 2048
rcrl $1, %eax
#
# Return long-long in EAX:EDX
#
popl %edi
popl %esi
popl %ecx # Restore registers used
popl %ebx
lock
decb (lockf) # Release lock
popf
ret
.end
#-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
Cheers,
Dick Johnson
Penguin : Linux version 2.4.18 on an i686 machine (797.90 BogoMips).
Windows-2000/Professional isn't.
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