[avr-libc-dev] "cli" and "sei" should clobber memory

[Paulo Marques]

Hi, all

I have a 32 bit variable that is updated on an interrupt. I don't want 
to declare it volatile as it is _not_ volatile most of the time.

I want to explicitly state the places where I want to do an "atomic" 
access to it.

So I wrote this simple function:

dword atomic_get_dword(dword *ptr)
{
        dword ret;
        cli();
        ret = *ptr;
        sei();
        return ret;
}

Whenever I want an atomic access to var X, I just do 
atomic_get_dword(&X) instead of accessing X directly.

The problem is that gcc reorders around the "sei" instruction, resulting 
in this output:

        cli();
 22e:   f8 94           cli
        ret = *ptr;
        sei();
 230:   78 94           sei
 232:   60 81           ld      r22, Z
 234:   71 81           ldd     r23, Z+1        ; 0x01
 236:   82 81           ldd     r24, Z+2        ; 0x02
 238:   93 81           ldd     r25, Z+3        ; 0x03
 23a:   08 95           ret

Changing the "sei" and "cli" definitions in avr/interrupt.h to:

# define sei()  __asm__ __volatile__ ("sei" ::: "memory")

# define cli()  __asm__ __volatile__ ("cli" ::: "memory")

fixes the problem. The output is then:

        cli();
 22e:   f8 94           cli
        ret = *ptr;
 230:   60 81           ld      r22, Z
 232:   71 81           ldd     r23, Z+1        ; 0x01
 234:   82 81           ldd     r24, Z+2        ; 0x02
 236:   93 81           ldd     r25, Z+3        ; 0x03
        sei();
 238:   78 94           sei
 23a:   08 95           ret

Basically this forces gcc to not reorder memory accesses around cli or 
sei calls.

This seems like the correct thing to do since we want to make sure that 
accesses to variables between "cli" and "sei" are not affect by interrupts.

This particular case might be solved by using a volatile var but others 
are not. For instance, if you want to take the value from one volatile 
var and add it to another atomically, you really need a "cli"/"sei" 
sequence.

Should I submit this as a patch? Where to?

--
Paulo Marques
Software Development Department - Grupo PIE, S.A.
Phone: +351 252 290600, Fax: +351 252 290601
Web: www.grupopie.com

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