From: Will Deacon
Sent: 18 November 2015 12:28 On Wed, Nov 18, 2015 at 12:11:25PM +0000, David Laight wrote:
From: Will Deacon
Sent: 18 November 2015 10:14 On Tue, Nov 17, 2015 at 08:17:17PM +0100, Arnd Bergmann wrote:
On Tuesday 17 November 2015 17:12:37 Will Deacon wrote:
On Tue, Nov 17, 2015 at 06:03:40PM +0100, Arnd Bergmann wrote:
On Tuesday 17 November 2015 16:44:53 Will Deacon wrote: > > 8<---- > > Subject: ARM64: make smp_load_acquire() work with const arguments > > > > smp_load_acquire() uses typeof() to declare a local variable for temporarily > > storing the output of the memory access. This fails when the argument is > > constant, because the assembler complains about using a constant register > > as output: > > > > arch/arm64/include/asm/barrier.h:71:3: error: read-only variable '___p1' > > used as 'asm' output > > Do you know the usage in the kernel causing this warning?
A newly introduced function in include/net/sock.h:
static inline int sk_state_load(const struct sock *sk) { return smp_load_acquire(&sk->sk_state); }
Hmm, maybe we could play a similar trick to READ_ONCE by declaring an anonymous union and writing through the non-const member?
Yes, I think that would work, if you think we need to care about the case where we read into a structure.
Can you come up with a patch for that?
Done:
http://lists.infradead.org/pipermail/linux-arm-kernel/2015-November/386094.h...
That patch forces a memory write-read and returns uninitialised stack for short reads.
Really? The disassembly looks fine to me. Do you have a concrete example of where you think it goes wrong, please?
Who knows what happens on big-endian systems.
The same thing as READ_ONCE? I'll test it there to make sure, but I don't see a problem.
Ah, god, it is absolutely horrid. But probably right :-(
Do all the lda variants zero extend to 64 bits ? If so maybe you could use a single 64 bit variable for the result of the read and then cast it to typeof(*p) to get the required sign extension for small integer types.
David